[java] Collision Detection between two images in Java

I have two characters displayed in a game I am writing, the player and the enemy. defined as such:

public void player(Graphics g) {
    g.drawImage(plimg, x, y, this);
}

public void enemy(Graphics g) {
    g.drawImage(enemy, 200, 200, this);
}

Then called with:

player(g);
enemy(g);

I am able to move player() around with the keyboard, but I am at a loss when trying to detect a collision between the two. A lot of people have said to use Rectangles, but being a beginner I cannot see how I would link this into my existing code. Can anyone offer some advice for me?

Here's a useful of an open source game that uses a lot of collisions: http://robocode.sourceforge.net/

You may take a look at the code and complement with the answers written here.

You don't want to have the collision check code inside the painting code. The painting needs to be fast. Collision can go in the game loop. Therefore you need an internal representation of the objects independent of their sprites.

Since Java doesn't have an intersect function (really!?) you can do collision detection by simply comparying the X and Y, Width and Height values of the bounding boxes (rectangle) for each of the objects that could potentially collide.

So... in the base object of each colliding object... i.e. if your player and enemy have a common base you can put a simple Rectangle object called something like BoundingBox. If the common base is a built in Java class then you'll need to create a class that extends the build in class and have the player and enemy objects extend your new class or are instances of that class.

At creation (and each tick or update) you'll need to set the BoundingBox paremeters for both your player and enemy. I don't have the Rectangle class infront of me but its most likely something like X, Y, Width and finally Height. X and Y are that objects location in your game world. The width and height are self explanatory I think. They'll most likely come out from the right of the players location though so, if the X and Y were bothe at 0 and your Width and Height were both at 256 you wouldn't see anything because the character would be at the top left outside of the screen.

Anyways... to detect a collision, you'll want to compare the attributes of the player and enemy BoundingBoxes. So something like this...

 if( Player.BoundingBox.X = Enemy.BoundingBox.X && If( Player.BoundingBox.Y = Enemy.BoundingBox.Y )
 {
      //Oh noes!  The enemy and player are on top of eachother.
 }

The logic can get sort of complicated but you'll need to compare the distances between each BoundingBox and compare locations.

First, use the bounding boxes as described by Jonathan Holland to find if you may have a collision.

From the (multi-color) sprites, create black and white versions. You probably already have these if your sprites are transparent (i.e. there are places which are inside the bounding box but you can still see the background). These are "masks".

Use Image.getRGB() on the mask to get at the pixels. For each pixel which isn't transparent, set a bit in an integer array (playerArray and enemyArray below). The size of the array is height if width <= 32 pixels, (width+31)/32*height otherwise. The code below is for width <= 32.

If you have a collision of the bounding boxes, do this:

// Find the first line where the two sprites might overlap
int linePlayer, lineEnemy;
if (player.y <= enemy.y) {
    linePlayer = enemy.y - player.y;
    lineEnemy = 0;
} else {
    linePlayer = 0;
    lineEnemy = player.y - enemy.y;
}
int line = Math.max(linePlayer, lineEnemy);

// Get the shift between the two
x = player.x - enemy.x;
int maxLines = Math.max(player.height, enemy.height);
for ( line < maxLines; line ++) {
    // if width > 32, then you need a second loop here
    long playerMask = playerArray[linePlayer];
    long enemyMask = enemyArray[lineEnemy];
    // Reproduce the shift between the two sprites
    if (x < 0) playerMask << (-x);
    else enemyMask << x;
    // If the two masks have common bits, binary AND will return != 0
    if ((playerMask & enemyMask) != 0) {
        // Contact!
    }

}

Links: JGame, Framework for Small Java Games

No need to use rectangles ... compare the coordinates of 2 players constantly.

like if(x1===x&&y1==y) remember to increase the range of x when ur comparing.

if ur rectangle width is 30 take as if (x1>x&&x2>x+30)..likewise y

is there a problem with:

Rectangle box1 = new Rectangle(100,100,100,100);
Rectangle box2 = new Rectangle(200,200,100,100);

// what this means is if any pixel in box2 enters (hits) box1
if (box1.contains(box2)) 
{
     // collision occurred
}

// your code for moving the boxes 

this can also be applied to circles:

Ellipse2D.Double ball1 = new Ellipse2D.Double(100,100,200,200);
Ellipse2D.Double ball2 = new Ellipse2D.Double(400,100,200,200);

// what this means is if any pixel on the circumference in ball2 touches (hits)
// ball1
    if (ball1.contains(ball2)) 
    {
         // collision occurred
    }

    // your code for moving the balls

to check whether youve hit the edge of a screen you could use the following:

Rectangle screenBounds = jpanel.getBounds();
Ellipse2D.Double ball = new Ellipse2D.Double(100,100,200,200);  // diameter 200
Rectangle ballBounds = ball.getBounds();

if (!screenBounds.contains(ballBounds))
{
// the ball touched the edge of the screen
}

First, use the bounding boxes as described by Jonathan Holland to find if you may have a collision.

From the (multi-color) sprites, create black and white versions. You probably already have these if your sprites are transparent (i.e. there are places which are inside the bounding box but you can still see the background). These are "masks".

Use Image.getRGB() on the mask to get at the pixels. For each pixel which isn't transparent, set a bit in an integer array (playerArray and enemyArray below). The size of the array is height if width <= 32 pixels, (width+31)/32*height otherwise. The code below is for width <= 32.

If you have a collision of the bounding boxes, do this:

// Find the first line where the two sprites might overlap
int linePlayer, lineEnemy;
if (player.y <= enemy.y) {
    linePlayer = enemy.y - player.y;
    lineEnemy = 0;
} else {
    linePlayer = 0;
    lineEnemy = player.y - enemy.y;
}
int line = Math.max(linePlayer, lineEnemy);

// Get the shift between the two
x = player.x - enemy.x;
int maxLines = Math.max(player.height, enemy.height);
for ( line < maxLines; line ++) {
    // if width > 32, then you need a second loop here
    long playerMask = playerArray[linePlayer];
    long enemyMask = enemyArray[lineEnemy];
    // Reproduce the shift between the two sprites
    if (x < 0) playerMask << (-x);
    else enemyMask << x;
    // If the two masks have common bits, binary AND will return != 0
    if ((playerMask & enemyMask) != 0) {
        // Contact!
    }

}

Links: JGame, Framework for Small Java Games

Since Java doesn't have an intersect function (really!?) you can do collision detection by simply comparying the X and Y, Width and Height values of the bounding boxes (rectangle) for each of the objects that could potentially collide.

So... in the base object of each colliding object... i.e. if your player and enemy have a common base you can put a simple Rectangle object called something like BoundingBox. If the common base is a built in Java class then you'll need to create a class that extends the build in class and have the player and enemy objects extend your new class or are instances of that class.

At creation (and each tick or update) you'll need to set the BoundingBox paremeters for both your player and enemy. I don't have the Rectangle class infront of me but its most likely something like X, Y, Width and finally Height. X and Y are that objects location in your game world. The width and height are self explanatory I think. They'll most likely come out from the right of the players location though so, if the X and Y were bothe at 0 and your Width and Height were both at 256 you wouldn't see anything because the character would be at the top left outside of the screen.

Anyways... to detect a collision, you'll want to compare the attributes of the player and enemy BoundingBoxes. So something like this...

 if( Player.BoundingBox.X = Enemy.BoundingBox.X && If( Player.BoundingBox.Y = Enemy.BoundingBox.Y )
 {
      //Oh noes!  The enemy and player are on top of eachother.
 }

The logic can get sort of complicated but you'll need to compare the distances between each BoundingBox and compare locations.

Here's a useful of an open source game that uses a lot of collisions: http://robocode.sourceforge.net/

You may take a look at the code and complement with the answers written here.

Here's the main class from my collision detection program.
You can see it run at: http://www.youtube.com/watch?v=JIXhCvXgjsQ

/**
 *
 * @author Tyler Griffin
 */
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.awt.GraphicsDevice.*;
import java.util.ArrayList;
import java.awt.Graphics;
import java.awt.geom.Line2D;


public class collision extends JFrame implements KeyListener, MouseMotionListener, MouseListener
{
    ArrayList everything=new ArrayList<tile>();

    int time=0, x, y, width, height, up=0, down=0, left=0, right=0, mouse1=0, mouse2=0;
    int mouseX, mouseY;

    GraphicsEnvironment environment = GraphicsEnvironment.getLocalGraphicsEnvironment();
    GraphicsDevice screen = environment.getDefaultScreenDevice();
    DisplayMode displayMode = screen.getDisplayMode();

    //private BufferStrategy strategy;

    JLayeredPane pane = new JLayeredPane();

     tile Tile;
     circle Circle;
     rectangle Rectangle;

         textPane text;

    public collision()
    {
        setUndecorated(screen.isFullScreenSupported());
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);
        setLayout(null);
        setResizable(false);
        screen.setFullScreenWindow(this);


        width=displayMode.getWidth();
        height=displayMode.getHeight();


          Circle=new circle(-(int)Math.round((double)height/7*2),-(int)Math.round((double)height/7*2),(int)Math.round((double)height/7*.85),this);
          Rectangle=new rectangle(-(int)Math.round((double)height/7*1.5),-(int)Math.round((double)height/7*1.5),(int)Math.round((double)height/7*1.5),(int)Math.round((double)height/7*1.5),this);
          Tile=Circle;
          Tile.move(mouseX-Tile.width/2, mouseY-Tile.height/2);
                  text=new textPane(0,0,width,height,this);

          everything.add(new circle((int)Math.round((double)width/100*75),(int)Math.round((double)height/100*15),(int)Math.round((double)width/100*10),this));
                  everything.add(new rectangle((int)Math.round((double)width/100*70),(int)Math.round((double)height/100*60),(int)Math.round((double)width/100*20),(int)Math.round((double)height/100*20),this));
                  //everything.add(new line(750,250,750,750,this));
                  /*everything.add(new line(width/700*419,height/700*68,width/700*495,height/700*345,this));
                  everything.add(new line(width/700*495,height/700*345,width/700*749,height/700*350,this));
                  everything.add(new line(width/700*749,height/700*350,width/700*549,height/700*519,this));
                  everything.add(new line(width/700*549,height/700*519,width/700*624,height/700*800,this));
                  everything.add(new line(width/700*624,height/700*800,width/700*419,height/700*638,this));
                  everything.add(new line(width/700*419,height/700*638,width/700*203,height/700*800,this));
                  everything.add(new line(width/700*203,height/700*800,width/700*279,height/700*519,this));
                  everything.add(new line(width/700*279,height/700*519,width/700*76,height/700*350,this));
                  everything.add(new line(width/700*76,height/700*350,width/700*333,height/700*345,this));
                  everything.add(new line(width/700*333,height/700*345,width/700*419,height/700*68,this));

                  everything.add(new line(width/950*419,height/700*68,width/950*624,height/700*800,this));
                  everything.add(new line(width/950*419,height/700*68,width/950*203,height/700*800,this));
                  everything.add(new line(width/950*76,height/700*350,width/950*624,height/700*800,this));
                  everything.add(new line(width/950*203,height/700*800,width/950*749,height/700*350,this));
                  everything.add(new rectangle(width/950*76,height/700*350,width/950*673,1,this));*/

                  everything.add(new line((int)Math.round((double)width/1350*419),(int)Math.round((double)height/1000*68),(int)Math.round((double)width/1350*624),(int)Math.round((double)height/1000*800),this));
                  everything.add(new line((int)Math.round((double)width/1350*419),(int)Math.round((double)height/1000*68),(int)Math.round((double)width/1350*203),(int)Math.round((double)height/1000*800),this));
                  everything.add(new line((int)Math.round((double)width/1350*76),(int)Math.round((double)height/1000*350),(int)Math.round((double)width/1350*624),(int)Math.round((double)height/1000*800),this));
                  everything.add(new line((int)Math.round((double)width/1350*203),(int)Math.round((double)height/1000*800),(int)Math.round((double)width/1350*749),(int)Math.round((double)height/1000*350),this));
                  everything.add(new rectangle((int)Math.round((double)width/1350*76),(int)Math.round((double)height/1000*350),(int)Math.round((double)width/1350*673),1,this));


        addKeyListener(this);
        addMouseMotionListener(this);
        addMouseListener(this);
    }

    public void keyReleased(KeyEvent e)
    {
        Object source=e.getSource();

        int released=e.getKeyCode();

        if (released==KeyEvent.VK_A){left=0;}
        if (released==KeyEvent.VK_W){up=0;}
        if (released==KeyEvent.VK_D){right=0;}
        if (released==KeyEvent.VK_S){down=0;}
    }//end keyReleased


    public void keyPressed(KeyEvent e)
    {
        Object source=e.getSource();

        int pressed=e.getKeyCode();

        if (pressed==KeyEvent.VK_A){left=1;}
        if (pressed==KeyEvent.VK_W){up=1;}
        if (pressed==KeyEvent.VK_D){right=1;}
        if (pressed==KeyEvent.VK_S){down=1;}

        if (pressed==KeyEvent.VK_PAUSE&&pressed==KeyEvent.VK_P)
        {
            //if (paused==0){paused=1;}
            //else paused=0;
        }
    }//end keyPressed

    public void keyTyped(KeyEvent e){}

//***********************************************************************************************

    public void mouseDragged(MouseEvent e)
    {
        mouseX=(e.getX());
        mouseY=(e.getY());

          //run();
    }

    public void mouseMoved(MouseEvent e)
    {
        mouseX=(e.getX());
        mouseY=(e.getY());

          //run();
    }

//***********************************************************************************************

    public void mousePressed(MouseEvent e)
    {
        if(e.getX()==0 && e.getY()==0){System.exit(0);}

    mouseX=(e.getX()+x);
        mouseY=(e.getY()+y);

        if(Tile instanceof circle)
        {
                Circle.move(0-Circle.width, 0-Circle.height);
                Circle.setBounds(Circle.x, Circle.y, Circle.width, Circle.height);
                Tile=Rectangle;
        }
        else
        {
                Rectangle.move(0-Rectangle.width, 0-Rectangle.height);
                Rectangle.setBounds(Rectangle.x, Rectangle.y, Rectangle.width, Rectangle.height);
                Tile=Circle;
        }

        Tile.move(mouseX-Tile.width/2, mouseY-Tile.height/2);
    }

    public void mouseReleased(MouseEvent e)
    {
         //run();
    }

    public void mouseEntered(MouseEvent e){}
    public void mouseExited(MouseEvent e){}

    public void mouseClicked(MouseEvent e){}

//***********************************************************************************************

    public void run()//run collision detection
    {
        while (this == this)
        {
            Tile.move(Tile.x + ((mouseX - (Tile.x + (Tile.width / 2))) / 10), Tile.y + ((mouseY - (Tile.y + (Tile.height / 2))) / 10));
            //Tile.move((mouseX - Tile.width / 2), mouseY - (Tile.height / 2));

            for (int i = 0; i < everything.size(); i++)
            {
                tile Temp = (tile) everything.get(i);

                if (Temp.x < (Tile.x + Tile.width) && (Temp.x + Temp.width) > Tile.x && Temp.y < (Tile.y + Tile.height) && (Temp.y + Temp.height) > Tile.y)//rectangles collided
                {
                    if (Temp instanceof rectangle)
                    {
                        if (Tile instanceof rectangle){rectangleRectangle(Temp);}
                        else {circleRectangle(Temp);}//Tile instanceof circle
                    }
                    else
                    {
                        if (Temp instanceof circle)
                        {
                            if (Tile instanceof rectangle) {rectangleCircle(Temp);}
                            else {circleCircle(Temp);}
                        }
                        else//line
                        {
                            if (Tile instanceof rectangle){rectangleLine(Temp);}
                            else{circleLine(Temp);}
                        }
                    }
                }//end if
            }//end for

            try {Thread.sleep(16L);}
            catch (Exception e) {}

            Tile.setBounds(Tile.x, Tile.y, Tile.width, Tile.height);
            //Rectangle.setBounds(x, y, width, height);
            //Circle.setBounds(x, y, width, height);
            repaint();

            text.out=" ";
        }//end while loop
    }//end run

//***************************************special collision detection/handling functions************************************************

    void rectangleRectangle(tile Temp)
    {
        int lapTop, lapBot, lapLeft, lapRight, small, scootX=0, scootY=0;

        lapTop=(Temp.y+Temp.height)-Tile.y;
        lapBot=(Tile.y+Tile.height)-Temp.y;
        lapLeft=(Temp.x+Temp.width)-Tile.x;
        lapRight=(Tile.x+Tile.width)-Temp.x;

        small=999999999;

        if (lapTop<small){small=lapTop; scootX=0; scootY=lapTop;}
        if (lapBot<small){small=lapBot; scootX=0; scootY=lapBot*-1;}
                if (lapLeft<small){small=lapLeft; scootX=lapLeft; scootY=0;}
                if (lapRight<small){small=lapRight; scootX=lapRight*-1; scootY=0;}

        Tile.move(Tile.x+scootX, Tile.y+scootY);text.out="collision detected!";
    }



    void circleRectangle(tile Temp)
    {
        if((Tile.x+Tile.width/2<=Temp.x+Temp.width && Tile.x+Tile.width/2>=Temp.x)||(Tile.y+Tile.height/2>=Temp.y && Tile.y+Tile.height/2<=Temp.y+Temp.height))
        {
            rectangleRectangle(Temp);
        }
        else//push from nearest corner
        {
            int x,y;
            if(Tile.x+Tile.width/2>Temp.x+Temp.width && Tile.y+Tile.height/2<Temp.y){x=Temp.x+Temp.width; y=Temp.y;}
            else if(Tile.x+Tile.width/2<Temp.x && Tile.y+Tile.height/2<Temp.y){x=Temp.x; y=Temp.y;}
            else if(Tile.x+Tile.width/2>Temp.x+Temp.width && Tile.y+Tile.height/2>Temp.y+Temp.height){x=Temp.x+Temp.width; y=Temp.y+Temp.height;}
            else {x=Temp.x; y=Temp.y+Temp.height;}

            double distance = Math.sqrt(Math.pow(Tile.x+(Tile.width/2) - x, 2) + Math.pow(Tile.y+(Tile.height/2) - y, 2));

            if((int)Math.round(distance)<Tile.height/2)
            {
                             double normY = ((Tile.y+(Tile.height/2) - y) / distance);
                             double normX = ((Tile.x+(Tile.width/2) - x) / distance);

                            Tile.move(x-Tile.width/2+(int)Math.round(normX*((Tile.width/2))) , y-Tile.height/2+(int)Math.round(normY*((Tile.height/2))));text.out="collision detected!";
            }
        }
    }



    void rectangleCircle(tile Temp)
    {
        if((Temp.x+Temp.width/2<=Tile.x+Tile.width && Temp.x+Temp.width/2>=Tile.x)||(Temp.y+Temp.height/2>=Tile.y && Temp.y+Temp.height/2<=Tile.y+Tile.height))
        {
            rectangleRectangle(Temp);
        }
        else//push from nearest corner
        {
            int x,y;
            if(Temp.x+Temp.width/2>Tile.x+Tile.width && Temp.y+Temp.height/2<Tile.y){x=Tile.x+Tile.width; y=Tile.y;}
            else if(Temp.x+Temp.width/2<Tile.x && Temp.y+Temp.height/2<Tile.y){x=Tile.x; y=Tile.y;}
            else if(Temp.x+Temp.width/2>Tile.x+Tile.width && Temp.y+Temp.height/2>Tile.y+Tile.height){x=Tile.x+Tile.width; y=Tile.y+Tile.height;}
            else {x=Tile.x; y=Tile.y+Tile.height;}

            double distance = Math.sqrt(Math.pow(Temp.x+(Temp.width/2) - x, 2) + Math.pow(Temp.y+(Temp.height/2) - y, 2));

            if((int)Math.round(distance)<Temp.height/2)
            {
             double normY = ((Temp.y+(Temp.height/2) - y) / distance);
             double normX = ((Temp.x+(Temp.width/2) - x) / distance);

             if(Temp.x+Temp.width/2>Tile.x+Tile.width && Temp.y+Temp.height/2<Tile.y){Tile.move((Temp.x+Temp.width/2)-(int)Math.round(normX*((Temp.width/2)))-Tile.width,(Temp.y+Temp.height/2)-(int)Math.round(normY*((Temp.height/2))));text.out="collision detected!";}
                else if(Temp.x+Temp.width/2<Tile.x && Temp.y+Temp.height/2<Tile.y){Tile.move((Temp.x+Temp.width/2)-(int)Math.round(normX*((Temp.width/2))),(Temp.y+Temp.height/2)-(int)Math.round(normY*((Temp.height/2))));text.out="collision detected!";}
                else if(Temp.x+Temp.width/2>Tile.x+Tile.width && Temp.y+Temp.height/2>Tile.y+Tile.height){Tile.move((Temp.x+Temp.width/2)-(int)Math.round(normX*((Temp.width/2)))-Tile.width,(Temp.y+Temp.height/2)-(int)Math.round(normY*((Temp.height/2)))-Tile.height);text.out="collision detected!";}
                else {Tile.move((Temp.x+Temp.width/2)-(int)Math.round(normX*((Temp.width/2))),(Temp.y+Temp.height/2)-(int)Math.round(normY*((Temp.height/2)))-Tile.height);text.out="collision detected!";}
            }
        }
    }




    void circleCircle(tile Temp)
    {
        double distance = Math.sqrt(Math.pow((Tile.x+(Tile.width/2)) - (Temp.x+(Temp.width/2)),2) + Math.pow((Tile.y+(Tile.height/2)) - (Temp.y+(Temp.height/2)), 2));

        if((int)distance<(Tile.width/2+Temp.width/2))
        {
                        double normX = ((Tile.x+(Tile.width/2)) - (Temp.x+(Temp.width/2))) / distance;
                        double normY = ((Tile.y+(Tile.height/2)) - (Temp.y+(Temp.height/2))) / distance;

            Tile.move((Temp.x+(Temp.width/2))+(int)Math.round(normX*(Tile.width/2+Temp.width/2))-(Tile.width/2) , (Temp.y+(Temp.height/2))+(int)Math.round(normY*(Tile.height/2+Temp.height/2))-(Tile.height/2));text.out="collision detected!";
        }
    }



    void circleLine(tile Temp)
    {
            line Line=(line)Temp;

            if (Line.x1 < (Tile.x + Tile.width) && (Line.x1) > Tile.x && Line.y1 < (Tile.y + Tile.height) && Line.y1 > Tile.y)//circle may be hitting one of the end points
            {
                rectangle rec=new rectangle(Line.x1, Line.y1, 1, 1, this);
                circleRectangle(rec);
                remove(rec);
            }

            if (Line.x2 < (Tile.x + Tile.width) && (Line.x2) > Tile.x && Line.y2 < (Tile.y + Tile.height) && Line.y2 > Tile.y)//circle may be hitting one of the end points
            {
                rectangle rec=new rectangle(Line.x2, Line.y2, 1, 1, this);
                circleRectangle(rec);
                remove(rec);
            }


            int x1=0, y1=0, x2=Tile.x+(Tile.width/2), y2=Tile.y+(Tile.height/2);

            x1=Tile.x+(Tile.width/2)-Line.height;//(int)Math.round(Line.xNorm*1000);
            x2=Tile.x+(Tile.width/2)+Line.height;
            if(Line.posSlope)
            {
                y1=Tile.y+(Tile.height/2)-Line.width;
                y2=Tile.y+(Tile.height/2)+Line.width;
            }
            else
            {
                y1=Tile.y+(Tile.height/2)+Line.width;
                y2=Tile.y+(Tile.height/2)-Line.width;
            }

            Point point=intersection((double)x1,(double)y1,(double)x2,(double)y2,(double)Line.x1,(double)Line.y1,(double)Line.x2,(double)Line.y2);//find intersection

            if (point.x < (Line.x + Line.width) && point.x > Line.x && point.y < (Line.y + Line.height) && point.y > Line.y)//line intersects within line segment
            {
                //if(point!=null){System.out.println(point.x+","+point.y);}
                double distance = Math.sqrt(Math.pow((Tile.x+(Tile.width/2)) - point.x,2) + Math.pow((Tile.y+(Tile.width/2)) - point.y, 2));

                if((int)distance<Tile.width/2)
                {
                    //System.out.println("hit");
                    double normX = ((Tile.x+(Tile.width/2)) - point.x) / distance;
                    double normY = ((Tile.y+(Tile.height/2)) - point.y) / distance;

                    Tile.move((point.x)+(int)Math.round(normX*(Tile.width/2))-(Tile.width/2) , (point.y)+(int)Math.round(normY*(Tile.height/2))-(Tile.height/2));text.out="collision detected!";
                    //System.out.println(point.x+","+point.y);
                }
            }

            //new bullet(this, (int)Math.round(tryX), (int)Math.round(tryY));
    }

        void rectangleLine(tile Temp)
    {
            line Line=(line)Temp;
            if(new Line2D.Double(Line.x1,Line.y1,Line.x2,Line.y2).intersects(new Rectangle(Tile.x,Tile.y,Tile.width,Tile.height)))
            {
                if (Line.x1 < (Tile.x + Tile.width) && (Line.x1) > Tile.x && Line.y1 < (Tile.y + Tile.height) && Line.y1 > Tile.y)//circle may be hitting one of the end points
                {
                    rectangle rec=new rectangle(Line.x1, Line.y1, 1, 1, this);
                    rectangleRectangle(rec);
                    remove(rec);
                }

                if (Line.x2 < (Tile.x + Tile.width) && (Line.x2) > Tile.x && Line.y2 < (Tile.y + Tile.height) && Line.y2 > Tile.y)//circle may be hitting one of the end points
                {
                    rectangle rec=new rectangle(Line.x2, Line.y2, 1, 1, this);
                    rectangleRectangle(rec);
                    remove(rec);
                }

                if(Line.posSlope)//positive sloped line
                {
                    //first we'll do the top left corner
                    int x1=Tile.x-Line.height;
                    int x2=Tile.x+Line.height;
                    int y1=Tile.y-Line.width;
                    int y2=Tile.y+Line.width;
                    Point topPoint=new Point(-99,-99), botPoint=new Point(-99,-99);
                    double topDistance=0, botDistance=0;

                    topPoint=intersection((double)x1,(double)y1,(double)x2,(double)y2,(double)Line.x1,(double)Line.y1,(double)Line.x2,(double)Line.y2);//find intersection

                    topDistance = Math.sqrt(Math.pow(Tile.x - topPoint.x,2) + Math.pow(Tile.y - topPoint.y, 2));

                    //new let's do the bottom right corner
                    x1=Tile.x+Tile.width-Line.height;
                    x2=Tile.x+Tile.width+Line.height;
                    y1=Tile.y+Tile.height-Line.width;
                    y2=Tile.y+Tile.height+Line.width;

                    botPoint=intersection((double)x1,(double)y1,(double)x2,(double)y2,(double)Line.x1,(double)Line.y1,(double)Line.x2,(double)Line.y2);//find intersection

                    botDistance = Math.sqrt(Math.pow((Tile.x+Tile.width) - botPoint.x,2) + Math.pow((Tile.y+Tile.height) - botPoint.y, 2));


                    if(topDistance<botDistance)
                    {
                        if(new Rectangle(Tile.x,Tile.y,Tile.width,Tile.height).contains(topPoint) && new Rectangle(Line.x,Line.y,Line.width,Line.height).contains(topPoint))
                        {
                            Tile.move(topPoint.x,topPoint.y);text.out="collision detected!";
                        }
                    }
                    else
                    {
                        if(new Rectangle(Tile.x,Tile.y,Tile.width,Tile.height).contains(botPoint) && new Rectangle(Line.x,Line.y,Line.width,Line.height).contains(botPoint))
                        {
                            Tile.move(botPoint.x-Tile.width,botPoint.y-Tile.height);text.out="collision detected!";
                        }
                    }
                }
                else//negative sloped lne
                {
                    //first we'll do the top right corner
                    int x1=Tile.x+Tile.width-Line.height;
                    int x2=Tile.x+Tile.width+Line.height;
                    int y1=Tile.y+Line.width;
                    int y2=Tile.y-Line.width;
                    Point topPoint=new Point(-99,-99), botPoint=new Point(-99,-99);
                    double topDistance=0, botDistance=0;

                    topPoint=intersection((double)x1,(double)y1,(double)x2,(double)y2,(double)Line.x1,(double)Line.y1,(double)Line.x2,(double)Line.y2);//find intersection

                    topDistance = Math.sqrt(Math.pow(Tile.x + Tile.width - topPoint.x,2) + Math.pow(Tile.y - topPoint.y, 2));

                    //new let's do the bottom left corner
                    x1=Tile.x-Line.height;
                    x2=Tile.x+Line.height;
                    y1=Tile.y+Tile.height+Line.width;
                    y2=Tile.y+Tile.height-Line.width;

                    botPoint=intersection((double)x1,(double)y1,(double)x2,(double)y2,(double)Line.x1,(double)Line.y1,(double)Line.x2,(double)Line.y2);//find intersection

                    botDistance = Math.sqrt(Math.pow(Tile.x - botPoint.x,2) + Math.pow((Tile.y+Tile.height) - botPoint.y, 2));


                    if(topDistance<botDistance)
                    {
                        if(new Rectangle(Tile.x,Tile.y,Tile.width,Tile.height).contains(topPoint) && new Rectangle(Line.x,Line.y,Line.width,Line.height).contains(topPoint))
                        {
                            Tile.move(topPoint.x-Tile.width,topPoint.y);text.out="collision detected!";
                        }
                    }
                    else
                    {
                        if(new Rectangle(Tile.x,Tile.y,Tile.width,Tile.height).contains(botPoint) && new Rectangle(Line.x,Line.y,Line.width,Line.height).contains(botPoint))
                        {
                            Tile.move(botPoint.x,botPoint.y-Tile.height);text.out="collision detected!";
                        }
                    }
                }
            }
    }

       public Point intersection(double x1, double y1, double x2, double y2,double x3, double y3, double x4, double y4)//I didn't write this. got it from http://www.ahristov.com/tutorial/geometry-games/intersection-lines.html (I altered it)
       {
            double d = (x1 - x2) * (y3 - y4) - (y1 - y2) * (x3 - x4);

            double xi = ((x3 - x4) * (x1 * y2 - y1 * x2) - (x1 - x2) * (x3 * y4 - y3 * x4)) / d;
            double yi = ((y3 - y4) * (x1 * y2 - y1 * x2) - (y1 - y2) * (x3 * y4 - y3 * x4)) / d;

            int x=(int)Math.round(xi);
            int y=(int)Math.round(yi);

            return new Point(x, y);
        }

//***************************************************************************************

    public static void main(String[] args)
    {
        final collision Collision=new collision();
          Collision.run();
    }//end main
}//end class

is there a problem with:

Rectangle box1 = new Rectangle(100,100,100,100);
Rectangle box2 = new Rectangle(200,200,100,100);

// what this means is if any pixel in box2 enters (hits) box1
if (box1.contains(box2)) 
{
     // collision occurred
}

// your code for moving the boxes 

this can also be applied to circles:

Ellipse2D.Double ball1 = new Ellipse2D.Double(100,100,200,200);
Ellipse2D.Double ball2 = new Ellipse2D.Double(400,100,200,200);

// what this means is if any pixel on the circumference in ball2 touches (hits)
// ball1
    if (ball1.contains(ball2)) 
    {
         // collision occurred
    }

    // your code for moving the balls

to check whether youve hit the edge of a screen you could use the following:

Rectangle screenBounds = jpanel.getBounds();
Ellipse2D.Double ball = new Ellipse2D.Double(100,100,200,200);  // diameter 200
Rectangle ballBounds = ball.getBounds();

if (!screenBounds.contains(ballBounds))
{
// the ball touched the edge of the screen
}

First, use the bounding boxes as described by Jonathan Holland to find if you may have a collision.

From the (multi-color) sprites, create black and white versions. You probably already have these if your sprites are transparent (i.e. there are places which are inside the bounding box but you can still see the background). These are "masks".

Use Image.getRGB() on the mask to get at the pixels. For each pixel which isn't transparent, set a bit in an integer array (playerArray and enemyArray below). The size of the array is height if width <= 32 pixels, (width+31)/32*height otherwise. The code below is for width <= 32.

If you have a collision of the bounding boxes, do this:

// Find the first line where the two sprites might overlap
int linePlayer, lineEnemy;
if (player.y <= enemy.y) {
    linePlayer = enemy.y - player.y;
    lineEnemy = 0;
} else {
    linePlayer = 0;
    lineEnemy = player.y - enemy.y;
}
int line = Math.max(linePlayer, lineEnemy);

// Get the shift between the two
x = player.x - enemy.x;
int maxLines = Math.max(player.height, enemy.height);
for ( line < maxLines; line ++) {
    // if width > 32, then you need a second loop here
    long playerMask = playerArray[linePlayer];
    long enemyMask = enemyArray[lineEnemy];
    // Reproduce the shift between the two sprites
    if (x < 0) playerMask << (-x);
    else enemyMask << x;
    // If the two masks have common bits, binary AND will return != 0
    if ((playerMask & enemyMask) != 0) {
        // Contact!
    }

}

Links: JGame, Framework for Small Java Games

Since Java doesn't have an intersect function (really!?) you can do collision detection by simply comparying the X and Y, Width and Height values of the bounding boxes (rectangle) for each of the objects that could potentially collide.

So... in the base object of each colliding object... i.e. if your player and enemy have a common base you can put a simple Rectangle object called something like BoundingBox. If the common base is a built in Java class then you'll need to create a class that extends the build in class and have the player and enemy objects extend your new class or are instances of that class.

At creation (and each tick or update) you'll need to set the BoundingBox paremeters for both your player and enemy. I don't have the Rectangle class infront of me but its most likely something like X, Y, Width and finally Height. X and Y are that objects location in your game world. The width and height are self explanatory I think. They'll most likely come out from the right of the players location though so, if the X and Y were bothe at 0 and your Width and Height were both at 256 you wouldn't see anything because the character would be at the top left outside of the screen.

Anyways... to detect a collision, you'll want to compare the attributes of the player and enemy BoundingBoxes. So something like this...

 if( Player.BoundingBox.X = Enemy.BoundingBox.X && If( Player.BoundingBox.Y = Enemy.BoundingBox.Y )
 {
      //Oh noes!  The enemy and player are on top of eachother.
 }

The logic can get sort of complicated but you'll need to compare the distances between each BoundingBox and compare locations.

You don't want to have the collision check code inside the painting code. The painting needs to be fast. Collision can go in the game loop. Therefore you need an internal representation of the objects independent of their sprites.

Use a rectangle to surround each player and enemy, the height and width of the rectangles should correspond to the object you're surrounding, imagine it being in a box only big enough to fit it.

Now, you move these rectangles the same as you do the objects, so they have a 'bounding box'

I'm not sure if Java has this, but it might have a method on the rectangle object called .intersects() so you'd do if(rectangle1.intersectS(rectangle2) to check to see if an object has collided with another.

Otherwise you can get the x and y co-ordinates of the boxes and using the height/width of them detect whether they've intersected yourself.

Anyway, you can use that to either do an event on intersection (make one explode, or whatever) or prevent the movement from being drawn. (revert to previous co-ordinates)

edit: here we go

boolean

intersects(Rectangle r) Determines whether or not this Rectangle and the specified Rectangle intersect.

So I would do (and don't paste this code, it most likely won't work, not done java for a long time and I didn't do graphics when I did use it.)

Rectangle rect1 = new Rectangle(player.x, player.y, player.width, player.height);

Rectangle rect2 = new Rectangle(enemy.x, enemy.y, enemy.width, enemy.height);

//detects when the two rectangles hit
if(rect1.intersects(rect2))
{

System.out.println("game over, g");
}

obviously you'd need to fit that in somewhere.

Here's a useful of an open source game that uses a lot of collisions: http://robocode.sourceforge.net/

You may take a look at the code and complement with the answers written here.

Use a rectangle to surround each player and enemy, the height and width of the rectangles should correspond to the object you're surrounding, imagine it being in a box only big enough to fit it.

Now, you move these rectangles the same as you do the objects, so they have a 'bounding box'

I'm not sure if Java has this, but it might have a method on the rectangle object called .intersects() so you'd do if(rectangle1.intersectS(rectangle2) to check to see if an object has collided with another.

Otherwise you can get the x and y co-ordinates of the boxes and using the height/width of them detect whether they've intersected yourself.

Anyway, you can use that to either do an event on intersection (make one explode, or whatever) or prevent the movement from being drawn. (revert to previous co-ordinates)

edit: here we go

boolean

intersects(Rectangle r) Determines whether or not this Rectangle and the specified Rectangle intersect.

So I would do (and don't paste this code, it most likely won't work, not done java for a long time and I didn't do graphics when I did use it.)

Rectangle rect1 = new Rectangle(player.x, player.y, player.width, player.height);

Rectangle rect2 = new Rectangle(enemy.x, enemy.y, enemy.width, enemy.height);

//detects when the two rectangles hit
if(rect1.intersects(rect2))
{

System.out.println("game over, g");
}

obviously you'd need to fit that in somewhere.

You don't want to have the collision check code inside the painting code. The painting needs to be fast. Collision can go in the game loop. Therefore you need an internal representation of the objects independent of their sprites.

Here's the main class from my collision detection program.
You can see it run at: http://www.youtube.com/watch?v=JIXhCvXgjsQ

/**
 *
 * @author Tyler Griffin
 */
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.awt.GraphicsDevice.*;
import java.util.ArrayList;
import java.awt.Graphics;
import java.awt.geom.Line2D;


public class collision extends JFrame implements KeyListener, MouseMotionListener, MouseListener
{
    ArrayList everything=new ArrayList<tile>();

    int time=0, x, y, width, height, up=0, down=0, left=0, right=0, mouse1=0, mouse2=0;
    int mouseX, mouseY;

    GraphicsEnvironment environment = GraphicsEnvironment.getLocalGraphicsEnvironment();
    GraphicsDevice screen = environment.getDefaultScreenDevice();
    DisplayMode displayMode = screen.getDisplayMode();

    //private BufferStrategy strategy;

    JLayeredPane pane = new JLayeredPane();

     tile Tile;
     circle Circle;
     rectangle Rectangle;

         textPane text;

    public collision()
    {
        setUndecorated(screen.isFullScreenSupported());
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);
        setLayout(null);
        setResizable(false);
        screen.setFullScreenWindow(this);


        width=displayMode.getWidth();
        height=displayMode.getHeight();


          Circle=new circle(-(int)Math.round((double)height/7*2),-(int)Math.round((double)height/7*2),(int)Math.round((double)height/7*.85),this);
          Rectangle=new rectangle(-(int)Math.round((double)height/7*1.5),-(int)Math.round((double)height/7*1.5),(int)Math.round((double)height/7*1.5),(int)Math.round((double)height/7*1.5),this);
          Tile=Circle;
          Tile.move(mouseX-Tile.width/2, mouseY-Tile.height/2);
                  text=new textPane(0,0,width,height,this);

          everything.add(new circle((int)Math.round((double)width/100*75),(int)Math.round((double)height/100*15),(int)Math.round((double)width/100*10),this));
                  everything.add(new rectangle((int)Math.round((double)width/100*70),(int)Math.round((double)height/100*60),(int)Math.round((double)width/100*20),(int)Math.round((double)height/100*20),this));
                  //everything.add(new line(750,250,750,750,this));
                  /*everything.add(new line(width/700*419,height/700*68,width/700*495,height/700*345,this));
                  everything.add(new line(width/700*495,height/700*345,width/700*749,height/700*350,this));
                  everything.add(new line(width/700*749,height/700*350,width/700*549,height/700*519,this));
                  everything.add(new line(width/700*549,height/700*519,width/700*624,height/700*800,this));
                  everything.add(new line(width/700*624,height/700*800,width/700*419,height/700*638,this));
                  everything.add(new line(width/700*419,height/700*638,width/700*203,height/700*800,this));
                  everything.add(new line(width/700*203,height/700*800,width/700*279,height/700*519,this));
                  everything.add(new line(width/700*279,height/700*519,width/700*76,height/700*350,this));
                  everything.add(new line(width/700*76,height/700*350,width/700*333,height/700*345,this));
                  everything.add(new line(width/700*333,height/700*345,width/700*419,height/700*68,this));

                  everything.add(new line(width/950*419,height/700*68,width/950*624,height/700*800,this));
                  everything.add(new line(width/950*419,height/700*68,width/950*203,height/700*800,this));
                  everything.add(new line(width/950*76,height/700*350,width/950*624,height/700*800,this));
                  everything.add(new line(width/950*203,height/700*800,width/950*749,height/700*350,this));
                  everything.add(new rectangle(width/950*76,height/700*350,width/950*673,1,this));*/

                  everything.add(new line((int)Math.round((double)width/1350*419),(int)Math.round((double)height/1000*68),(int)Math.round((double)width/1350*624),(int)Math.round((double)height/1000*800),this));
                  everything.add(new line((int)Math.round((double)width/1350*419),(int)Math.round((double)height/1000*68),(int)Math.round((double)width/1350*203),(int)Math.round((double)height/1000*800),this));
                  everything.add(new line((int)Math.round((double)width/1350*76),(int)Math.round((double)height/1000*350),(int)Math.round((double)width/1350*624),(int)Math.round((double)height/1000*800),this));
                  everything.add(new line((int)Math.round((double)width/1350*203),(int)Math.round((double)height/1000*800),(int)Math.round((double)width/1350*749),(int)Math.round((double)height/1000*350),this));
                  everything.add(new rectangle((int)Math.round((double)width/1350*76),(int)Math.round((double)height/1000*350),(int)Math.round((double)width/1350*673),1,this));


        addKeyListener(this);
        addMouseMotionListener(this);
        addMouseListener(this);
    }

    public void keyReleased(KeyEvent e)
    {
        Object source=e.getSource();

        int released=e.getKeyCode();

        if (released==KeyEvent.VK_A){left=0;}
        if (released==KeyEvent.VK_W){up=0;}
        if (released==KeyEvent.VK_D){right=0;}
        if (released==KeyEvent.VK_S){down=0;}
    }//end keyReleased


    public void keyPressed(KeyEvent e)
    {
        Object source=e.getSource();

        int pressed=e.getKeyCode();

        if (pressed==KeyEvent.VK_A){left=1;}
        if (pressed==KeyEvent.VK_W){up=1;}
        if (pressed==KeyEvent.VK_D){right=1;}
        if (pressed==KeyEvent.VK_S){down=1;}

        if (pressed==KeyEvent.VK_PAUSE&&pressed==KeyEvent.VK_P)
        {
            //if (paused==0){paused=1;}
            //else paused=0;
        }
    }//end keyPressed

    public void keyTyped(KeyEvent e){}

//***********************************************************************************************

    public void mouseDragged(MouseEvent e)
    {
        mouseX=(e.getX());
        mouseY=(e.getY());

          //run();
    }

    public void mouseMoved(MouseEvent e)
    {
        mouseX=(e.getX());
        mouseY=(e.getY());

          //run();
    }

//***********************************************************************************************

    public void mousePressed(MouseEvent e)
    {
        if(e.getX()==0 && e.getY()==0){System.exit(0);}

    mouseX=(e.getX()+x);
        mouseY=(e.getY()+y);

        if(Tile instanceof circle)
        {
                Circle.move(0-Circle.width, 0-Circle.height);
                Circle.setBounds(Circle.x, Circle.y, Circle.width, Circle.height);
                Tile=Rectangle;
        }
        else
        {
                Rectangle.move(0-Rectangle.width, 0-Rectangle.height);
                Rectangle.setBounds(Rectangle.x, Rectangle.y, Rectangle.width, Rectangle.height);
                Tile=Circle;
        }

        Tile.move(mouseX-Tile.width/2, mouseY-Tile.height/2);
    }

    public void mouseReleased(MouseEvent e)
    {
         //run();
    }

    public void mouseEntered(MouseEvent e){}
    public void mouseExited(MouseEvent e){}

    public void mouseClicked(MouseEvent e){}

//***********************************************************************************************

    public void run()//run collision detection
    {
        while (this == this)
        {
            Tile.move(Tile.x + ((mouseX - (Tile.x + (Tile.width / 2))) / 10), Tile.y + ((mouseY - (Tile.y + (Tile.height / 2))) / 10));
            //Tile.move((mouseX - Tile.width / 2), mouseY - (Tile.height / 2));

            for (int i = 0; i < everything.size(); i++)
            {
                tile Temp = (tile) everything.get(i);

                if (Temp.x < (Tile.x + Tile.width) && (Temp.x + Temp.width) > Tile.x && Temp.y < (Tile.y + Tile.height) && (Temp.y + Temp.height) > Tile.y)//rectangles collided
                {
                    if (Temp instanceof rectangle)
                    {
                        if (Tile instanceof rectangle){rectangleRectangle(Temp);}
                        else {circleRectangle(Temp);}//Tile instanceof circle
                    }
                    else
                    {
                        if (Temp instanceof circle)
                        {
                            if (Tile instanceof rectangle) {rectangleCircle(Temp);}
                            else {circleCircle(Temp);}
                        }
                        else//line
                        {
                            if (Tile instanceof rectangle){rectangleLine(Temp);}
                            else{circleLine(Temp);}
                        }
                    }
                }//end if
            }//end for

            try {Thread.sleep(16L);}
            catch (Exception e) {}

            Tile.setBounds(Tile.x, Tile.y, Tile.width, Tile.height);
            //Rectangle.setBounds(x, y, width, height);
            //Circle.setBounds(x, y, width, height);
            repaint();

            text.out=" ";
        }//end while loop
    }//end run

//***************************************special collision detection/handling functions************************************************

    void rectangleRectangle(tile Temp)
    {
        int lapTop, lapBot, lapLeft, lapRight, small, scootX=0, scootY=0;

        lapTop=(Temp.y+Temp.height)-Tile.y;
        lapBot=(Tile.y+Tile.height)-Temp.y;
        lapLeft=(Temp.x+Temp.width)-Tile.x;
        lapRight=(Tile.x+Tile.width)-Temp.x;

        small=999999999;

        if (lapTop<small){small=lapTop; scootX=0; scootY=lapTop;}
        if (lapBot<small){small=lapBot; scootX=0; scootY=lapBot*-1;}
                if (lapLeft<small){small=lapLeft; scootX=lapLeft; scootY=0;}
                if (lapRight<small){small=lapRight; scootX=lapRight*-1; scootY=0;}

        Tile.move(Tile.x+scootX, Tile.y+scootY);text.out="collision detected!";
    }



    void circleRectangle(tile Temp)
    {
        if((Tile.x+Tile.width/2<=Temp.x+Temp.width && Tile.x+Tile.width/2>=Temp.x)||(Tile.y+Tile.height/2>=Temp.y && Tile.y+Tile.height/2<=Temp.y+Temp.height))
        {
            rectangleRectangle(Temp);
        }
        else//push from nearest corner
        {
            int x,y;
            if(Tile.x+Tile.width/2>Temp.x+Temp.width && Tile.y+Tile.height/2<Temp.y){x=Temp.x+Temp.width; y=Temp.y;}
            else if(Tile.x+Tile.width/2<Temp.x && Tile.y+Tile.height/2<Temp.y){x=Temp.x; y=Temp.y;}
            else if(Tile.x+Tile.width/2>Temp.x+Temp.width && Tile.y+Tile.height/2>Temp.y+Temp.height){x=Temp.x+Temp.width; y=Temp.y+Temp.height;}
            else {x=Temp.x; y=Temp.y+Temp.height;}

            double distance = Math.sqrt(Math.pow(Tile.x+(Tile.width/2) - x, 2) + Math.pow(Tile.y+(Tile.height/2) - y, 2));

            if((int)Math.round(distance)<Tile.height/2)
            {
                             double normY = ((Tile.y+(Tile.height/2) - y) / distance);
                             double normX = ((Tile.x+(Tile.width/2) - x) / distance);

                            Tile.move(x-Tile.width/2+(int)Math.round(normX*((Tile.width/2))) , y-Tile.height/2+(int)Math.round(normY*((Tile.height/2))));text.out="collision detected!";
            }
        }
    }



    void rectangleCircle(tile Temp)
    {
        if((Temp.x+Temp.width/2<=Tile.x+Tile.width && Temp.x+Temp.width/2>=Tile.x)||(Temp.y+Temp.height/2>=Tile.y && Temp.y+Temp.height/2<=Tile.y+Tile.height))
        {
            rectangleRectangle(Temp);
        }
        else//push from nearest corner
        {
            int x,y;
            if(Temp.x+Temp.width/2>Tile.x+Tile.width && Temp.y+Temp.height/2<Tile.y){x=Tile.x+Tile.width; y=Tile.y;}
            else if(Temp.x+Temp.width/2<Tile.x && Temp.y+Temp.height/2<Tile.y){x=Tile.x; y=Tile.y;}
            else if(Temp.x+Temp.width/2>Tile.x+Tile.width && Temp.y+Temp.height/2>Tile.y+Tile.height){x=Tile.x+Tile.width; y=Tile.y+Tile.height;}
            else {x=Tile.x; y=Tile.y+Tile.height;}

            double distance = Math.sqrt(Math.pow(Temp.x+(Temp.width/2) - x, 2) + Math.pow(Temp.y+(Temp.height/2) - y, 2));

            if((int)Math.round(distance)<Temp.height/2)
            {
             double normY = ((Temp.y+(Temp.height/2) - y) / distance);
             double normX = ((Temp.x+(Temp.width/2) - x) / distance);

             if(Temp.x+Temp.width/2>Tile.x+Tile.width && Temp.y+Temp.height/2<Tile.y){Tile.move((Temp.x+Temp.width/2)-(int)Math.round(normX*((Temp.width/2)))-Tile.width,(Temp.y+Temp.height/2)-(int)Math.round(normY*((Temp.height/2))));text.out="collision detected!";}
                else if(Temp.x+Temp.width/2<Tile.x && Temp.y+Temp.height/2<Tile.y){Tile.move((Temp.x+Temp.width/2)-(int)Math.round(normX*((Temp.width/2))),(Temp.y+Temp.height/2)-(int)Math.round(normY*((Temp.height/2))));text.out="collision detected!";}
                else if(Temp.x+Temp.width/2>Tile.x+Tile.width && Temp.y+Temp.height/2>Tile.y+Tile.height){Tile.move((Temp.x+Temp.width/2)-(int)Math.round(normX*((Temp.width/2)))-Tile.width,(Temp.y+Temp.height/2)-(int)Math.round(normY*((Temp.height/2)))-Tile.height);text.out="collision detected!";}
                else {Tile.move((Temp.x+Temp.width/2)-(int)Math.round(normX*((Temp.width/2))),(Temp.y+Temp.height/2)-(int)Math.round(normY*((Temp.height/2)))-Tile.height);text.out="collision detected!";}
            }
        }
    }




    void circleCircle(tile Temp)
    {
        double distance = Math.sqrt(Math.pow((Tile.x+(Tile.width/2)) - (Temp.x+(Temp.width/2)),2) + Math.pow((Tile.y+(Tile.height/2)) - (Temp.y+(Temp.height/2)), 2));

        if((int)distance<(Tile.width/2+Temp.width/2))
        {
                        double normX = ((Tile.x+(Tile.width/2)) - (Temp.x+(Temp.width/2))) / distance;
                        double normY = ((Tile.y+(Tile.height/2)) - (Temp.y+(Temp.height/2))) / distance;

            Tile.move((Temp.x+(Temp.width/2))+(int)Math.round(normX*(Tile.width/2+Temp.width/2))-(Tile.width/2) , (Temp.y+(Temp.height/2))+(int)Math.round(normY*(Tile.height/2+Temp.height/2))-(Tile.height/2));text.out="collision detected!";
        }
    }



    void circleLine(tile Temp)
    {
            line Line=(line)Temp;

            if (Line.x1 < (Tile.x + Tile.width) && (Line.x1) > Tile.x && Line.y1 < (Tile.y + Tile.height) && Line.y1 > Tile.y)//circle may be hitting one of the end points
            {
                rectangle rec=new rectangle(Line.x1, Line.y1, 1, 1, this);
                circleRectangle(rec);
                remove(rec);
            }

            if (Line.x2 < (Tile.x + Tile.width) && (Line.x2) > Tile.x && Line.y2 < (Tile.y + Tile.height) && Line.y2 > Tile.y)//circle may be hitting one of the end points
            {
                rectangle rec=new rectangle(Line.x2, Line.y2, 1, 1, this);
                circleRectangle(rec);
                remove(rec);
            }


            int x1=0, y1=0, x2=Tile.x+(Tile.width/2), y2=Tile.y+(Tile.height/2);

            x1=Tile.x+(Tile.width/2)-Line.height;//(int)Math.round(Line.xNorm*1000);
            x2=Tile.x+(Tile.width/2)+Line.height;
            if(Line.posSlope)
            {
                y1=Tile.y+(Tile.height/2)-Line.width;
                y2=Tile.y+(Tile.height/2)+Line.width;
            }
            else
            {
                y1=Tile.y+(Tile.height/2)+Line.width;
                y2=Tile.y+(Tile.height/2)-Line.width;
            }

            Point point=intersection((double)x1,(double)y1,(double)x2,(double)y2,(double)Line.x1,(double)Line.y1,(double)Line.x2,(double)Line.y2);//find intersection

            if (point.x < (Line.x + Line.width) && point.x > Line.x && point.y < (Line.y + Line.height) && point.y > Line.y)//line intersects within line segment
            {
                //if(point!=null){System.out.println(point.x+","+point.y);}
                double distance = Math.sqrt(Math.pow((Tile.x+(Tile.width/2)) - point.x,2) + Math.pow((Tile.y+(Tile.width/2)) - point.y, 2));

                if((int)distance<Tile.width/2)
                {
                    //System.out.println("hit");
                    double normX = ((Tile.x+(Tile.width/2)) - point.x) / distance;
                    double normY = ((Tile.y+(Tile.height/2)) - point.y) / distance;

                    Tile.move((point.x)+(int)Math.round(normX*(Tile.width/2))-(Tile.width/2) , (point.y)+(int)Math.round(normY*(Tile.height/2))-(Tile.height/2));text.out="collision detected!";
                    //System.out.println(point.x+","+point.y);
                }
            }

            //new bullet(this, (int)Math.round(tryX), (int)Math.round(tryY));
    }

        void rectangleLine(tile Temp)
    {
            line Line=(line)Temp;
            if(new Line2D.Double(Line.x1,Line.y1,Line.x2,Line.y2).intersects(new Rectangle(Tile.x,Tile.y,Tile.width,Tile.height)))
            {
                if (Line.x1 < (Tile.x + Tile.width) && (Line.x1) > Tile.x && Line.y1 < (Tile.y + Tile.height) && Line.y1 > Tile.y)//circle may be hitting one of the end points
                {
                    rectangle rec=new rectangle(Line.x1, Line.y1, 1, 1, this);
                    rectangleRectangle(rec);
                    remove(rec);
                }

                if (Line.x2 < (Tile.x + Tile.width) && (Line.x2) > Tile.x && Line.y2 < (Tile.y + Tile.height) && Line.y2 > Tile.y)//circle may be hitting one of the end points
                {
                    rectangle rec=new rectangle(Line.x2, Line.y2, 1, 1, this);
                    rectangleRectangle(rec);
                    remove(rec);
                }

                if(Line.posSlope)//positive sloped line
                {
                    //first we'll do the top left corner
                    int x1=Tile.x-Line.height;
                    int x2=Tile.x+Line.height;
                    int y1=Tile.y-Line.width;
                    int y2=Tile.y+Line.width;
                    Point topPoint=new Point(-99,-99), botPoint=new Point(-99,-99);
                    double topDistance=0, botDistance=0;

                    topPoint=intersection((double)x1,(double)y1,(double)x2,(double)y2,(double)Line.x1,(double)Line.y1,(double)Line.x2,(double)Line.y2);//find intersection

                    topDistance = Math.sqrt(Math.pow(Tile.x - topPoint.x,2) + Math.pow(Tile.y - topPoint.y, 2));

                    //new let's do the bottom right corner
                    x1=Tile.x+Tile.width-Line.height;
                    x2=Tile.x+Tile.width+Line.height;
                    y1=Tile.y+Tile.height-Line.width;
                    y2=Tile.y+Tile.height+Line.width;

                    botPoint=intersection((double)x1,(double)y1,(double)x2,(double)y2,(double)Line.x1,(double)Line.y1,(double)Line.x2,(double)Line.y2);//find intersection

                    botDistance = Math.sqrt(Math.pow((Tile.x+Tile.width) - botPoint.x,2) + Math.pow((Tile.y+Tile.height) - botPoint.y, 2));


                    if(topDistance<botDistance)
                    {
                        if(new Rectangle(Tile.x,Tile.y,Tile.width,Tile.height).contains(topPoint) && new Rectangle(Line.x,Line.y,Line.width,Line.height).contains(topPoint))
                        {
                            Tile.move(topPoint.x,topPoint.y);text.out="collision detected!";
                        }
                    }
                    else
                    {
                        if(new Rectangle(Tile.x,Tile.y,Tile.width,Tile.height).contains(botPoint) && new Rectangle(Line.x,Line.y,Line.width,Line.height).contains(botPoint))
                        {
                            Tile.move(botPoint.x-Tile.width,botPoint.y-Tile.height);text.out="collision detected!";
                        }
                    }
                }
                else//negative sloped lne
                {
                    //first we'll do the top right corner
                    int x1=Tile.x+Tile.width-Line.height;
                    int x2=Tile.x+Tile.width+Line.height;
                    int y1=Tile.y+Line.width;
                    int y2=Tile.y-Line.width;
                    Point topPoint=new Point(-99,-99), botPoint=new Point(-99,-99);
                    double topDistance=0, botDistance=0;

                    topPoint=intersection((double)x1,(double)y1,(double)x2,(double)y2,(double)Line.x1,(double)Line.y1,(double)Line.x2,(double)Line.y2);//find intersection

                    topDistance = Math.sqrt(Math.pow(Tile.x + Tile.width - topPoint.x,2) + Math.pow(Tile.y - topPoint.y, 2));

                    //new let's do the bottom left corner
                    x1=Tile.x-Line.height;
                    x2=Tile.x+Line.height;
                    y1=Tile.y+Tile.height+Line.width;
                    y2=Tile.y+Tile.height-Line.width;

                    botPoint=intersection((double)x1,(double)y1,(double)x2,(double)y2,(double)Line.x1,(double)Line.y1,(double)Line.x2,(double)Line.y2);//find intersection

                    botDistance = Math.sqrt(Math.pow(Tile.x - botPoint.x,2) + Math.pow((Tile.y+Tile.height) - botPoint.y, 2));


                    if(topDistance<botDistance)
                    {
                        if(new Rectangle(Tile.x,Tile.y,Tile.width,Tile.height).contains(topPoint) && new Rectangle(Line.x,Line.y,Line.width,Line.height).contains(topPoint))
                        {
                            Tile.move(topPoint.x-Tile.width,topPoint.y);text.out="collision detected!";
                        }
                    }
                    else
                    {
                        if(new Rectangle(Tile.x,Tile.y,Tile.width,Tile.height).contains(botPoint) && new Rectangle(Line.x,Line.y,Line.width,Line.height).contains(botPoint))
                        {
                            Tile.move(botPoint.x,botPoint.y-Tile.height);text.out="collision detected!";
                        }
                    }
                }
            }
    }

       public Point intersection(double x1, double y1, double x2, double y2,double x3, double y3, double x4, double y4)//I didn't write this. got it from http://www.ahristov.com/tutorial/geometry-games/intersection-lines.html (I altered it)
       {
            double d = (x1 - x2) * (y3 - y4) - (y1 - y2) * (x3 - x4);

            double xi = ((x3 - x4) * (x1 * y2 - y1 * x2) - (x1 - x2) * (x3 * y4 - y3 * x4)) / d;
            double yi = ((y3 - y4) * (x1 * y2 - y1 * x2) - (y1 - y2) * (x3 * y4 - y3 * x4)) / d;

            int x=(int)Math.round(xi);
            int y=(int)Math.round(yi);

            return new Point(x, y);
        }

//***************************************************************************************

    public static void main(String[] args)
    {
        final collision Collision=new collision();
          Collision.run();
    }//end main
}//end class

You don't want to have the collision check code inside the painting code. The painting needs to be fast. Collision can go in the game loop. Therefore you need an internal representation of the objects independent of their sprites.

Use a rectangle to surround each player and enemy, the height and width of the rectangles should correspond to the object you're surrounding, imagine it being in a box only big enough to fit it.

Now, you move these rectangles the same as you do the objects, so they have a 'bounding box'

I'm not sure if Java has this, but it might have a method on the rectangle object called .intersects() so you'd do if(rectangle1.intersectS(rectangle2) to check to see if an object has collided with another.

Otherwise you can get the x and y co-ordinates of the boxes and using the height/width of them detect whether they've intersected yourself.

Anyway, you can use that to either do an event on intersection (make one explode, or whatever) or prevent the movement from being drawn. (revert to previous co-ordinates)

edit: here we go

boolean

intersects(Rectangle r) Determines whether or not this Rectangle and the specified Rectangle intersect.

So I would do (and don't paste this code, it most likely won't work, not done java for a long time and I didn't do graphics when I did use it.)

Rectangle rect1 = new Rectangle(player.x, player.y, player.width, player.height);

Rectangle rect2 = new Rectangle(enemy.x, enemy.y, enemy.width, enemy.height);

//detects when the two rectangles hit
if(rect1.intersects(rect2))
{

System.out.println("game over, g");
}

obviously you'd need to fit that in somewhere.

Here's a useful of an open source game that uses a lot of collisions: http://robocode.sourceforge.net/

You may take a look at the code and complement with the answers written here.

No need to use rectangles ... compare the coordinates of 2 players constantly.

like if(x1===x&&y1==y) remember to increase the range of x when ur comparing.

if ur rectangle width is 30 take as if (x1>x&&x2>x+30)..likewise y

Use a rectangle to surround each player and enemy, the height and width of the rectangles should correspond to the object you're surrounding, imagine it being in a box only big enough to fit it.

Now, you move these rectangles the same as you do the objects, so they have a 'bounding box'

I'm not sure if Java has this, but it might have a method on the rectangle object called .intersects() so you'd do if(rectangle1.intersectS(rectangle2) to check to see if an object has collided with another.

Otherwise you can get the x and y co-ordinates of the boxes and using the height/width of them detect whether they've intersected yourself.

Anyway, you can use that to either do an event on intersection (make one explode, or whatever) or prevent the movement from being drawn. (revert to previous co-ordinates)

edit: here we go

boolean

intersects(Rectangle r) Determines whether or not this Rectangle and the specified Rectangle intersect.

So I would do (and don't paste this code, it most likely won't work, not done java for a long time and I didn't do graphics when I did use it.)

Rectangle rect1 = new Rectangle(player.x, player.y, player.width, player.height);

Rectangle rect2 = new Rectangle(enemy.x, enemy.y, enemy.width, enemy.height);

//detects when the two rectangles hit
if(rect1.intersects(rect2))
{

System.out.println("game over, g");
}

obviously you'd need to fit that in somewhere.