[java] Why does the JFrame setSize() method not set the size correctly?

So I've been programming in java for a semester or so, and I've had this problem a few times and finally got around to asking.

If I make a JFrame and then set the size, like setSize(400,800) for example. The frame is not actually 800 pixels long. From what I can tell it is actually more like 770 (or maybe 769) pixels long. Also, if you set the vertical size very low (below 30), the frame doesn't even show up, only the top window bar from the OS and the frame doesn't get bigger until you go to a value over 30 (so setSize(400,0) looks the same as setSize(400,20)). Why is this, it's not hard to fix but its weird and I'm curious why this is?

If you need more information about anything just ask and I'll get it to you.

The top border of frame is of size 30.You can write code for printing the coordinate of any point on the frame using MouseInputAdapter.You will find when the cursor is just below the top border of the frame the y coordinate is not zero , its close to 30.Hence if you give size to frame 300 * 300 , the size available for putting the components on the frame is only 300 * 270.So if you need to have size 300 * 300 ,give 300 * 330 size of the frame.

There are lots of good reasons for setting the size of a frame. One is to remember the last size the user set, and restore those settings. I have this code which seems to work for me:

package javatools.swing;
import java.util.prefs.*;
import java.awt.*;
import java.awt.event.*;

import javax.swing.JFrame;

public class FramePositionMemory {
    public static final String WIDTH_PREF = "-width";

    public static final String HEIGHT_PREF = "-height";

    public static final String XPOS_PREF = "-xpos";

    public static final String YPOS_PREF = "-ypos";
    String prefix;
    Window frame;
    Class<?> cls;

    public FramePositionMemory(String prefix, Window frame, Class<?> cls) {
        this.prefix = prefix;
        this.frame = frame;
        this.cls = cls;
    }

    public void loadPosition() {
        Preferences prefs = (Preferences)Preferences.userNodeForPackage(cls);
    //  Restore the most recent mainframe size and location
        int width = prefs.getInt(prefix + WIDTH_PREF, frame.getWidth());
        int height = prefs.getInt(prefix + HEIGHT_PREF, frame.getHeight());
        System.out.println("WID: " + width + " HEI: " + height);
        Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
        int xpos = (screenSize.width - width) / 2;
        int ypos = (screenSize.height - height) / 2;
        xpos = prefs.getInt(prefix + XPOS_PREF, xpos);
        ypos = prefs.getInt(prefix + YPOS_PREF, ypos);
        frame.setPreferredSize(new Dimension(width, height));
        frame.setLocation(xpos, ypos);
        frame.pack();
    }

    public void storePosition() {
        Preferences prefs = (Preferences)Preferences.userNodeForPackage(cls);
        prefs.putInt(prefix + WIDTH_PREF, frame.getWidth());
        prefs.putInt(prefix + HEIGHT_PREF, frame.getHeight());
        Point loc = frame.getLocation();
        prefs.putInt(prefix + XPOS_PREF, (int)loc.getX());
        prefs.putInt(prefix + YPOS_PREF, (int)loc.getY());
        System.out.println("STORE: " + frame.getWidth() + " " + frame.getHeight() + " " + loc.getX() + " " + loc.getY());
    }
}
public class Main {
void main(String[] args) {
        JFrame frame = new Frame();
        // SET UP YOUR FRAME HERE.
        final FramePositionMemory fm = new FramePositionMemory("scannacs2", frame, Main.class);
        frame.setSize(400, 400); // default size in the absence of previous setting
        fm.loadPosition();

        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        Runtime.getRuntime().addShutdownHook(new Thread() {
            @Override
            public void run() {
                fm.storePosition();
            }
        });
        frame.setVisible(true);
    }
}

}

I know that this question is about 6+ years old, but the answer by @Kyle doesn't work.

Using this

setSize(width - (getInsets().left + getInsets().right), height - (getInsets().top + getInsets().bottom));

But this always work in any size:

setSize(width + 14, height + 7);

If you don't want the border to border, and only want the white area, here:

setSize(width + 16, height + 39);

Also this only works on Windows 10, for MacOS users, use @ben's answer.

On OS X, you need to take into account existing window decorations. They add 22 pixels to the height. So on a JFrame, you need to tell the program this:

frame.setSize(width, height + 22);

JFrame SetSize() contains the the Area + Border.

I think you have to set the size of ContentPane of that

jFrame.getContentPane().setSize(800,400);

So I would advise you to use JPanel embedded in a JFrame and you draw on that JPanel. This would minimize your problem.

JFrame jf = new JFrame();
JPanel jp = new JPanel();
jp.setPreferredSize(new Dimension(400,800));// changed it to preferredSize, Thanks!
jf.getContentPane().add( jp );// adding to content pane will work here. Please read the comment bellow.
jf.pack();

I am reading this from Javadoc

The JFrame class is slightly incompatible with Frame. Like all other JFC/Swing top-level containers, a JFrame contains a JRootPane as its only child. The content pane provided by the root pane should, as a rule, contain all the non-menu components displayed by the JFrame. This is different from the AWT Frame case. For example, to add a child to an AWT frame you'd write:

frame.add(child);

However using JFrame you need to add the child to the JFrame's content pane instead:

frame.getContentPane().add(child);