In Java, classes you write are Objects. Constructors construct those objects. For example if I have an Apple.class like so:
public class Apple {
//instance variables
String type; // macintosh, green, red, ...
/**
* This is the default constructor that gets called when you use
* Apple a = new Apple(); which creates an Apple object named a.
*/
public Apple() {
// in here you initialize instance variables, and sometimes but rarely
// do other functionality (at least with basic objects)
this.type = "macintosh"; // the 'this' keyword refers to 'this' object. so this.type refers to Apple's 'type' instance variable.
}
/**
* this is another constructor with a parameter. You can have more than one
* constructor as long as they have different parameters. It creates an Apple
* object when called using Apple a = new Apple("someAppleType");
*/
public Apple(String t) {
// when the constructor is called (i.e new Apple() ) this code is executed
this.type = t;
}
/**
* methods in a class are functions. They are whatever functionality needed
* for the object
*/
public String someAppleRelatedMethod(){
return "hello, Apple class!";
}
public static void main(String[] args) {
// construct an apple
Apple a = new Apple("green");
// 'a' is now an Apple object and has all the methods and
// variables of the Apple class.
// To use a method from 'a':
String temp = a.someAppleRelatedMethod();
System.out.println(temp);
System.out.println("a's type is " + a.type);
}
}
Hopefully I explained everything in the comments of the code, but here is a summary: Constructors 'construct' an object of type of the class. The constructor must be named the same thing as the class. They are mostly used for initializing instance varibales Methods are functionality of the objects.