4.5.4 Initial Values of Variables

Every variable in a Java program must have a value before its value is used:

• Each class variable, instance variable, or array component is initialized with a default value when it is created (§15.8, §15.9, §20.3.6):
  • For type byte, the default value is zero, that is, the value of (byte)0.
  • For type short, the default value is zero, that is, the value of (short)0.
  • For type int, the default value is zero, that is, 0.
  • For type long, the default value is zero, that is, 0L.
  • For type float, the default value is positive zero, that is, 0.0f.
  • For type double, the default value is positive zero, that is, 0.0d.
  • For type char, the default value is the null character, that is, '\u0000'.
  • For type boolean, the default value is false.
  • For all reference types (§4.3), the default value is null.
• Each method parameter (§8.4.1) is initialized to the corresponding argument value provided by the invoker of the method (§15.11).
• Each constructor parameter (§8.6.1) is initialized to the corresponding argument value provided by a class instance creation expression (§15.8) or explicit constructor invocation (§8.6.5).
• An exception-handler parameter (§14.18) is initialized to the thrown object representing the exception (§11.3, §14.16).
• A local variable (§14.3, §14.12) must be explicitly given a value before it is used, by either initialization (§14.3) or assignment (§15.25), in a way that can be verified by the compiler using the rules for definite assignment (§16).

The example program:

class Point {
	static int npoints;
	int x, y;
	Point root;
}


class Test {
	public static void main(String[] args) {
		System.out.println("npoints=" + Point.npoints);
		Point p = new Point();
		System.out.println("p.x=" + p.x + ", p.y=" + p.y);
		System.out.println("p.root=" + p.root);
	}
}

prints:

npoints=0
p.x=0, p.y=0
p.root=null

illustrating the default initialization of npoints, which occurs when the class Point is prepared (§12.3.2), and the default initialization of x, y, and root, which occurs when a new Point is instantiated. See §12 for a full description of all aspects of loading, linking, and initialization of classes and interfaces, plus a description of the instantiation of classes to make new class instances.