Looking for explanation

[Luke Kamble]

Hi, can someone explain me please why this

public class StaticTest {
	private final static StaticTest getInstance = new StaticTest();
	private final static  int a = 5;
	public static void main(String[] args) {
		StaticTest.getInstance();
	}
	private StaticTest() {
		System.out.println(a); //prints 5
	}
	static StaticTest getInstance(){
		return getInstance;
	}
}

prints 5. But if I ommit final(like: private static int a = 5;) it prints 0;
Then if I chacge my singleton pattern to this

public class StaticTest {
	private final static StaticTest getInstance = null;
	private final static  int a = 5;
	public static void main(String[] args) {
		StaticTest.getInstance();
	}
	private StaticTest() {
		System.out.println(a); //prints 5
	}
	static StaticTest getInstance(){
		return getInstance == null? (getInstance = new StaticTest()) : getInstance;
	}
}

works fine with or without final near a = 5.(it prints 5)

[Manish Singh]

Read the topics like static and instance initializes.

[Henry Wong]

Static variables are initialized in the order that they are encountered in the source. So, if you try to use a static variable to initialize another static variable, and that variable is later in source, then you will get the default value.

Final variables that are initialized during declaration, to a compile time constant, are themselves, compile time constants. And since the compiler knows their value are compile time, it is always correct.

Henry

[Rob Spoor]

The order is important. If you omit final in the first example, the instance is created before a is given its value of 5. That's why it prints 0 - before the main method is even executed. By making the field final you're turning it into a compile time constant, and compile time constants get substituted by the compiler. In the compiled code with the final keyword in place, field "a" no longer exists. Instead line 8 says "System.out.println(5);".

[Luke Kamble]

Thank you guys, now I get it.
Just another question, class files are loaded on first useage or when the vm is started?

[Rob Spoor]

First usage. You can verify this using the following example:

public class Test
{
    public static void main(String[] args) throws Exception
    {
        System.out.println("First print");
        Test2 t = new Test2(); // loads class Test2
        System.out.println("Second print");
    }
}

class Test2
{
    static
    {
        // executed when the class is loaded
        System.out.println("Test2 is loaded");
    }
}