Working with enum

[Astha Sharma]

Hello,

Here is a code using enums-

enum Test {a("aa"), c("aa"), b("aa");
 private Test(String string) {
	// TODO Auto-generated constructor stub
	 this.string = string;
}
 String string;
 }
public class TestEnum {
	Test bar;
	public static void main(String[]args) {
		TestEnum test = new TestEnum();
		test.go();
		
	}
	void go(){
		if(bar==null){
			System.out.println("null");
		}
		else{
			System.out.println("not null");
		}
		System.out.println(Test.a.string);
		System.out.println(bar.a.string);
	}
}

output of this code is-

null
aa
aa

My query is that bar reference has never been initialized. If it's value is null, how line no. 23 is able to compile? Why it's not throwing NullPointerException?

[Arnab Sunny Rakshit]

May be its a compiler syntax trick exactly like the following code.
t is null but still no NPE because referring to a static member through an instance variable is just a syntax trick. Compiler substitutes the instance variable with its Classname.

public class Count{
	static Thread t;
     public static void main(String args[]) throws InterruptedException{
          t.sleep(1000);
     }
}

[Astha Sharma]

Thanks Arnab

[Mahbub Rahman]

In your enum declaration, each of the enum constant is implicitly declared as follows:

public static final Test a = new Test("aa");

Think it will help you

[Mala Gupta]

Astha,

Try to decompile Test.class using a decompiler (like jad). Here's what you get:

// Decompiled by Jad v1.5.8f. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://www.kpdus.com/jad.html
// Decompiler options: packimports(3) 
// Source File Name:   Test.java


final class Test extends Enum
{

    public static Test[] values()
    {
        return (Test[])$VALUES.clone();
    }

    public static Test valueOf(String s)
    {
        return (Test)Enum.valueOf(Test, s);
    }

    private Test(String s, int i, String s1)
    {
        super(s, i);
        string = s1;
    }

    public static final Test a;
    public static final Test c;
    public static final Test b;
    String string;
    private static final Test $VALUES[];

    static 
    {
        a = new Test("a", 0, "aa");
        c = new Test("c", 1, "aa");
        b = new Test("b", 2, "aa");
        $VALUES = (new Test[] {
            a, c, b
        });
    }
}

This should help clear a lot of other related doubts.

With respect,
Mala