Understanding static variable initialization in Java

Question

public class InstanceBuilder {

    private static final InstanceBuilder INSTANCE = new InstanceBuilder();

    private static String name = null;

    private InstanceBuilder() {
        System.out.println("Setting cons()");
        name = "Testing";
    }

    public static String getName() {
        return name;
    }
}

public class Driver {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        System.out.println("value is " + InstanceBuilder.getName());
    }

}

Output:

Setting cons()
value is null

Why does it print value as null even though I have set the static variable in constructor and it is called as expected. If I try to print in constructor, it is printing Testing, but if I call from public static method, it is null. I know if I change it to INSTANCE.name, it works. But I want to understand why it is not working if I directly access static variable since same copy should be shared. What I am missing here?

Somnath Musib · Accepted Answer · 2016-09-16 11:29:40Z

Because the value of name is getting modified after the constructor invocation as per the declaration order.

Lets see what is happening:

1) When you call InstanceBuilder.getName(), InstanceBuilder class is getting loaded. As part of that process INSTANCE and name instance variables are getting initialized.

2) While initializing INSTANCE,

private static final InstanceBuilder INSTANCE = new InstanceBuilder();

constructor of InstanceBuilder class is getting invoked & Setting cons() statement is getting printed and name variable is initialized with Testing.

3) Next name is again getting re-initialized with null due to the below statement

private static String name = null;

so when the method call returns to Driver class, value of name is null and null is getting printed. So even though name is declared as static, static has no role in that context.

Note:

Check the below link on the order of which class members should be declared

http://www.oracle.com/technetwork/java/javase/documentation/codeconventions-141855.html#1852

Jekin Kalariya · Accepted Answer · 2016-09-16 03:26:37Z

0

Here value is null because static initialization occurs as its declare in order, So First your

`private static final InstanceBuilder INSTANCE = new InstanceBuilder();`

code execute and assign value to "testing", then your

 `private static String name = null;`

code exceute and override value to null(as static variable have coppy per class only), so final value will be null.

So here this behavior is just beacuse of order of execution of static variable

answered Sep 16, 2016 at 3:26

Jekin Kalariya

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1 Comment

Kars Over a year ago

Thanks, Got it where I was wrong from your explaination.

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