JAVA snippet: Output Explanation

Question

I was going through java code snippets and this one snippet I am not able to figure out why the output is 2?

 package com.instanceofjava;     
    public class A{  
     static int a = 1111;
     static
     {
            a = a-- - --a;
     }        
     {
            a = a++ + ++a;
     }     
     public static void main(String[] args)  {     
           System.out.println(a);     
        }     
    }

Can somebody explain why the output is 2 for this code snippet.

Eran · Accepted Answer · 2015-10-07 08:33:01Z

6

Since you don't create an instance of your class, only the static initializer block is executed (the expression a = a++ + ++a; in the instance initializer block is not executed).

First a is initialized to 1111 (as a result of static int a = 1111;).

Then the static initializer block is executed and the following assignment takes place :

a = a-- - --a;

a-- decrements a and returns the previous value 1111. --a decrements the previously decremented value (1110) and returns the new value 1109.

Hence the expression is evaluated to :

a = 1111 - 1109 = 2

answered Oct 7, 2015 at 8:33

Eran

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6 Comments

Arthur Eirich Over a year ago

'a-- decrements a and returns the previous value 1111'. If a-- decrements a why it then returns 1111 again?

Akki619 Over a year ago

so a-- makes the value as 1110 and --a makes it 1109 but since a-- is there so decremented value will be used next time and a-- will be 1111 only?

Eran Over a year ago

@ArthurEirich the postfix decrement operator a-- decrements the value of a but returns the previous value of a. That's the definition of this operator.

Eran Over a year ago

@Akki619 Yes, a-- will return the original value of a - 1111 - even though it decrements it to 1110.

Eran Over a year ago

@SSH Whether the static keyword is missing from the second block or was intentionally left out is another question. Without the static keyword, it is an instance initializer block, which would only get executed when you create an instance of the class. Since no instance of A is ever created, it is not executed.

|

Suresh Atta · Accepted Answer · 2015-10-07 08:42:29Z

3

The key point here to note is only static block executes and initialization block never executed here.

Hence the code

  static
     {

     }

Executed and giving result 2.

Just to check with you can remove the whole initilization block and run

public class A{
    static int a = 1111;
    static
    {
        a = a-- - --a;
        System.out.println(a);
    }

    public static void main(String[] args)  {
        System.out.println(a);
    }

And run the code. Gives you the same out put.

And coming to the part decrement

a-- means: Decrement a AFTER evaluating the expression.

--a means: Decrement variable BEFORE evaluating the expression.

Hence the line a = a-- - --a; equals to

a = 1111 - 1109

edited Oct 7, 2015 at 8:42

answered Oct 7, 2015 at 8:36

Suresh Atta

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