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javavariable-assignmentatomic

Increment operation mechanism

As many developers know, increment operator isn't atomic.

For instance:

public void incrementId(){ // id being an int field
   id++;
}

Actually, this corresponds to three distinct operations:

int temp = id;
id = id + 1;  
id = temp;

Besides, this method behaves similarly:

    public void incrementId(){ // id being an int field
       id = id + 1;  // three steps also here
    }

My question is:

What is the real difference behind the scene between following both operations:

id = id + 1; //three steps => non atomic

id = anotherIntVariable + 1; // one step => atomic

What concept forces the compiler to translate the first one into 3 steps and not the other?

Solution

  • What concept forces the compiler to translate the first one into 3 steps and not the other?

    it is not, id = xyz + 1 will be compiled to the following byte code:

     7  iload_2 [xyz]
 8  iconst_1
 9  iadd
10  istore_1 [id]

    It is easy to see from the byte code that the above is not "one step"