Java – Difference Between Operators `+=` vs `=+`

assignment-operatorjavaoperators

I just realized that I was using =+ instead of the operator += and my program was doing all sorts of weird and unexpected things.
Eclipse didn't give me an error of any kind so I assume that =+ is a legitimate operator, but there is no reference to that in my book.

My question is:
What does =+ do and under what circumstances would you use it?

Best Answer

A common syntax is:

 +=

This is the add and assignment operator, which adds right-hand expression to the left-hand variable then assigns the result to left-hand variable. For example:

 int i = 1;
 int j = 2;
 i += j;

 // Output: 3
 System.out.println( i )

A far less common syntax is:

=+

Usually this is written as two different operators, separated by a space:

= +

Without the space, it looks as follows:

int i = 1;
int j = 2;
    
i =+ j;

// Output: 2
System.out.println(i);

An idiomatic way to write this is to shift the unary operator to the right-hand side:

int i = 1;
int j = 2;
    
i = +j;

// Output: 2
System.out.println(i);

Now it's easy to see that i is being assigned to the positive value of j. However, + is superfluous, so it's often dropped, resulting in i = j, effectively the equivalent of i = +1 * j. In contrast is the negative unary operator:

int i = 1;
int j = 2;
    
i = -j;

// Output: -2
System.out.println(i);

Here, the - would be necessary because it inverts the signedness of j, effectively the equivalent of i = -1 * j.

See the operators tutorial for more details.

Related Question