Question

I have seen this symbol/operator in a block of code:

a+=1;

But I cannot figure out what it does. Can someone help me please?

Answer 1

From the Java Language Specification:

A compound assignment expression of the form E1 op= E2 is equivalent to E1 = (T) ((E1) op (E2)), where T is the type of E1, except that E1 is evaluated only once.

The last phrase is important if the left-hand side has side effects:

array[i++] += 1;

This is not equivalent to:

array[i++] = array[i++] + 1;

The first expression will increment i once. The second will increment i twice and will assign the right-hand value to a different element of array than will the first expression.

I should note that these kind of side-effect statements are not good programming form, despite the fact that you often find them used.

The cast is also important because the type of (E1) op (E2) may not be assignment-compatible with E1. For example, if a is of type short, then a++ is not equivalent to a = a + 1. The latter will not compile because the type of a + 1 is int and cannot be assigned to a short variable without a cast. That’s why the spec in this case says that a++ is equivalent to a = (short) ((a) + (1)). The same thing goes if a is of type char or byte.