implicit conversion in java operator +=

Question

I've found that java compile has a non-expected behavior regarding assignment and self assignment statements using an int and a float.

The following code block illustrates the error.

    int i = 3;
    float f = 0.1f;

    i += f;              // no compile error, but i = 3
    i = i + f;           // COMPILE ERROR

• In the self assignment i += f the compile does not issue an error, but the result of the exaluation is an int with value 3, and the variable i maintains the value 3.

• In the i = i + f expression the compiler issues an error with "error: possible loss of precision" message.

Can someone explain this behavior.

EDIT: I've posted this code block in https://compilr.com/cguedes/java-autoassignment-error/Program.java

Answer 1

http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.26.2

The Java Language Specification says:

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.

So i += f is equivalent to i = (int) (i + f).