Why doesn't the post increment operator work on a method that returns an int?

public void increment(){
    int zero = 0;

    int oneA = zero++; // Compiles

    int oneB = 0++; // Doesn't compile

    int oneC = getInt()++; // Doesn't compile
}

private int getInt(){
    return 0;
}

They are all int's, why won't B & C compile? Is it to do with the way ++ operator differs from = 0 + 1;?

Invalid argument to operation ++/--

Solution 1:

i++ is an assignment to a variable i.

In your case, zero++ is an equivalent to zero = zero + 1. So 0++ would mean 0 = 0 + 1, which makes no sense, as well as getInt() = getInt() + 1.

More accurately :

int oneA = zero++;

means

int oneA = zero;
zero = zero + 1; // OK, oneA == 0, zero == 1

int oneB = 0++;

means

int oneB = 0;
0 = 0 + 1; // wrong, can't assign value to a value.

int oneC = getInt()++;

means

int oneC = getInt();
getInt() = getInt() + 1; // wrong, can't assign value to a method return value.

From a more general point of view, a variable is a L-value, meaning that it refers to a memory location, and can therefore be assigned. L in L-value stands for left side of the assignment operator (i.e. =), even if L-values can be found either on the left side or the right side of the assignment operator (x = y for instance).

The opposite is R-value (R stands for right side of the assignment operator). R-values can be used only on the right side of assignment statements, to assign something to a L-value. Typically, R-values are literals (numbers, characters strings...) and methods.

Solution 2:

Because as stated in JLS:

The result of the postfix expression must be a variable of a type that is convertible (§5.1.8) to a numeric type, or a compile-time error occurs.

Solution 3:

getInt() is not int

getInt() returns int

++ operator does two things increment + assignment

So for ++ operator to work you need a variable to store the result of increment operation which 0 and getInt() both are not.