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

Question

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 ++/--

Answer 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 

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int oneB = 0++; 

means

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

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int oneC = getInt()++; 

means

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

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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.

Answer 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.