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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 ++/--
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The post increment operator is used to increment the value of some variable after using it in an expression. In the post increment the value is used inside the expression, then incremented by one. if the expression is a = b++; and b is holding 5 at first, then a will also hold 5.
Description. If used postfix, with operator after operand (for example, x++ ), the increment operator increments and returns the value before incrementing. If used prefix, with operator before operand (for example, ++x ), the increment operator increments and returns the value after incrementing.
2) Post-increment operator: A post-increment operator is used to increment the value of the variable after executing the expression completely in which post-increment is used. In the Post-Increment, value is first used in an expression and then incremented. Syntax: a = x++;
x++ is post increment; this means that the value of x is used then it is incremented. If it is so, then x=0 should be used and the answer should be true.
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.
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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.
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