Question

I am trying to understand the difference between index = index--; and index=--index; I checked the output of index = index--; assignment in the below code block (first example):

public static void main(String[] args) {
    int index = 10;
    for (int i = 0; i < 10; i++) {
        index = index--;
        System.out.println("Index:" + index);
    }//end of for loop
}// end of main method

Output:

Index:10
Index:10
Index:10
Index:10
Index:10
Index:10
Index:10
Index:10
Index:10
Index:10

When I use the index=--index; assignment (second example)

public static void main(String[] args) {
        int index = 10;
        for (int i = 0; i < 10; i++) {
            index = --index;
            System.out.println("Index:" + index);
        }
    }// end of main method

Output:

Index:9
Index:8
Index:7
Index:6
Index:5
Index:4
Index:3
Index:2
Index:1
Index:0

I have two questions. In the second example Eclipse throws a warning: assignment index = --index;(The assignment to variable index has no effect) But actually it does have an effect on the variable. It decreases its value by 1. Why does Eclipse gives such a warning message?

In the first example, the variable is not affected by the operation (and Eclipse gives no warning). I wonder why Eclipse has no message, and I don’t understand why this assignment has no effect over the variable index index = index--;

Answer 1

Both = and -- alter the value of index. In the case of Java, the -- is guaranteed to happen before the =, so in the case of index = index--; the return value of -- is the old value of index and overwrites the decrement of index while in the case of index = --index; the return value of -- is already the new value of index so the assignment has no effect and index stays decremented.

Note that in C the statement index = index--; has undefined behaviour and compilers are allowed to decrement index, ignore it or write code that reformats your hard disk (although the latter case is extremely unlikely).