Why Does `j = j ` Result in `j` Remaining 0 in Java?

Post Increment Operator in Java

In Java, the post increment operator ( ) increments the value of a variable by one after its evaluation. This behavior can lead to unexpected results, as exemplified by the code provided in "Java Puzzlers":

<code class="java">public class Test22 {
    public static void main(String[] args) {
        int j = 0;
        for (int i = 0; i < 100; i++) {
            j = j++;
        }
        System.out.println(j); // prints 0

        int a = 0, b = 0;
        a = b++;
        System.out.println(a);
        System.out.println(b); // prints 1
    }
}</code>

The confusion arises when examining the statement j = j . According to the Java Language Specification (JLS), this statement is equivalent to:

<code class="java">temp = j;
j = j + 1;
j = temp;</code>

However, this explanation contradicts the result of a = b , which assigns 0 to a and increments b to 1. To resolve this discrepancy, it's crucial to note that a = b is evaluated as follows:

<code class="java">temp = b;
b = b + 1;
a = temp;</code>

This means that post increment assignments of the form lhs = rhs are equivalent to:

<code class="java">temp = rhs;
rhs = rhs + 1;
lhs = temp;</code>

Applying this rule to both j = j and a = b clarifies the results observed in the code. j = j effectively assigns the value of j (0) to temp, increments j to 1, and then assigns temp (0) back to j. This explains why j prints 0 despite the increment operator.

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