why i = i++ doesn’t increment the variable?

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Why i = i++ Doesn’t Increment the Variable

Summary

Root Cause

  • Operator precedence: Post-increment (i++) returns the original value before incrementing the variable
  • Execution order:

Why This Happens in Real Systems

  • Attempts to compact code into fewer lines
  • Accidental refactoring where increment logic gets merged with assignment
  • Misunderstanding of operator behavior during quick fixes
  • Copy-pasting code snippets without understanding execution flow

Real-World Impact

  • Finite loops → Infinite loops causing resource exhaustion
  • Off-by-one errors → Broken aggregation/validation logic
  • Silent data corruption → Accounting errors, sensor data loss
  • Debugging costs → Non-obvious failures requiring tracing

Example or Code

Assignment-chained post-increment operations (e.g., `i = i++`) fail to increment variables due to the order of operations in Java/Kotlin. The expression overwrites the incremented value with the original value, resulting in a no-op.
  1. The right-hand expression `i++` evaluates to the current value of `i`  
  2. Post-increment: `i` is incremented in memory  
  3. Assignment: The original value (from step 1) overwrites `i`, discarding the increment

java
// Broken Version
int i = 0;
while (i < 5) {
i = i++; // Fails to increment
System.out.println(i); // Output: 0 forever
}

// Fixed Version
int i = 0;
while (i < 5) {
i++; // Correct
// OR i = i + 1;
System.out.println(i); // Output: 1,2,3,4,5
}

How Senior Engineers Fix It

  • Eliminate assignment: Replace i = i++ with standalone i++
  • Use atomic assignments: Leverage AtomicInteger for thread-safe increments
  • Extract operations: Separate increments from complex expressions
  • Assert and test: Add pre/post condition checks around critical values
  • Ban anti-patterns: Enforce static analysis rules against assignment chaining

Why Juniors Miss It

  • False intuition: Assumes = happens after ++ visually
  • Syntax familiarity: Mistakenly treats x++ as equivalent to x += 1
  • Focus on simplicity: Overlooks hidden steps in compound expressions
  • Lack of bytecode awareness: Unfamiliar with JVM stack operations behind assignments
  • Testing gaps: Validates with non-boundary cases (e.g., single iteration)