i++ vs ++i in Java: Which Is Faster in Loops? Bytecode Deep Dive

Earlier a friend was asked in an interview whether to use i++ or ++i in a for loop. The interviewer suggested considering execution efficiency.

In a for loop, should we use i++ or ++i?

We examine the two forms from a bytecode perspective. Both post‑increment i++ and pre‑increment ++i are not atomic operations, so we look at the generated bytecode.

Thought Process

We start with a simple method that assigns a value and performs i++:

public static void main(String[] args) {
    int i = 3;
    int j = i++;
    System.out.println(j);
}

Using javap to decompile we obtain the following bytecode:

The key instructions involve loading the variable and the iinc operation, which updates the local variable directly.

2: iload_1
3: iinc    1, 1

This translates roughly to:

int temp = i;
i = i + 1;

Next we replace i++ with ++i and repeat the analysis.

public static void main(String[] args) {
    int i = 3;
    int j = ++i;
    System.out.println(j);
}

Decompiling yields:

The bytecode for ++i consists of a single iinc instruction:

2: iinc    1, 1

Thus ++i avoids the extra temporary variable introduced by i++, giving a slight efficiency advantage when the result value is not used.

Increment in a for Loop

We compare two loops, one using i++ and the other ++i:

// i++ post‑increment
public class ForIpp {
    public static void main(String[] args) {
        for (int i = 0; i < 5; i++) {
            System.out.println(i);
        }
    }
}

// ++i pre‑increment
public class ForPpi {
    public static void main(String[] args) {
        for (int i = 0; i < 5; ++i) {
            System.out.println(i);
        }
    }
}

Decompiling both classes shows identical bytecode, both using the iinc instruction for the loop increment.

This occurs because the Java compiler optimizes the loop increment to the pre‑increment form regardless of the source syntax.

Conclusion

In typical Java for‑loops, the JVM compiles both i++ and ++i to the same iinc bytecode instruction, making their runtime performance effectively identical.

Therefore, the two increment styles have the same efficiency in a for loop.

Note: The code was tested on Java 1.8.0_261‑b12.