Java Trap – Autoboxing and Unboxing

The below program is a typical Java trap, and also a popular Java interview question. It has no compiler error or warning but running very slow.

Can you spot the problem?

package com.mkyong;

import java.time.Duration;
import java.time.Instant;

public class JavaSum {

    public static void main(String[] args) {

        Instant start = Instant.now();

        Long total = 0L;
        for (long l = 0; l < Integer.MAX_VALUE; l++) {
            total += l;
        }

        Instant end = Instant.now();

        System.out.println(Duration.between(start, end).getSeconds() + " seconds");
        System.out.println("Total: " + total);

    }
}

Output

6 seconds
Total: 2305843005992468481

P.S The elapsed time may vary.

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Answer

The mixed-used of primitive type and boxed primitive type (wrapper class), the boxed primitive type will auto-unboxed (Long -> long). In the above example, the Long total variable is repeatedly boxed and unboxed, causing the slow running of the program.

  // Long -> unboxed -> long
  // long -> boxed   -> Long
  Long total = 0L; // boxed primitive type (wrapper class)
  for (long l = 0; l < Integer.MAX_VALUE; l++) { // here is primitive type!
      // Long = Long (unboxed) -> long + long
      // Long = (boxed) long
      total += l;
  }

To make it faster, update the Long total to long total:

  long total = 0L;
  for (long l = 0; l < Integer.MAX_VALUE; l++) {
    total += l;  // no more boxed and unboxed, and it run much faster!
  }

Back to basic:

1. Autoboxing – int -> Integer, long -> Long, convert long primitive type to boxed primitive type (wrapper class) Long.
2. Unboxing – Integer -> int, Long -> long, convert Long boxed primitive type (wrapper class) to primitive type long.

References

• Autoboxing and Unboxing