Wrapper Classes and Autoboxing

Every primitive type has a corresponding wrapper class in java.lang: Integer, Long, Double, Float, Byte, Short, Character, and Boolean. Wrapper classes are objects, so they can be stored in collections such as ArrayList, passed as generic type arguments, and set to null. They also provide useful constants (Integer.MAX_VALUE) and parsing methods (Integer.parseInt). Autoboxing is the automatic conversion the Java compiler performs between a primitive and its wrapper type. When you assign an int to an Integer variable, the compiler inserts a call to Integer.valueOf. Unboxing is the reverse: converting an Integer to an int by calling intValue. Autoboxing makes code more concise but has subtle pitfalls: comparing two Integer objects with == checks reference equality, not value equality, and unboxing a null reference throws a NullPointerException. Integer caching is a related detail: the JVM caches Integer instances for values between -128 and 127. Two calls to Integer.valueOf(100) return the same cached object, so == returns true. But Integer.valueOf(200) creates a new object each time, so == returns false. Always use equals() for comparing wrapper objects.

Example

import java.util.ArrayList;
import java.util.List;

public class AutoboxingDemo {

    public static void main(String[] args) {

        // --- Autoboxing: primitive -> wrapper (compiler inserts Integer.valueOf) ---
        Integer boxed = 42;          // equivalent to Integer.valueOf(42)
        int     unboxed = boxed;     // equivalent to boxed.intValue()

        // --- Wrapper utility methods ---
        int parsed  = Integer.parseInt("123");
        int max     = Integer.MAX_VALUE;
        String bin  = Integer.toBinaryString(255);   // "11111111"
        String hex  = Integer.toHexString(255);      // "ff"

        System.out.printf("parsed: %d, max: %d%n", parsed, max);
        System.out.printf("255 in binary: %s, hex: %s%n", bin, hex);

        // --- Integer caching trap ---
        Integer a = 127;
        Integer b = 127;
        Integer x = 200;
        Integer y = 200;

        System.out.println("127 == 127 (cached)  : " + (a == b));   // true
        System.out.println("200 == 200 (uncached): " + (x == y));   // false
        System.out.println("200.equals(200)      : " + x.equals(y)); // true

        // --- NullPointerException from unboxing null ---
        Integer nullInt = null;
        try {
            int boom = nullInt; // NullPointerException here
        } catch (NullPointerException e) {
            System.out.println("Caught NPE from unboxing null Integer");
        }

        // --- Autoboxing in collections ---
        List<Integer> numbers = new ArrayList<>();
        for (int i = 1; i <= 5; i++) {
            numbers.add(i);         // autoboxed on add
        }
        int sum = 0;
        for (int n : numbers) {     // unboxed on retrieval
            sum += n;
        }
        System.out.println("Sum 1-5: " + sum);
    }
}

HTML

Full Editor

Related Resources

Java Reference

Complete tag & property list

Java How-To Guides

Step-by-step practical guides

Java Exercises

Practice what you've learned