Wrapper Classes

Autoboxing and unboxing

• Java 5+ converts int ↔ Integer automatically: Integer x = 5; and int y = x;.
• Integer.valueOf(int) is preferred over new Integer(int) (deprecated). parseInt returns a primitive; valueOf(String) returns a wrapper.

public class WrapperBasics {
    public static void main(String[] args) {
        Integer boxed = 42;          // autobox
        int raw = boxed;             // unbox
        int parsed = Integer.parseInt("42");
        Integer cached = Integer.valueOf(42);
        System.out.println(raw + parsed + cached);
    }
}

The Integer cache

• Integer.valueOf(i) (and autoboxing) reuses cached objects for **-128 to 127** inclusive. Outside that range, a new object is typical.
• So Integer a = 100; Integer b = 100; a == b is true, but Integer c = 200; Integer d = 200; c == d is often false. Always use equals.

Null unboxing, compare, and identity

• Unboxing null throws NullPointerException: Integer x = null; int y = x;.
• Do not mix == on wrappers. Use equals or intValue() / compare. Integer.compare(a, b) is null-hostile if you pass wrappers that are null.
• Boolean.parseBoolean("true") is case-insensitive; any other string is false (no exception).
• Optional cache size for Integer can be widened with -XX:AutoBoxCacheMax, but never depend on it in code.

Cache vs equals

public class IntegerCacheDemo {
    public static void main(String[] args) {
        Integer a = 127;
        Integer b = 127;
        Integer c = 128;
        Integer d = 128;
        System.out.println(a == b);       // true (cache)
        System.out.println(c == d);       // false (typically)
        System.out.println(c.equals(d));  // true
        Integer missing = null;
        System.out.println(missing == null);
    }
}