Q: Explain autoboxing, unboxing, and the Integer cache.

Answer:

Autoboxing & Unboxing

• Autoboxing: automatic conversion of primitive → wrapper (int → Integer).
• Unboxing: wrapper → primitive (Integer → int).

Integer boxed = 10;       // autoboxing: Integer.valueOf(10)
int unboxed = boxed;      // unboxing: boxed.intValue()

List<Integer> nums = new ArrayList<>();
nums.add(5);              // autoboxing — primitive can't go in generic
int first = nums.get(0);  // unboxing

The Integer Cache (-128 to 127)

Integer.valueOf(int) caches values in [-128, 127]. Same reference returned for cached values.

Integer a = 100;
Integer b = 100;
System.out.println(a == b);   // true  — same cached reference

Integer c = 200;
Integer d = 200;
System.out.println(c == d);   // false — new objects, outside cache

System.out.println(c.equals(d)); // true — always use .equals() for wrappers

Cache upper bound configurable via -XX:AutoBoxCacheMax=N.

Pitfalls

1. NullPointerException on unboxing

Integer x = null;
int y = x;  // 💥 NPE — unboxing null

2. Performance — boxing in tight loops

Long sum = 0L;                    // ❌ Long, not long
for (long i = 0; i < 1_000_000; i++) {
    sum += i;                     // boxes/unboxes every iteration
}

Use long primitive → 10x+ faster.

3. == vs .equals() on wrappers

Integer a = 1000, b = 1000;
if (a == b) { ... }        // ❌ reference compare — false outside cache
if (a.equals(b)) { ... }   // ✅ value compare

4. Conditional expression unboxing

Integer i = null;
int x = true ? i : 0;  // 💥 NPE — ternary unboxes Integer

When Boxing Happens

• Generics: List<Integer>, Map<String, Long>
• Object params: Object o = 5;
• Collection ops: set.contains(42)
• Reflection: method.invoke(obj, 1) — args are Object[]

Best Practices

• Primitives in hot paths.
• Wrappers only when nullability or generics needed.
• Always .equals() for wrapper comparison.
• Watch ternary + null returns for NPE.