オートボックスとは何か、またその落とし穴は何か？

Integer boxed = 42;       // autoboxing: int → Integer (Integer.valueOf(42))
int unboxed = boxed;       // auto-unboxing: Integer → int (boxed.intValue())

List<Integer> nums = new ArrayList<>();
nums.add(5);               // autoboxes int 5 → Integer (collections need objects)
int x = nums.get(0);       // auto-unboxes Integer → int

Integer a = 1000;
Integer b = 1000;
a == b;            // ❌ FALSE — different Integer OBJECTS (reference comparison)
a.equals(b);       // ✅ true — value comparison

// the WORSE trap — the Integer cache makes small values seem to work:
Integer c = 100, d = 100;
c == d;            // TRUE — Java CACHES Integers from -128 to 127 (same object)
Integer e = 200, f = 200;
e == f;            // FALSE — outside the cache range → different objects

Integer value = null;       // a wrapper can be null
int x = value;              // 💥 NullPointerException — unboxing null calls null.intValue()

Map<String, Integer> map = new HashMap<>();
int count = map.get("missing");   // 💥 NPE — get() returns null, then unboxing fails

// ❌ autoboxing in a hot loop — creates millions of Integer objects (GC pressure, slow)
Long sum = 0L;                          // WRONG type — wrapper
for (long i = 0; i < 1_000_000; i++) {
    sum += i;                            // unbox, add, re-box → new Long each iteration!
}
// ✅ use primitives in hot paths
long sum = 0L;                          // primitive — no boxing