int vs Integer — Primitive vs Wrapper and Autoboxing

Direct Answer

int is a 32-bit primitive value and cannot be null. Integer is an immutable wrapper object and can be null; it is needed by generics and APIs that use reference types. Java can autobox and unbox between them, but unboxing null throws NullPointerException, and wrapper identity must not be compared with == as a value test.

Code Example

This is the everyday reason wrappers and autoboxing exist: collections can only hold objects, so the int literals you add are boxed on the way in and unboxed on the way out — all without you writing a single conversion.

import java.util.*;
public class BoxingDemo {
    public static void main(String[] args) {
        List<Integer> scores = new ArrayList<>();
        scores.add(95);          // autobox: int 95 -> Integer.valueOf(95)
        scores.add(80);          // autobox again
        int first = scores.get(0); // unbox: Integer -> int
        int sum = scores.get(0) + scores.get(1); // unbox both, then add
        System.out.println("First: " + first + ", Sum: " + sum);
    }
}
// Output:
// First: 95, Sum: 175

Notice List<Integer> — never List<int>. Every add(95) boxes the primitive into an object, and every get unboxes it back. Convenient, but in a tight loop that hidden boxing allocates a fresh object each time, which is why performance-sensitive code prefers a primitive int[] over a List<Integer>.

Common Mistakes & Pitfalls

• Unboxing a null wrapper: Integer x = map.get("missing"); int y = x; throws a NullPointerException because get returned null and unboxing called intValue() on it. Always treat a wrapper from a map, query, or API as possibly null.
• Comparing Integer with ==: == compares object references, not values. It appears to work for small numbers because of the Integer cache (−128 to 127 are shared objects), then fails for 1000 == 1000. Use .equals() or unbox to int first.
• Using Integer everywhere by habit: a wrapper allocates an object and can be null, so prefer plain int for counters, loop variables, and fields that always have a value — reach for Integer only when you genuinely need null or generics.
• Boxing in hot loops: Integer sum = 0; for (...) sum += x; re-boxes on every iteration, creating thousands of throwaway objects. Use an int accumulator instead.