# equals() & hashCode() — Core Java

Source: https://www.geekswithgeeks.com/en/core-java/java-equals-hashcode

> Override equals() and hashCode() correctly and together so your objects behave in HashMaps, HashSets, and everywhere equality matters.

## Default is identity

`Object`'s default `equals()` is just `==` (reference identity), and default `hashCode()` is derived from that identity. If two objects should be "equal" by their **content** (e.g. two `Point(1,2)`), you must override both methods yourself.

## The contract

The golden rule: **if `a.equals(b)` is true, then `a.hashCode() == b.hashCode()` must also be true.** Break this and objects silently vanish inside `HashMap`/`HashSet` lookups. Use the same fields in both methods.

```java
public final class Point {
    private final int x, y;
    // ... constructor, getters ...

    @Override public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Point p)) return false;
        return x == p.x && y == p.y;
    }
    @Override public int hashCode() {
        return Objects.hash(x, y);
    }
}
```

Quick check

**Quiz:** You override equals() but forget hashCode(). What breaks?

- [ ] Nothing, they're independent
- [ ] Compilation fails
- [x] "Equal" objects can land in different HashMap buckets and lookups fail
- [ ] equals() itself throws at runtime

*Answer:* "Equal" objects can land in different HashMap buckets and lookups fail. Hash-based collections use hashCode() to pick a bucket first, then equals() within it. Mismatched hashCode() means equal objects can be looked for in the wrong bucket and never be found.