Java.Collections.How does HashMap work when trying to store two elements in it by keys with the same hashCode(), but for which equals() == false?

🎯 TL;DR:

✅ HashMap can store multiple keys with the same hashCode() as long as equals() returns false — because it uses both hashCode() and equals() to handle collisions properly.

Let’s break it down 👇

🧠 Scenario:

You have two objects:

Object a;
Object b;

// They both return the same hash code:
a.hashCode() == b.hashCode()  ✅

// But are not equal:
a.equals(b) == false          ✅

Now you do:

map.put(a, "valueA");
map.put(b, "valueB");

❓ What happens?

✅ Both entries will be stored in the same bucket
✅ HashMap will NOT overwrite one with the other
✅ Retrieval still works correctly

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🔧 How Does HashMap Handle This?

Internally:

1. Java calculates the bucket index using hashCode():

index = (n - 1) & hash

1. If two keys have the same index, it checks the linked list or tree in that bucket.
2. For each entry, it checks:
   • If the hash is equal
   • Then if equals() returns true

If equals() is false, it:

• Keeps searching the bucket chain (or tree)
• Eventually adds the new entry to the end of the chain

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🧪 Real Example:

class CustomKey {
    private final String name;

    CustomKey(String name) {
        this.name = name;
    }

    public int hashCode() {
        return 42; // Force collision!
    }

    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof CustomKey)) return false;
        return !this.name.equals(((CustomKey) o).name); // Reverse logic!
    }

    public String toString() {
        return name;
    }
}

public class Demo {
    public static void main(String[] args) {
        Map<CustomKey, String> map = new HashMap<>();
        CustomKey a = new CustomKey("Alice");
        CustomKey b = new CustomKey("Bob");

        map.put(a, "Value A");
        map.put(b, "Value B");

        System.out.println(map);          // Both entries appear
        System.out.println(map.get(a));   // "Value A"
        System.out.println(map.get(b));   // "Value B"
    }
}

✅ Even though both keys have hashCode() = 42,
✅ They are not equal, so HashMap treats them as distinct entries

✅ Why This Works Well

HashMap uses this combo check:

if (e.hash == hash && (e.key.equals(key)))

• So same hash doesn’t mean same key
• Collision resolution (via chaining or treeing) handles same-hash-but-unequal keys

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🧠 TL;DR (Again):

✅ You can store multiple keys with the same hashCode() in a HashMap
✅ As long as equals() says they’re different, they’ll be stored separately
✅ HashMap handles this gracefully using chaining or tree bins inside each bucket