Why Is String a Good Key for HashMap in Java | Tricky Java Interview Questions — Part 16

Hello Developers,

Welcome to Part 16 of the Tricky Java Interview Questions series.

Today’s question dives into one of Java’s most-used data structures:

Why is String considered a good key for HashMap in Java?

You’ve probably used Map<String, Object> or Map<String, String> a hundred times. But do you know why it works so well?

Let’s explore the reasons — both technical and practical.

What Makes a Good Key for a HashMap?

To work well as a key in a HashMap, a class must:

1. Be immutable
2. Have a good hashCode() implementation
3. Have a correct equals() method
4. Not change while it’s being used as a key

Let’s see how String checks all those boxes — and why that makes it ideal.

Reason 1: String is Immutable

Immutability is the most important factor.

Map<String, String> map = new HashMap<>();
String key = "user123";

map.put(key, "John");
key = key.toUpperCase(); // changes reference, not the map key

System.out.println(map.get("user123")); // still prints "John"

If a key changes after insertion, the hash value changes too — and you won’t be able…