The Map Interface and HashMap Internals

A Map is like a dictionary — every key maps to exactly one value. Keys must be unique, but values can repeat freely.

Basic Map Usage

Map marks = new HashMap<>();
marks.put("Vishwas", 95);
marks.put("Rahul", 88);
marks.put("Vishwas", 99); // overwrites the previous value for this key

System.out.println(marks.get("Vishwas"));      // 99
System.out.println(marks.containsKey("Rahul")); // true

How HashMap Works Internally

• Each key's hashCode() determines which "bucket" (array index) it's stored in
• Multiple keys can land in the same bucket — this is called a collision
• Since Java 8, buckets with many collisions switch from a linked list to a balanced tree for better worst-case lookup performance

HashMap vs TreeMap vs LinkedHashMap

Iterating a Map

Map scores = new HashMap<>();
scores.put("Alice", 95);
scores.put("Bob", 87);

for (Map.Entry entry : scores.entrySet()) {
    System.out.println(entry.getKey() + " -> " + entry.getValue());
}

equals() and hashCode() Contract

If you override equals() on a custom key class, you must also override hashCode() — otherwise two "equal" objects might land in different buckets and break lookups entirely.

Interview tip: A classic question is "What happens if hashCode() is the same for two different keys?" — Answer: they land in the same bucket, and equals() is then used to tell them apart. This is a normal collision, not an error.