In Java, which two methods must be implemented correctly in a key object that will be used in a HashMap to ensure proper hashing and lookup behaviour?

Introduction / Context:
Hash based collections in Java, such as HashMap and HashSet, rely on the hashing mechanism of key objects to store and retrieve entries efficiently. To work correctly, the key objects must implement certain methods consistently. This question asks about which methods are essential for a key that will be used in a HashMap, a very common topic in Java interviews.

Given Data / Assumptions:

• The collection in question is HashMap from the Java Collections Framework.
• We are focusing on the behaviour of key objects.
• The goal is to ensure that keys can be stored and retrieved reliably.

Concept / Approach:
HashMap uses the hashCode of a key to decide which bucket to place the entry in and uses equals to resolve collisions by comparing keys within the same bucket. Therefore, any class used as a key must provide a consistent and compatible implementation of both equals and hashCode. The general contract states that if two objects are equal according to equals, then they must have the same hashCode. Failing to implement these methods correctly can result in lost entries or unexpected behaviour during lookups.

Step-by-Step Solution:
Step 1: Recall that HashMap is a hash table based implementation of the Map interface. Step 2: Understand that hashing involves computing an integer value from the key using the hashCode method. Step 3: Recognise that when multiple keys map to the same bucket, the equals method is used to differentiate between keys. Step 4: Conclude that both equals and hashCode must be implemented consistently to guarantee correct insertion and retrieval. Step 5: Select the option that lists equals and hashCode as the two required methods.

Verification / Alternative check:
Many official Java tutorials and books emphasise that any class used as a key in a HashMap must override equals and hashCode. You can also verify this by creating a simple class without overriding these methods and use instances of that class as keys. You will quickly observe incorrect behaviour when trying to look up entries using a different but logically equal instance.

Why Other Options Are Wrong:
Option B: toString and clone are useful methods but have no direct effect on hashing behaviour or key comparison. Option C: finalize and wait are related to garbage collection and thread coordination and are not used by HashMap for lookup. Option D: notify and notifyAll are also thread coordination methods and are unrelated to hash based collections.

Common Pitfalls:
A frequent mistake is to override equals without overriding hashCode, violating the contract and causing keys to vanish from HashMap buckets. Another pitfall is to base hashCode on mutable fields that change while the object is in the map, which leads to inconsistent hashing. Ensuring that both equals and hashCode are overridden and based on stable fields is essential for correct behaviour.

Final Answer:
For a key object in a HashMap, the two methods that must be implemented correctly are equals and hashCode.