Java · Intermediate level

Intermediate level · Module 2: The Java Collections Framework · Lesson 3 of 3

equals, hashCode and Safe Iteration

Make your own objects work correctly as Set elements and Map keys, and remove items safely while iterating.

In this lesson you will

  • Implement equals and hashCode consistently
  • Remove items safely with Iterator or removeIf

Key concepts

equals contracthashCodeConcurrentModificationExceptionIterator.remove

The lesson

HashSet and HashMap use hashCode to find a bucket and equals to compare items within it. If you override equals but not hashCode, two equal objects can land in different buckets and the set will contain duplicates. Always override both, using the same fields.

Objects.hash(name, phone) builds a suitable hashCode, and IntelliJ can generate equals and hashCode for you. Use fields that do not change while the object is in a set; changing a key field later makes the object impossible to find.

Removing from a list inside an enhanced for loop throws ConcurrentModificationException. Use an Iterator and call iterator.remove(), or use list.removeIf(condition), which is shorter and clearer.

Step by step

  1. Create a Customer class without equals/hashCode and add two equal customers to a HashSet.
  2. Observe the duplicate, then generate equals and hashCode.
  3. Repeat and confirm the set holds one customer.
  4. Remove inactive customers with removeIf.

Practice activity

Write a member register that stores members in a HashSet, rejects duplicates by phone number and removes expired members safely.

Expected result: Adding a member with an existing phone number has no effect, and expired members are removed without exceptions.

Check your understanding

  1. What goes wrong if you override equals but not hashCode?

    Show answer

    Equal objects can be stored twice in hash-based collections because they get different hash codes.

  2. Which method removes all list items that match a condition?

    Show answer

    removeIf.

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