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equals-hashcode

equals and hashCode

Last reviewed May 28, 2026 Content v20260528
Track mode
server_compiled
Means
Compiled runner
Reading
~1 min
Level
intermediate

This lesson

This lesson teaches equals and hashCode: the syntax, APIs, and habits you need before advancing in Java.

Teams ship equals and hashCode on every Java codebase—skipping it leaves gaps in debugging and code reviews.

You will apply equals and hashCode in contexts like: Spring Boot APIs, banking systems, Android (with Kotlin), and batch/data pipelines on the JVM.

Write Java with a public class (lessons use Main), click Run on server—the dev runner runs javac then java; fix compile errors from stderr (LEARNING_RUNNER_ENABLED=true).

When you can explain the previous lesson's ideas without copying starter code.

Objects compared by reference with ==. equals defines value equality; hashCode supports hash-based collections. Override both together with a consistent contract.

Contract (simplified)

  • If a.equals(b), then a.hashCode() == b.hashCode()
  • Reflexive, symmetric, transitive, consistent equals

Records (Java 16+)

record Point(int x, int y) {} generates equals, hashCode, and toString automatically—prefer for data carriers.

Important interview questions and answers

  1. Q: Why override hashCode with equals?
    A: HashMap/HashSet break if equal objects hash differently.
  2. Q: Default equals behavior?
    A: Inherited from Object—reference equality like ==.

Self-check

  1. What happens if two equal objects have different hash codes?
  2. When should you use a record instead of a class?

Interview tip Lesson completion confidence

Can you explain this lesson in 30 seconds without reading notes?

Not saved yet.

Playground

Runs on the configured server runner (dev: npm run runner with LEARNING_RUNNER_ENABLED=true). Output appears below the editor.

Check yourself

Multiple choice — immediate feedback.

Discussion

Past discussion is visible to everyone. Only logged-in users can post comments and replies.

Starter discussion topics

  • Equals without hashCode bug?
  • Reference vs value equality?

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