equals and hashCode contract

Intermediate Algorithms and Data Structures English

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Created by Best · 16.08.2026 at 09:13 UTC

The Contract: if a.equals(b), then a.hashCode() and b.hashCode() must be equal. equals must be reflexive, symmetric, and transitive.

HashSet and HashMap use hashCode for bucket selection and equals for precision. Violations lead to "lost" elements.

Practice: all fields used by equals should also be included in hashCode. Avoid mutable fields as keys.

Contract in the JDK API: Object.equals [1].

Where used

Without a correct contract, HashMap/HashSet, ORM entities in sets, and deduplication in pipelines break. Each domain class that serves as a map key (UserId, Coordinate, Money) needs both methods to be consistent.

Depth

Business equality is modeled by equals. It must be reflexive, symmetric, transitive, and consistent over unchanged states. The hash code condenses the same relevant properties for quick selection.

If two objects are business equal, they must land in the same hash area. The reverse does not hold because different objects may collide. Therefore, a equality check still follows the hash check.

Inheritance complicates symmetric equality when a subclass has additional relevant fields. Immutable value objects or composition often provide a clearer contract than an extendable equality hierarchy.

Difficulty levels

Check the properties of an equivalence relation.
Distinguish acceptable collisions from contract violations.
Assess equality in an inheritance hierarchy.

Pitfalls

Overriding only one of the two methods damages hash-based collections. Mutable equality fields also make already inserted keys unreliable.

Sources

[1]https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Object.html#equals(java.lang.Object)Return to text

University approvals: 0

Tasks

Question 1

Which rule is part of the equals/hashCode contract?

Different hashCodes enforce equals true

equals true requires the same hashCodes.

hashCode may always change when equals is true.

is allowed to throw null

Question 2

Implement equals and hashCode for Point with int x,y (only these fields).

Hint

Override equals(Object) and hashCode() with @Override. Objects.hash(x, y) is a suitable JDK helper method.

public class Point {
  public final int x;
  public final int y;
  public Point(int x, int y) { this.x = x; this.y = y; }
  @Override public boolean equals(Object o) {
    // TODO
    return false;
  }
  @Override public int hashCode() {
    // TODO
    return 0;
  }
}

Starter code is prefilled; replace TODO blocks with your solution.

Runtime output (stdout/stderr)

1 test case will be used for grading

Run checks runtime behavior only. Final correctness is evaluated when you submit.

Question 3

Which situation is allowed after the hash contract?

Objects of the same type lie in different hash ranges.

Different objects have the same hash value.

equals changes its result without changing the state.

equals is not reflexive.

Card Info

Topic: Algorithms and Data Structures
Difficulty: Intermediate
Completed: 0 users

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