The java.lang.Object Class: toString(), equals(), and hashCode()

Intermediate14 min read•Updated: 2026-10-22

👑 The java.lang.Object Class: toString, equals, and hashCode

Introduction

In Java, java.lang.Object is the root ancestor of every single class. If a class does not explicitly extend another class, the compiler automatically inserts extends Object.

🔄 The Big Three Methods to Override

1
toString(): Returns a readable string representation of the object (default returns ClassName@HexHash).
2
equals(Object obj): Determines logical equality between two objects (default checks reference ==).
3
hashCode(): Returns an integer hash value used by hash-based collections (HashMap, HashSet).

💻 Full Implementation & The Equals-HashCode Contract

The Hash Contract: If a.equals(b) is true, then a.hashCode() MUST be identical to b.hashCode()!

1import java.util.Objects;
2 
3public class Student {
  private int rollNumber;
  private String name;
6 
  public Student(int roll, String name) {
      this.rollNumber = roll;
      this.name = name;
  }
11 
  // 1. Override toString()
  @Override
  public String toString() {
      return "Student[Roll=" + rollNumber + ", Name='" + name + "']";
  }
17 
  // 2. Override equals()
  @Override
  public boolean equals(Object o) {
      if (this == o) return true; // Reference identity check
      if (o == null || getClass() != o.getClass()) return false; // Null and class type check
      Student student = (Student) o; // Cast
      return rollNumber == student.rollNumber && Objects.equals(name, student.name);
  }
26 
  // 3. Override hashCode()
  @Override
  public int hashCode() {
      return Objects.hash(rollNumber, name);
  }
32 
  public static void main(String[] args) {
      Student s1 = new Student(101, "Aarav");
      Student s2 = new Student(101, "Aarav");
36 
      System.out.println("String representation: " + s1);
      System.out.println("Are s1 and s2 equal?   " + s1.equals(s2)); // true!
      System.out.println("s1 Hash: " + s1.hashCode() + " | s2 Hash: " + s2.hashCode());
  }
41}

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Encapsulation: Access Modifiers (private, default, protected, public) and Getters/Setters	Inheritance with extends and the super Keyword

Practice Quiz

Test your understanding of this lesson with 1 questions. Each question has one correct answer.

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💻 Full Implementation & The Equals-HashCode Contract

The Hash Contract: If a.equals(b) is true, then a.hashCode() MUST be identical to b.hashCode()!

1import java.util.Objects;
2 
3public class Student {
  private int rollNumber;
  private String name;
6 
  public Student(int roll, String name) {
      this.rollNumber = roll;
      this.name = name;
  }
11 
  // 1. Override toString()
  @Override
  public String toString() {
      return "Student[Roll=" + rollNumber + ", Name='" + name + "']";
  }
17 
  // 2. Override equals()
  @Override
  public boolean equals(Object o) {
      if (this == o) return true; // Reference identity check
      if (o == null || getClass() != o.getClass()) return false; // Null and class type check
      Student student = (Student) o; // Cast
      return rollNumber == student.rollNumber && Objects.equals(name, student.name);
  }
26 
  // 3. Override hashCode()
  @Override
  public int hashCode() {
      return Objects.hash(rollNumber, name);
  }
32 
  public static void main(String[] args) {
      Student s1 = new Student(101, "Aarav");
      Student s2 = new Student(101, "Aarav");
36 
      System.out.println("String representation: " + s1);
      System.out.println("Are s1 and s2 equal?   " + s1.equals(s2)); // true!
      System.out.println("s1 Hash: " + s1.hashCode() + " | s2 Hash: " + s2.hashCode());
  }
41}

Previous Lesson

Next Lesson

Encapsulation: Access Modifiers (private, default, protected, public) and Getters/Setters

Inheritance with extends and the super Keyword