Object Class Methods in Java
In Java, every class implicitly inherits from the Object class, which is the root class of the Java class hierarchy.
That means all Java classes — whether built-in or user-defined — automatically have Object class methods available.
The three most commonly used methods are:
toString()– returns a string representation of an object.equals()– compares two objects for equality.hashCode()– returns an integer value (hash code) for efficient object comparison in hash-based collections.
These methods are fundamental for object comparison, debugging, and collection frameworks.
1. toString() Method
- The
toString()method returns a textual (string) representation of an object. - It’s automatically called when you print an object using
System.out.println().
Default Behavior
By default, it returns:
ClassName@HexadecimalHashCode
Example – Default toString()
class Student {
int id;
String name;
Student(int id, String name) {
this.id = id;
this.name = name;
}
}
public class ToStringExample {
public static void main(String[] args) {
Student s = new Student(101, "Aman");
System.out.println(s); // Implicitly calls toString()
}
}
Output (Default):
Student@5ca881b5
Overriding toString()
You can override toString() to display meaningful information about your object.
class Student {
int id;
String name;
Student(int id, String name) {
this.id = id;
this.name = name;
}
@Override
public String toString() {
return "Student[id=" + id + ", name=" + name + "]";
}
}
public class ToStringOverride {
public static void main(String[] args) {
Student s = new Student(101, "Aman");
System.out.println(s);
}
}
Output (Overridden):
Student[id=101, name=Aman]
2. equals() Method
The equals() method checks whether two objects are equal in terms of content.
Default Behavior
The default equals() method in Object compares references, not actual content:
Object.equals(Object obj) → returns true if both references point to the same memory location.
Example – Default equals()
class Demo {
int value;
Demo(int value) {
this.value = value;
}
}
public class EqualsExample {
public static void main(String[] args) {
Demo d1 = new Demo(10);
Demo d2 = new Demo(10);
System.out.println(d1.equals(d2)); // false (different objects)
}
}
Output:
false
Overriding equals()
To compare object content, override the equals() method.
class Demo {
int value;
Demo(int value) {
this.value = value;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null || getClass() != obj.getClass())
return false;
Demo d = (Demo) obj;
return value == d.value;
}
}
public class EqualsOverride {
public static void main(String[] args) {
Demo d1 = new Demo(10);
Demo d2 = new Demo(10);
System.out.println(d1.equals(d2)); // true
}
}
Output:
true
3. hashCode() Method
The hashCode() method returns an integer value (hash code) used by hash-based collections like:
HashMapHashSetHashtable
Default Behavior
The default implementation returns a unique integer (usually derived from memory address).
Overriding hashCode()
Whenever you override equals(), you should also override hashCode() to maintain consistency — otherwise, hash-based collections may behave unpredictably.
class Demo {
int value;
Demo(int value) {
this.value = value;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null || getClass() != obj.getClass())
return false;
Demo d = (Demo) obj;
return value == d.value;
}
@Override
public int hashCode() {
return Integer.hashCode(value);
}
}
public class HashCodeExample {
public static void main(String[] args) {
Demo d1 = new Demo(10);
Demo d2 = new Demo(10);
System.out.println("Equals: " + d1.equals(d2));
System.out.println("HashCodes: " + d1.hashCode() + " & " + d2.hashCode());
}
}
Output:
Equals: true
HashCodes: 10 & 10
Rule:
If two objects are equal (equals() returns true), they must have the same hash code.
However, different objects can have the same hash code (hash collision).
Key Differences Between Methods
| Method | Purpose | Default Behavior | Common Override Use |
|---|---|---|---|
toString() | Converts object to String | Prints class name + hash code | To display readable info |
equals() | Compares two objects | Compares memory references | Compare actual data |
hashCode() | Returns integer hash code | Based on memory address | For HashSet, HashMap, etc. |
Real-Life Example
import java.util.HashSet;
class Employee {
int id;
String name;
Employee(int id, String name) {
this.id = id;
this.name = name;
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (!(o instanceof Employee)) return false;
Employee e = (Employee) o;
return id == e.id && name.equals(e.name);
}
@Override
public int hashCode() {
return id + name.hashCode();
}
@Override
public String toString() {
return "Employee[id=" + id + ", name=" + name + "]";
}
}
public class ObjectMethodsDemo {
public static void main(String[] args) {
HashSet<Employee> set = new HashSet<>();
set.add(new Employee(1, "Aman"));
set.add(new Employee(1, "Aman")); // Duplicate
System.out.println(set); // Only one object stored
}
}
Output:
[Employee[id=1, name=Aman]]
Because equals() and hashCode() are properly overridden, duplicates are avoided.
Points to Remember
- All classes in Java implicitly extend
Object. - Override
toString(),equals(), andhashCode()for custom behavior. - Always override
hashCode()when you overrideequals(). - Use
Objects.equals()andObjects.hash()(Java 7+) for null-safe comparisons. - These methods are crucial for collection operations, object comparison, and debugging.