Java Notes

Object Class in Java

Complete guide to the java.lang.Object class — toString(), equals(), hashCode(), clone(), getClass(), finalize(), and how every Java class inherits from Object.

java.lang.Object is the root class of the entire Java class hierarchy. Every class in Java — whether you write extends Object or not — inherits from Object. It provides essential methods that every object needs.

java example

// These are EQUIVALENT:
public class MyClass { }
public class MyClass extends Object { }

Methods of Object Class

Method	Purpose	Override?
`toString()`	String representation	Always
`equals(Object)`	Content equality	When needed
`hashCode()`	Hash for collections	With equals()
`getClass()`	Runtime type info	Never (final)
`clone()`	Object copying	When needed
`finalize()`	Pre-GC cleanup	Never (deprecated)
`wait()`	Thread synchronization	Rarely
`notify()`	Thread synchronization	Rarely
`notifyAll()`	Thread synchronization	Rarely

toString() — String Representation

Default toString() returns ClassName@hexHashCode, which is useless:

java example

public class Student {
    private String name;
    private int age;
    private double gpa;

    public Student(String name, int age, double gpa) {
        this.name = name;
        this.age = age;
        this.gpa = gpa;
    }
}

public class Main {
    public static void main(String[] args) {
        Student s = new Student("Alice", 20, 3.8);
        System.out.println(s);  // Default: Student@1b6d3586 (useless!)
    }
}

Always override toString():

java example

public class Student {
    private String name;
    private int age;
    private double gpa;

    public Student(String name, int age, double gpa) {
        this.name = name;
        this.age = age;
        this.gpa = gpa;
    }

    @Override
    public String toString() {
        return String.format("Student{name='%s', age=%d, gpa=%.1f}", name, age, gpa);
    }
}

public class Main {
    public static void main(String[] args) {
        Student s = new Student("Alice", 20, 3.8);
        System.out.println(s);  // Student{name='Alice', age=20, gpa=3.8}

        // toString() is called automatically in:
        System.out.println("Student: " + s);  // String concatenation
        String.format("Data: %s", s);          // Format
        System.out.printf("Info: %s%n", s);    // Printf
    }
}

Output

Student{name='Alice', age=20, gpa=3.8}
Student: Student{name='Alice', age=20, gpa=3.8}

equals() — Content Equality

Default equals() uses == (reference comparison). Override for meaningful comparison:

Example

public class Point { private int x, y; public Point(int x, int y) { this.x = x; this.y = y; } @Override public boolean equals(Object obj) { // 1. Same reference? Definitely equal. if (this == obj) return true; // 2. Null or different class? Not equal. if (obj == null || getClass() != obj.getClass()) return false; // 3. Cast and compare fields Point other = (Point) obj; return this.x == other.x && this.y == other.y; } @Override public int hashCode() { return Objects.hash(x, y); // MUST override when equals is overridden } @Override public String toString() { return "(" + x + ", " + y + ")"; } } public class Main { public static void main(String[] args) { Point p1 = new Point(3, 4); Point p2 = new Point(3, 4); Point p3 = new Point(5, 6); Point p4 = p1; System.out.println("p1 == p2: " + (p1 == p2)); // false (different objects) System.out.println("p1.equals(p2): " + p1.equals(p2)); // true (same content) System.out.println("p1.equals(p3): " + p1.equals(p3)); // false (different content) System.out.println("p1 == p4: " + (p1 == p4)); // true (same reference) } }

Output

p1 == p2: false
p1.equals(p2): true
p1.equals(p3): false
p1 == p4: true

The equals() Contract

Your equals() must be:

Reflexive: x.equals(x) is always true
Symmetric: if x.equals(y) then y.equals(x)
Transitive: if x.equals(y) and y.equals(z) then x.equals(z)
Consistent: multiple calls return same result (if objects unchanged)
Null-safe: x.equals(null) is always false

hashCode() — Hash for Collections

Rule: If you override equals(), you MUST override hashCode().

Equal objects must have the same hash code. This is critical for HashMap, HashSet, etc.

Example

public class Employee { private String id; private String name; private String department; public Employee(String id, String name, String department) { this.id = id; this.name = name; this.department = department; } @Override public boolean equals(Object obj) { if (this == obj) return true; if (obj == null || getClass() != obj.getClass()) return false; Employee other = (Employee) obj; return Objects.equals(id, other.id); // Equal if same ID } @Override public int hashCode() { return Objects.hash(id); // Based on same fields as equals } @Override public String toString() { return id + ": " + name + " (" + department + ")"; } } public class Main { public static void main(String[] args) { Set<Employee> team = new HashSet<>(); team.add(new Employee("E001", "Alice", "Engineering")); team.add(new Employee("E002", "Bob", "Marketing")); team.add(new Employee("E001", "Alice", "Engineering")); // Duplicate! System.out.println("Team size: " + team.size()); // 2, not 3 team.forEach(System.out::println); // HashMap lookup works with equals/hashCode Map<Employee, Double> salaries = new HashMap<>(); salaries.put(new Employee("E001", "Alice", "Engineering"), 95000.0); // Different object, same equals() — finds it! Double salary = salaries.get(new Employee("E001", "Alice", "Eng")); System.out.println("Alice's salary: $" + salary); } }

Output

Team size: 2
E001: Alice (Engineering)
E002: Bob (Marketing)
Alice's salary: $95000.0

getClass() — Runtime Type Information

java example

public class Main {
    public static void main(String[] args) {
        Object obj = "Hello";
        System.out.println(obj.getClass());              // class java.lang.String
        System.out.println(obj.getClass().getName());    // java.lang.String
        System.out.println(obj.getClass().getSimpleName()); // String

        // Useful for type checking
        Animal animal = new Dog("Rex", 3);
        System.out.println(animal.getClass().getSimpleName()); // Dog

        // getClass() vs instanceof
        System.out.println(animal instanceof Animal);     // true
        System.out.println(animal instanceof Dog);        // true
        System.out.println(animal.getClass() == Animal.class); // false (exact class is Dog)
        System.out.println(animal.getClass() == Dog.class);    // true
    }
}

Output

class java.lang.String
java.lang.String
String
Dog
true
true
false
true

clone() — Object Copying

To use clone(), your class must implement Cloneable and override clone():

java example

public class Address implements Cloneable {
    private String city;
    private String street;

    public Address(String city, String street) {
        this.city = city;
        this.street = street;
    }

    @Override
    public Address clone() {
        try {
            return (Address) super.clone();
        } catch (CloneNotSupportedException e) {
            throw new RuntimeException(e);
        }
    }

    @Override
    public String toString() { return street + ", " + city; }

    public void setCity(String city) { this.city = city; }
}

public class Person implements Cloneable {
    private String name;
    private int age;
    private Address address;

    public Person(String name, int age, Address address) {
        this.name = name;
        this.age = age;
        this.address = address;
    }

    // Shallow clone — address is shared!
    @Override
    public Person clone() {
        try {
            Person cloned = (Person) super.clone();
            cloned.address = this.address.clone();  // Deep clone the address!
            return cloned;
        } catch (CloneNotSupportedException e) {
            throw new RuntimeException(e);
        }
    }

    @Override
    public String toString() {
        return name + ", " + age + ", " + address;
    }
}

public class Main {
    public static void main(String[] args) {
        Person original = new Person("Alice", 30, new Address("NYC", "123 Main St"));
        Person copy = original.clone();

        System.out.println("Original: " + original);
        System.out.println("Copy: " + copy);

        // Modify copy's address
        copy.address.setCity("LA");
        System.out.println("After modifying copy:");
        System.out.println("Original: " + original);  // Unchanged (deep clone)
        System.out.println("Copy: " + copy);
    }
}

Output

Original: Alice, 30, 123 Main St, NYC
Copy: Alice, 30, 123 Main St, NYC
After modifying copy:
Original: Alice, 30, 123 Main St, NYC
Copy: Alice, 30, 123 Main St, LA

Complete Example: Well-Designed Class

Example

public class Product { private final String sku; private String name; private double price; private int stock; public Product(String sku, String name, double price, int stock) { this.sku = sku; this.name = name; this.price = price; this.stock = stock; } // Getters public String getSku() { return sku; } public String getName() { return name; } public double getPrice() { return price; } public int getStock() { return stock; } @Override public String toString() { return String.format("Product[sku=%s, name=%s, price=%.2f, stock=%d]", sku, name, price, stock); } @Override public boolean equals(Object obj) { if (this == obj) return true; if (obj == null || getClass() != obj.getClass()) return false; Product product = (Product) obj; return Objects.equals(sku, product.sku); // Products equal by SKU } @Override public int hashCode() { return Objects.hash(sku); } }

Common Mistakes

Overriding equals() without hashCode():

``java // ❌ HashMap/HashSet will BREAK @Override public boolean equals(Object o) { ... } // hashCode() still returns default — different for equal objects! ``

Using == instead of equals() for objects:

``java String a = new String("test"); String b = new String("test"); if (a == b) { } // ❌ false — different references if (a.equals(b)) { } // ✅ true — same content ``

Wrong equals() signature:

``java // ❌ This OVERLOADS (not overrides) Object.equals(Object)! public boolean equals(Point other) { ... } // ✅ Correct signature @Override public boolean equals(Object obj) { ... } ``

toString() not overridden:

``java System.out.println(myObject); // "MyClass@7852e922" — useless for debugging! ``

Best Practices

Always override toString() — it helps with debugging immensely
Override equals() when objects have logical equality (same ID, same content)
Always override hashCode() when you override equals()
Use Objects.hash() and Objects.equals() for clean implementations
Consider making classes final or documenting the equals() contract for subclasses
Prefer copy constructor over clone() for object copying
Use Java records (Java 16+) for simple data classes — they auto-generate these methods

Interview Questions

Q1: Why must hashCode() be overridden when equals() is overridden? Collections like HashMap use hashCode to find the bucket, then equals to confirm identity. If equal objects have different hash codes, HashMap won't find them in the right bucket.

Q2: Can two unequal objects have the same hashCode()? Yes. This is called a hash collision and is normal. But equal objects MUST have the same hash code.

Q3: What does getClass() return? The runtime Class<?> object representing the actual type. Useful for reflection and exact type checking.

Q4: Why is finalize() deprecated? It's unreliable (GC timing is unpredictable), causes performance issues, and can resurrect objects. Use try-with-resources and Cleaner instead.

Q5: What is the difference between shallow and deep cloning? Shallow: copies fields directly (references still point to same objects). Deep: recursively clones all referenced objects too.

Q6: Why does Object have wait() and notify()? For thread synchronization. Any object can serve as a monitor/lock. These methods enable threads to coordinate via shared objects.

Q7: Can we prevent a class from being used as a HashMap key? Technically no, but if equals/hashCode aren't properly overridden, it won't work correctly as a key. The default identity-based behavior might not be what you want.

Q8: What is the contract between equals() and hashCode()? If two objects are equal via equals(), they MUST have the same hashCode(). If hashCodes differ, objects MUST be unequal. Equal hashCodes don't guarantee equality (collisions allowed). Breaking this contract causes HashMap/HashSet to malfunction.

Q9: What does the finalize() method do? Why is it deprecated? finalize() was called by GC before reclaiming memory. It's deprecated (Java 9+) because: (1) no guarantee when/if it runs, (2) performance overhead, (3) can resurrect objects, (4) runs on a single thread causing bottlenecks. Use try-with-resources or Cleaner instead.

Q10: Explain wait(), notify(), and notifyAll() from Object class. These are for inter-thread communication. wait() releases the lock and suspends the thread until notified. notify() wakes one waiting thread. notifyAll() wakes all waiting threads. All three must be called from synchronized context. They enable producer-consumer patterns.

Q11: What is the getClass() method used for? getClass() returns the runtime class (Class<?> object) of the instance. Used for reflection, type checking, and logging. Unlike instanceof, it checks exact type: obj.getClass() == MyClass.class is true only for MyClass, not subclasses.

Q12: Why is clone() method in Object class protected? It's protected to prevent arbitrary cloning. Classes must explicitly opt-in by: (1) implementing Cloneable marker interface, and (2) overriding clone() with public access. This design ensures classes control their own cloning behavior.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Object Class in Java.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

java-master-course, java master course, java, master, course, oops, object, class

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Java Notes

Object Class in Java

Complete guide to the java.lang.Object class — toString(), equals(), hashCode(), clone(), getClass(), finalize(), and how every Java class inherits from Object.

java example

// These are EQUIVALENT:
public class MyClass { }
public class MyClass extends Object { }

Methods of Object Class

Method	Purpose	Override?
`toString()`	String representation	Always
`equals(Object)`	Content equality	When needed
`hashCode()`	Hash for collections	With equals()
`getClass()`	Runtime type info	Never (final)
`clone()`	Object copying	When needed
`finalize()`	Pre-GC cleanup	Never (deprecated)
`wait()`	Thread synchronization	Rarely
`notify()`	Thread synchronization	Rarely
`notifyAll()`	Thread synchronization	Rarely

toString() — String Representation

Default toString() returns ClassName@hexHashCode, which is useless:

java example

public class Student {
    private String name;
    private int age;
    private double gpa;

    public Student(String name, int age, double gpa) {
        this.name = name;
        this.age = age;
        this.gpa = gpa;
    }
}

public class Main {
    public static void main(String[] args) {
        Student s = new Student("Alice", 20, 3.8);
        System.out.println(s);  // Default: Student@1b6d3586 (useless!)
    }
}

Always override toString():

java example

public class Student {
    private String name;
    private int age;
    private double gpa;

    public Student(String name, int age, double gpa) {
        this.name = name;
        this.age = age;
        this.gpa = gpa;
    }

    @Override
    public String toString() {
        return String.format("Student{name='%s', age=%d, gpa=%.1f}", name, age, gpa);
    }
}

public class Main {
    public static void main(String[] args) {
        Student s = new Student("Alice", 20, 3.8);
        System.out.println(s);  // Student{name='Alice', age=20, gpa=3.8}

        // toString() is called automatically in:
        System.out.println("Student: " + s);  // String concatenation
        String.format("Data: %s", s);          // Format
        System.out.printf("Info: %s%n", s);    // Printf
    }
}

Output

Student{name='Alice', age=20, gpa=3.8}
Student: Student{name='Alice', age=20, gpa=3.8}

equals() — Content Equality

Default equals() uses == (reference comparison). Override for meaningful comparison:

Example

Output

p1 == p2: false
p1.equals(p2): true
p1.equals(p3): false
p1 == p4: true

The equals() Contract

Your equals() must be:

Reflexive: x.equals(x) is always true
Symmetric: if x.equals(y) then y.equals(x)
Transitive: if x.equals(y) and y.equals(z) then x.equals(z)
Consistent: multiple calls return same result (if objects unchanged)
Null-safe: x.equals(null) is always false

hashCode() — Hash for Collections

Rule: If you override equals(), you MUST override hashCode().

Equal objects must have the same hash code. This is critical for HashMap, HashSet, etc.

Example

Output

Team size: 2
E001: Alice (Engineering)
E002: Bob (Marketing)
Alice's salary: $95000.0

getClass() — Runtime Type Information

java example

public class Main {
    public static void main(String[] args) {
        Object obj = "Hello";
        System.out.println(obj.getClass());              // class java.lang.String
        System.out.println(obj.getClass().getName());    // java.lang.String
        System.out.println(obj.getClass().getSimpleName()); // String

        // Useful for type checking
        Animal animal = new Dog("Rex", 3);
        System.out.println(animal.getClass().getSimpleName()); // Dog

        // getClass() vs instanceof
        System.out.println(animal instanceof Animal);     // true
        System.out.println(animal instanceof Dog);        // true
        System.out.println(animal.getClass() == Animal.class); // false (exact class is Dog)
        System.out.println(animal.getClass() == Dog.class);    // true
    }
}

Output

class java.lang.String
java.lang.String
String
Dog
true
true
false
true

clone() — Object Copying

To use clone(), your class must implement Cloneable and override clone():

java example

public class Address implements Cloneable {
    private String city;
    private String street;

    public Address(String city, String street) {
        this.city = city;
        this.street = street;
    }

    @Override
    public Address clone() {
        try {
            return (Address) super.clone();
        } catch (CloneNotSupportedException e) {
            throw new RuntimeException(e);
        }
    }

    @Override
    public String toString() { return street + ", " + city; }

    public void setCity(String city) { this.city = city; }
}

public class Person implements Cloneable {
    private String name;
    private int age;
    private Address address;

    public Person(String name, int age, Address address) {
        this.name = name;
        this.age = age;
        this.address = address;
    }

    // Shallow clone — address is shared!
    @Override
    public Person clone() {
        try {
            Person cloned = (Person) super.clone();
            cloned.address = this.address.clone();  // Deep clone the address!
            return cloned;
        } catch (CloneNotSupportedException e) {
            throw new RuntimeException(e);
        }
    }

    @Override
    public String toString() {
        return name + ", " + age + ", " + address;
    }
}

public class Main {
    public static void main(String[] args) {
        Person original = new Person("Alice", 30, new Address("NYC", "123 Main St"));
        Person copy = original.clone();

        System.out.println("Original: " + original);
        System.out.println("Copy: " + copy);

        // Modify copy's address
        copy.address.setCity("LA");
        System.out.println("After modifying copy:");
        System.out.println("Original: " + original);  // Unchanged (deep clone)
        System.out.println("Copy: " + copy);
    }
}

Output

Original: Alice, 30, 123 Main St, NYC
Copy: Alice, 30, 123 Main St, NYC
After modifying copy:
Original: Alice, 30, 123 Main St, NYC
Copy: Alice, 30, 123 Main St, LA

Complete Example: Well-Designed Class

Example

Common Mistakes

Overriding equals() without hashCode():

``java // ❌ HashMap/HashSet will BREAK @Override public boolean equals(Object o) { ... } // hashCode() still returns default — different for equal objects! ``

Using == instead of equals() for objects:

``java String a = new String("test"); String b = new String("test"); if (a == b) { } // ❌ false — different references if (a.equals(b)) { } // ✅ true — same content ``

Wrong equals() signature:

``java // ❌ This OVERLOADS (not overrides) Object.equals(Object)! public boolean equals(Point other) { ... } // ✅ Correct signature @Override public boolean equals(Object obj) { ... } ``

toString() not overridden:

``java System.out.println(myObject); // "MyClass@7852e922" — useless for debugging! ``

Best Practices

Always override toString() — it helps with debugging immensely
Override equals() when objects have logical equality (same ID, same content)
Always override hashCode() when you override equals()
Use Objects.hash() and Objects.equals() for clean implementations
Consider making classes final or documenting the equals() contract for subclasses
Prefer copy constructor over clone() for object copying
Use Java records (Java 16+) for simple data classes — they auto-generate these methods

Interview Questions

Q2: Can two unequal objects have the same hashCode()? Yes. This is called a hash collision and is normal. But equal objects MUST have the same hash code.

Q3: What does getClass() return? The runtime Class<?> object representing the actual type. Useful for reflection and exact type checking.

Q4: Why is finalize() deprecated? It's unreliable (GC timing is unpredictable), causes performance issues, and can resurrect objects. Use try-with-resources and Cleaner instead.

Q5: What is the difference between shallow and deep cloning? Shallow: copies fields directly (references still point to same objects). Deep: recursively clones all referenced objects too.

Q6: Why does Object have wait() and notify()? For thread synchronization. Any object can serve as a monitor/lock. These methods enable threads to coordinate via shared objects.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Object Class in Java.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

java-master-course, java master course, java, master, course, oops, object, class

Continue learning this concept

PreviousMethod Overriding in Java NextObject Cloning in Java