Java Object Class

The Object class is the root of the class hierarchy. Every class has Object as a superclass. All objects, including arrays, implement the methods of this class.

If a class does not explicitly extend any other class, it directly extends Object.

public class MyClass {
    // implicitly extends Object
}

Common Methods of Object Class

1. toString()

Returns a string representation of the object. By default, it returns ClassName@HashCode. It is recommended to override this method to provide meaningful output.

class Student {
    String name;
    int age;

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

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

public class Main {
    public static void main(String[] args) {
        Student s = new Student("John", 20);
        System.out.println(s); // Calls toString() automatically
    }
}

2. equals(Object obj)

Indicates whether some other object is "equal to" this one. The default implementation checks for reference equality (==). You should override it to check for logical equality (content equality).

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;
    Student student = (Student) o;
    return age == student.age && name.equals(student.name);
}

3. hashCode()

Returns a hash code value for the object. This method is supported for the benefit of hash tables such as those provided by HashMap.

[!IMPORTANT] If you override equals(), you must override hashCode() as well. Equal objects must have equal hash codes.

@Override
public int hashCode() {
    return Objects.hash(name, age);
}

4. getClass()

Returns the runtime class of this Object.

Student s = new Student("John", 20);
System.out.println(s.getClass().getName()); // Output: Student

5. clone()

Creates and returns a copy of this object. The class must implement the Cloneable interface to use this method, otherwise CloneNotSupportedException is thrown.

class Student implements Cloneable {
    // ...
    public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

6. finalize() (Deprecated)

Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.

[!WARNING] Deprecated since Java 9. Do not use.

Key Takeaways

• Root Class: Object is the parent of all classes in Java.
• Overriding: Always override toString(), equals(), and hashCode() for your custom data classes.
• Contract: Respect the contract between equals() and hashCode().