Object class in Java is present in java.lang package. Every class in Java is directly or indirectly derived from the Object class. If a class does not extend any other class then it is a direct child class of the Java Object class and if it extends another class then it is indirectly derived. The Object class provides several methods such as toString(),equals(), hashCode(), and many others. Hence, the Object class acts as a root of the inheritance hierarchy in any Java Program.
Example: Here, we will use the toString() and hashCode() methods to provide a custom string representation for a class.
Java
// Java Code to demonstrate Object class
class Person {
String n; //name
// Constructor
public Person(String n) {
this.n = n;
}
// Override toString() for a
// custom string representation
@Override
public String toString() {
return "Person{name:'" + n + "'}";
}
public static void main(String[] args) {
Person p = new Person("Geek");
// Custom string representation
System.out.println(p.toString());
// Default hash code value
System.out.println(p.hashCode());
}
}
OutputPerson{name:'Geek'}
321001045
Explanation: In the above example, we override the toString() method to provide a custom string representation of the Person class and use the hashCode() method to display the default hash code value of the object.
Object Class Methods
The Object class provides multiple methods which are as follows:
- toString() method
- hashCode() method
- equals(Object obj) method
- finalize() method
- getClass() method
- clone() method
- wait(), notify() notifyAll() (Concurrency methods)
1. toString() Method
The toString() provides a String representation of an object and is used to convert an object to a String. The default toString() method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object.
Note: Whenever we try to print any Object reference, then internally toString() method is called.
Example:
public class Student {
public String toString() {
return "Student object";
}
}
Explanation: The toString() method is overridden to return a custom string representation of the Student object.
2. hashCode() Method
For every object, JVM generates a unique number which is a hashcode. It returns distinct integers for distinct objects. A common misconception about this method is that the hashCode() method returns the address of the object, which is not correct. It converts the internal address of the object to an integer by using an algorithm. The hashCode() method is native because in Java it is impossible to find the address of an object, so it uses native languages like C/C++ to find the address of the object.
Use of hashCode() method:
It returns a hash value that is used to search objects in a collection. JVM(Java Virtual Machine) uses the hashcode method while saving objects into hashing-related data structures like HashSet, HashMap, Hashtable, etc. The main advantage of saving objects based on hash code is that searching becomes easy.
Note: Override of hashCode() method needs to be done such that for every object we generate a unique number. For example, for a Student class, we can return the roll no. of a student from the hashCode() method as it is unique.
Example:
public class Student {
int roll;
@Override
public int hashCode() {
return roll;
}
}
Explanation: The hashCode() method is overridden to return a custom hash value based on the roll of the Student object.
3. equals(Object obj) Method
The equals() method compares the given object with the current object. It is recommended to override this method to define custom equality conditions.
Note: It is generally necessary to override the hashCode() method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.
Example:
public class Student {
int roll;
@Override
public boolean equals(Object o) {
if (o instanceof Student) {
return this.roll == ((Student) o).roll;
}
return false;
}
}
Explanation: The equals() method is overridden to compare roll between two Student objects.
4. getClass() method
The getClass() method returns the class object of "this" object and is used to get the actual runtime class of the object. It can also be used to get metadata of this class. The returned Class object is the object that is locked by static synchronized methods of the represented class. As it is final so we don't override it.
Example:
Java
// Demonstrate working of getClass()
public class Geeks {
public static void main(String[] args)
{
Object o = new String("GeeksForGeeks");
Class c = o.getClass();
System.out.println("Class of Object o is: "
+ c.getName());
}
}
OutputClass of Object o is: java.lang.String
Explanation: The getClass() method is used to print the runtime class of the "o" object.
Note: After loading a .class file, JVM will create an object of the type java.lang.Class in the Heap area. We can use this class object to get Class level information. It is widely used in Reflection
5. finalize() method
The finalize() method is called just before an object is garbage collected. It is called the Garbage Collector on an object when the garbage collector determines that there are no more references to the object. We should override finalize() method to dispose of system resources, perform clean-up activities and minimize memory leaks. For example, before destroying the Servlet objects web container, always called finalize method to perform clean-up activities of the session.
Note: The finalize method is called just once on an object even though that object is eligible for garbage collection multiple times.
Example:
Java
// Demonstrate working of finalize()
public class Geeks {
public static void main(String[] args) {
Geeks t = new Geeks();
System.out.println(t.hashCode());
t = null;
// calling garbage collector
System.gc();
System.out.println("end");
}
@Override protected void finalize()
{
System.out.println("finalize method called");
}
}
Output1510467688
end
finalize method called
Explanation: The finalize() method is called just before the object is garbage collected.
6. clone() method
The clone() method creates and returns a new object that is a copy of the current object.
Example:
public class Book implements Cloneable {
private String t; //title
public Book(String t) {
this.t = t;
}
@Override
public Object clone() throws CloneNotSupportedException {
return super.clone();
}
}
Explanation: The clone() method is overridden to return a cloned copy of the Book object.
7. Concurrency Methods: wait(), notify(), and notifyAll()
These methods are related to thread Communication in Java. They are used to make threads wait or notify others in concurrent programming.
Example of using all the Object Class Methods in Java
Java
import java.io.*;
public class Book implements Cloneable {
private String t; // title
private String a; // author
private int y; // year
public Book(String t, String a, int y)
{
this.t = t;
this.a = a;
this.y = y;
}
// Override the toString method
@Override public String toString()
{
return t + " by " + a + " (" + y + ")";
}
// Override the equals method
@Override public boolean equals(Object o)
{
if (o == null || !(o instanceof Book)) {
return false;
}
Book other = (Book)o;
return this.t.equals(other.getTitle())
&& this.a.equals(other.getAuthor())
&& this.y == other.getYear();
}
// Override the hashCode method
@Override public int hashCode()
{
int res = 17;
res = 31 * res + t.hashCode();
res = 31 * res + a.hashCode();
res = 31 * res + y;
return res;
}
// Override the clone method
@Override public Book clone()
{
try {
return (Book)super.clone();
}
catch (CloneNotSupportedException e) {
throw new AssertionError();
}
}
// Override the finalize method
@Override protected void finalize() throws Throwable
{
System.out.println("Finalizing " + this);
}
public String getTitle() { return t; }
public String getAuthor() { return a; }
public int getYear() { return y; }
public static void main(String[] args)
{
// Create a Book object and print its details
Book b1 = new Book(
"The Hitchhiker's Guide to the Galaxy",
"Douglas Adams", 1979);
System.out.println(b1);
// Create a clone of the Book object and print its
// details
Book b2 = b1.clone();
System.out.println(b2);
// Check if the two objects are equal
System.out.println("b1 equals b2: "
+ b1.equals(b2));
// Get the hash code of the two objects
System.out.println("b1 hash code: "
+ b1.hashCode());
System.out.println("b2 hash code: "
+ b2.hashCode());
// Set book1 to null to trigger garbage collection
// and finalize method
b1 = null;
System.gc();
}
}
OutputThe Hitchhiker's Guide to the Galaxy by Douglas Adams (1979)
The Hitchhiker's Guide to the Galaxy by Douglas Adams (1979)
b1 equals b2: true
b1 hash code: 1840214527
b2 hash code: 1840214527
Explanation: The above example demonstrates the use of toString(), equals(), hashCode(), and clone() methods in the Book class.
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Queue Interface In JavaThe Queue Interface is a part of java.util package and extends the Collection interface. It stores and processes the data in order means elements are inserted at the end and removed from the front. Key Features:Most implementations, like PriorityQueue, do not allow null elements.Implementation Class
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PriorityQueue in JavaThe PriorityQueue class in Java is part of the java.util package. It implements a priority heap-based queue that processes elements based on their priority rather than the FIFO (First-In-First-Out) concept of a Queue.Key Points:The PriorityQueue is based on the Priority Heap. The elements of the pri
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Deque Interface in JavaDeque Interface present in java.util package is a subtype of the queue interface. The Deque is related to the double-ended queue that supports adding or removing elements from either end of the data structure. It can either be used as a queue(first-in-first-out/FIFO) or as a stack(last-in-first-out/
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Map Interface in JavaIn Java, the Map Interface is part of the java.util package and represents a mapping between a key and a value. The Java Map interface is not a subtype of the Collections interface. So, it behaves differently from the rest of the collection types.Key Features:No Duplicates in Keys: Keys should be un
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HashMap in JavaIn Java, HashMap is part of the Java Collections Framework and is found in the java.util package. It provides the basic implementation of the Map interface in Java. HashMap stores data in (key, value) pairs. Each key is associated with a value, and you can access the value by using the corresponding
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Java LinkedHashMapLinkedHashMap in Java implements the Map interface of the Collections Framework. It stores key-value pairs while maintaining the insertion order of the entries. It maintains the order in which elements are added.Stores unique key-value pairs.Maintains insertion order.Allows one null key and multiple
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Hashtable in JavaHashtable class, introduced as part of the Java Collections framework, implements a hash table that maps keys to values. Any non-null object can be used as a key or as a value. To successfully store and retrieve objects from a hashtable, the objects used as keys must implement the hashCode method an
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Java Dictionary ClassDictionary class in Java is an abstract class that represents a collection of key-value pairs, where keys are unique and used to access the values. It was part of the Java Collections Framework and it was introduced in Java 1.0 but has been largely replaced by the Map interface since Java 1.2.Stores
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SortedSet Interface in Java with ExamplesThe SortedSet interface is present in java.util package extends the Set interface present in the collection framework. It is an interface that implements the mathematical set. This interface contains the methods inherited from the Set interface and adds a feature that stores all the elements in this
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Java Comparator InterfaceThe Comparator interface in Java is used to sort the objects of user-defined classes. The Comparator interface is present in java.util package. This interface allows us to define custom comparison logic outside of the class for which instances we want to sort. The comparator interface is useful when
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Java Comparable InterfaceThe Comparable interface in Java is used to define the natural ordering of objects for a user-defined class. It is part of the java.lang package and it provides a compareTo() method to compare instances of the class. A class has to implement a Comparable interface to define its natural ordering.Exam
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Java Comparable vs ComparatorIn Java, both Comparable and Comparator interfaces are used for sorting objects. The main difference between Comparable and Comparator is:Comparable: It is used to define the natural ordering of the objects within the class.Comparator: It is used to define custom sorting logic externally.Difference
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Java IteratorAn Iterator in Java is an interface used to traverse elements in a Collection sequentially. It provides methods like hasNext(), next(), and remove() to loop through collections and perform manipulation. An Iterator is a part of the Java Collection Framework, and we can use it with collections like A
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