How to Create Objects in Java
Java is an object-oriented programming language used when developing desktop, mobile, and web applications. In Java, everything revolves around the object. A Java class can, therefore, be regarded as an object template. <!--more--> In Java, we can create Objects in various ways:
- Using a new keyword
- Using the
newInstance() method of the Class class - Using the newInstance() method of the Constructor class
- Using Object Serialization and Deserialization
- Using the clone() method
Using a new keyword
This is the most common and basic way of creating an object in Java. In this method, we can call constructors with parameters or with no arguments. The new keyword allows us to create a new object.
Syntax
class_name object_name = new class_name();
Example
In this example, we will create an object using the new keyword.
public class Main{
String s = "Hello World";
public static void main(String args[]) {
Main a = new Main(); //creating an object
System.out.println(a.s); //Outputs Hello World
}
}
Using the newInstance() method of the Class
In this method, a no-argument constructor is called by the newInstance() function to create an object. The newInstance() method is regarded as a reflective way to create an object since it belongs to the java.lang.reflect.Constructor class.
The Class.forName() is used to load the class dynamically.
The newInstance() method uses the following syntax:
public T newInstance() throws InstantiationException,IllegalAcccessException
- If the method or class is not accessible, it returns the
IllegalAccessException. - If the Class represents a primitive data type, an interface, an abstract, an array class, or if the class does not have a no-arg constructor, it returns an
InstantiationExceptionmessage.
The example below helps us understand the newInstance() method.
public class Main
{
String a = "Hello World";
public static void main(String[] args) {
try{
Class b = Class.forName("Main");
Main c = (Main) b.newInstance();
System.out.println(c.a);
} catch (ClassNotFoundException ex) {
ex.printStackTrace();
} catch (InstantiationException ex){
ex.printStackTrace();
} catch (IllegalAccessException ex) {
ex.printStackTrace();
}
}
}
Using the newInstance() method of the Constructor Class
This method also uses the newInstance() function which is provided by the constructor and used when creating an object. This time the constructor is parameterized.
Syntax
public T newInstance(Objects...initargs)
The following exceptions are returned by the newInstance() method:
- Incase the constructor cannot be accessed, the method throws IllegalAccessException.
- If the number in the formal and actual parameters differ, it throws IllegalArgumentException.
- If an exception is thrown by the constructor, it throws InvocationTargetException.
- If the provoked initialization fails, it throws ExceptionInInitializerError.
Example
Let's look at an example of how to use the newInstance() method.
import java.lang.reflect.Constructor;
import java.lang.reflect.Constructor;
public class Main {
String str="Hello World";
public static void main(String args[]) {
try{
Constructor<Main> obj =Main.class.getConstructor();
Main myObj = obj.newInstance();
System.out.println(myObj.str);
}catch(Exception ex) {
ex.printStackTrace();
}
}
}
Using object serialization and deserialization
A new separate object is created when we serialize and then deserialize an object. To create an object, this method does not need any constructor.
We shall use the Serializable interface in Java for serializing and deserializing new objects.
Object serialization
Serialization involves converting an object state into a byte stream. We use the writeObject() method to serialize an Object.
Syntax when serializing an object
public final void writeObject(Object obj) throws IOException
Object deserialization
Deserialization is the process of using the byte stream to recreate an object in Java. We shall use the readObject() method to deserialize an object.
Syntax when deserializing an object
public final Object readObject() throws IOException
Example
In this example, we are using the serialization and deserialization method to create an object.
import java.io.*;
class Main implements Serializable
{
public int a;
public String b;
public Main(int a, String b)
{
this.a = a;
this.b = b;
}
}
class Example1 {
public static void main(String[] args)
{
Main obj = new Main(5, "Engineering Education");
String filename = "OurExample.ser"; //name of the file which should have .ser
/*-----------------Serialization----------*/
try
{
FileOutputStream newFile = new FileOutputStream(filename); //We are saving the object in the file
ObjectOutputStream out = new ObjectOutputStream(newFile);
out.writeObject(obj); //object being serialized
out.close(); //ObjectOutputStream closed
newFile.close(); //File closed
System.out.println("We have serialized the Object ");
}
catch(IOException ex)
{
ex.printStackTrace();
}
Main obj1 = null;
/*-----------------Deserialization--------*/
try
{
FileInputStream newFile = new FileInputStream(filename); // Here, we are reading the object from the file
ObjectInputStream is = new ObjectInputStream(newFile);
obj = (Main)is.readObject(); //object deserialized
is.close(); //ObjectInputStream closed
newFile.close(); //File closed
System.out.println("We have deserialized the Object");
System.out.println("number = " + obj.a);
System.out.println("string = " + obj.b);
}
catch(IOException ex)
{
System.out.println("Am an IOException");
}
catch(ClassNotFoundException ex)
{
System.out.println("Am an ClassNotFoundException");
}
}
}
Using the clone() method
Whenever the clone() method is called, it creates a new object, and then gets all the content in the old object copied to it.
A constructor does not get invoked when we use the
clone()method to create an object. We implement theCloneableclass to use theclone()method, as shown below.
Syntax
object.clone()
Example
Let's look at an example of how to create an object using the clone method.
class Student implements Cloneable{
int regno;
String name;
Student(int regno,String name){
this.regno=regno;
this.name=name;
}
public Object clone()throws CloneNotSupportedException{
return super.clone();
}
public static void main(String args[]){
try{
Student s1=new Student(203,"Geoffrey");
Student s2=(Student)s1.clone(); //cloning the s1 object
System.out.println(s1.regno+" "+s1.name); //prints the name from the original object
System.out.println(s2.regno+" "+s2.name); //prints the name from the cloned object
}
catch(CloneNotSupportedException c){
// this error is throne when the cloning process fails
}
}
}
Conclusion
In this article, we have looked at the different ways to create objects in Java. We can now easily create Objects in Java without using the new Keyword method.
Happy coding!
Peer Review Contributions by: Wanja Mike
