5 ways to create objects in Java-javaTutorial-php.cn

As Java developers, we create many objects every day, but we usually use dependency management systems, such as Spring, to create objects. However there are many ways to create objects, which we will learn in this article.

There are 5 ways to create objects in Java. Their examples and their bytecodes are given below

5 ways to create objects in Java

If you run the program at the end, you will find that methods 1, 2, and 3 use constructors to create objects, and methods 4 and 5 do not call constructors.

1. Use the new keyword

This is the most common and simplest way to create objects. In this way, we can call any constructor (parameterless and parameterized).

Employee emp1 = new Employee();

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0: new #19 // class org/programming/mitra/exercises/Employee 3: dup 4: invokespecial #21 // Method org/programming/mitra/exercises/Employee."":()V

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2. Use the newInstance method of the Class class

We can also use the newInstance method of the Class class to create objects. This newInstance method calls the parameterless constructor to create the object.

We can create objects by calling the newInstance method in the following ways:

Employee emp2 = (Employee) Class.forName("org.programming.mitra.exercises.Employee").newInstance();

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Employee emp2 = Employee.class.newInstance();

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51: invokevirtual #70 // Method java/lang/Class.newInstance:()Ljava/lang/Object;

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3. Use the newInstance method of the Constructor class

and Much like the newInstance method of the Class class, there is also a newInstance method in the java.lang.reflect.Constructor class to create objects. We can call parameterized and private constructors through this newInstance method.

Constructor constructor = Employee.class.getConstructor(); Employee emp3 = constructor.newInstance();

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111: invokevirtual #80 // Method java/lang/reflect/Constructor.newInstance:([Ljava/lang/Object;)Ljava/lang/Object;

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The newInstance method internally calls Constructor’s newInstance method. This is also the reason why many frameworks, such as Spring, Hibernate, Struts, etc., use the latter.

4. Use the clone method

Whenever we call the clone method of an object, the jvm will create a new object and copy all the contents of the previous object into it. Creating an object using the clone method does not call any constructor.

To use the clone method, we need to first implement the Cloneable interface and implement the clone method it defines.

Employee emp4 = (Employee) emp3.clone();’

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162: invokevirtual #87 // Method org/programming/mitra/exercises/Employee.clone ()Ljava/lang/Object;

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5. Use deserialization

When we serialize and deserialize an object, jvm will create a separate object for us. During deserialization, the jvm creates the object and does not call any constructor.

In order to deserialize an object, we need to make our class implement the Serializable interface.

ObjectInputStream in = new ObjectInputStream(new FileInputStream("data.obj")); Employee emp5 = (Employee) in.readObject();

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261: invokevirtual #118 // Method java/io/ObjectInputStream.readObject:()Ljava/lang/Object;

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We can see from the above bytecode fragment that except for the first method, the other four methods are all converted into invokevirtual (direct method of creating an object), and the first method is converted into two calls , new and invokespecial (constructor call).

Example

Let us take a look at creating objects for the following Employee class:

class Employee implements Cloneable, Serializable { private static final long serialVersionUID = 1L; private String name; public Employee() { System.out.println("Employee Constructor Called..."); } public String getName() { return name; } public void setName(String name) { this.name = name; } @Override public int hashCode() { final int prime = 31; int result = 1; result = prime * result + ((name == null) ? 0 : name.hashCode()); return result; } @Override public boolean equals(Object obj) { if (this == obj) return true; if (obj == null) return false; if (getClass() != obj.getClass()) return false; Employee other = (Employee) obj; if (name == null) { if (other.name != null) return false; } else if (!name.equals(other.name)) return false; return true; } @Override public String toString() { return "Employee [name=" + name + "]"; } @Override public Object clone() { Object obj = null; try { obj = super.clone(); } catch (CloneNotSupportedException e) { e.printStackTrace(); } return obj; } }

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In the following Java program, we will create Employee objects in 5 ways. You can find the code from GitHub.

public class ObjectCreation { public static void main(String... args) throws Exception { // By using new keyword Employee emp1 = new Employee(); emp1.setName("Naresh"); System.out.println(emp1 + ", hashcode : " + emp1.hashCode()); // By using Class class's newInstance() method Employee emp2 = (Employee) Class.forName("org.programming.mitra.exercises.Employee") .newInstance(); // Or we can simply do this // Employee emp2 = Employee.class.newInstance(); emp2.setName("Rishi"); System.out.println(emp2 + ", hashcode : " + emp2.hashCode()); // By using Constructor class's newInstance() method Constructor constructor = Employee.class.getConstructor(); Employee emp3 = constructor.newInstance(); emp3.setName("Yogesh"); System.out.println(emp3 + ", hashcode : " + emp3.hashCode()); // By using clone() method Employee emp4 = (Employee) emp3.clone(); emp4.setName("Atul"); System.out.println(emp4 + ", hashcode : " + emp4.hashCode()); // By using Deserialization // Serialization ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("data.obj")); out.writeObject(emp4); out.close(); //Deserialization ObjectInputStream in = new ObjectInputStream(new FileInputStream("data.obj")); Employee emp5 = (Employee) in.readObject(); in.close(); emp5.setName("Akash"); System.out.println(emp5 + ", hashcode : " + emp5.hashCode()); } }

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The program will output:

public class ObjectCreation { Employee Constructor Called... Employee [name=Naresh], hashcode : -1968815046 Employee Constructor Called... Employee [name=Rishi], hashcode : 78970652 Employee Constructor Called... Employee [name=Yogesh], hashcode : -1641292792 Employee [name=Atul], hashcode : 2051657 Employee [name=Akash], hashcode : 63313419

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