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5 Different Ways to Create Objects in Java

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5 Different Ways to Create Objects in Java

A list of five ways to create objects in Java, how they interact with constructors, and an example of how to utilize all of these methods.

· Java Zone ·
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While being a Java developer we usually create lots of objects daily, but we always use the new or dependency management systems e.g. Spring to create these objects. However, there are more ways to create objects which we are going to study in this article.

There are total 5 core ways to create objects in Java which are explained below with their example followed by bytecode of the line which is creating the object. However, lots of Apis are out there are which creates objects for us but these Apis will also are using one of these 5 core ways indirectly e.g. Spring BeanFactory.


If you will execute program given in the end, you will see method 1, 2, 3 uses the constructor to create the object while 4, 5 doesn’t call the constructor to create the object.

1. Using new keywords

It is the most common and regular way to create an object and a very simple one also. By using this method we can call whichever constructor we want to call (no-arg constructor as well as parameterized).

Employee emp1 = new Employee();
 0: new           #19          // class org/programming/mitra/exercises/Employee
 3: dup
 4: invokespecial #21          // Method org/programming/mitra/exercises/Employee."":()V

2. Using newInstance() method of Class class 

We can also use the newInstance() method of a Class class to create an object. This newInstance() method calls the no-arg constructor to create the object.

We can create an object by newInstance() in the following way:

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

Or

Employee emp2 = Employee.class.newInstance();
51: invokevirtual    #70    // Method java/lang/Class.newInstance:()Ljava/lang/Object;

3. Using newInstance() method of Constructor class

Similar to the newInstance() method of Class class, There is one newInstance() method in the java.lang.reflect.Constructor class which we can use to create objects. We can also call the parameterized constructor, and private constructor by using this newInstance() method.

Constructor<Employee> constructor = Employee.class.getConstructor();
Employee emp3 = constructor.newInstance();
111: invokevirtual  #80  // Method java/lang/reflect/Constructor.newInstance:([Ljava/lang/Object;)Ljava/lang/Object;

Both newInstance() methods are known as reflective ways to create objects. In fact newInstance() method of Class class internally uses newInstance() method of Constructor class. That's why the later one is preferred and also used by different frameworks like Spring, Hibernate, Struts etc. To know the differences between both newInstance() methods read Creating objects through Reflection in Java with Example.

4. Using clone() method:

Whenever we call clone() on any object JVM actually creates a new object for us and copy all content of the previous object into it. Creating an object using the clone method does not invoke any constructor.

To use the clone() method on an object we need to implements Cloneable and define clone() method in it.

Employee emp4 = (Employee) emp3.clone();
162: invokevirtual #87  // Method org/programming/mitra/exercises/Employee.clone ()Ljava/lang/Object;

Java cloning is the most debatable topic in Java community and it surely does have its drawbacks but it is still the most popular and easy way of creating a copy of any object until that object is full filling mandatory conditions of Java cloning. I have covered cloning in details in a 3 article long  Java Cloning Series which includes articles like Java Cloning And Types Of Cloning (Shallow And Deep) In Details With Example, Java Cloning - Copy Constructor Versus Cloning, Java Cloning - Even Copy Constructors Are Not Sufficient. Please go ahead and read them if you want to know more about cloning.

5. Using Deserialization:

Whenever we serialize and then deserialize an object JVM creates a separate object for us. In deserialization, JVM doesn’t use any constructor to create the object. To deserialize an object we need to implement the Serializable interface in our class.

ObjectInputStream in = new ObjectInputStream(new FileInputStream("data.obj"));
Employee emp5 = (Employee) in.readObject();
261: invokevirtual  #118   // Method java/io/ObjectInputStream.readObject:()Ljava/lang/Object;

As we can see in above bytecodes all 4 methods call get converted to invokevirtual (object creation is directly handled by these methods) except the first one which got converted to two calls one is new and other is invokespecial (call to the constructor).

I have discussed serialization and deserialization in more details in serialization series which includes articles like Everything You Need To Know About Java Serialization, How To Customize Serialization In Java By Using Externalizable Interface, How To Deep Clone An Object Using Java In Memory Serialization. Please go ahead and read them if you want to know more about serialization.

Example:

Let’s consider an Employee class for which we are going to create the objects:

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 boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Employee employee = (Employee) o;
        return Objects.equals(name, employee.name);
    }

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

    @Override
    public String toString() { return String.format("Employee{name='%s'}", name); }

    @Override
    public Object clone() {
        Object obj = null;
        try {
            obj = super.clone();
        } catch (CloneNotSupportedException e) {
            e.printStackTrace();
        }
        return obj;
    }
}

In the below Java program, we are going to create Employee objects in all 5 ways.

public class ObjectCreation {
    public static void main(String... args) throws Exception {

        // 1. Using new keyword
        Employee emp1 = new Employee();
        emp1.setName("emp1");


        // 2. Using Class class's newInstance() method
        Employee emp2 = Employee.class.newInstance();
        emp2.setName("emp2");


        // 3. Using Constructor class's newInstance() method
        Constructor<Employee> constructor = Employee.class.getConstructor();
        Employee emp3 = constructor.newInstance();
        emp3.setName("emp3");

        // 4. Using clone() method
        Employee emp4 = (Employee) emp3.clone();
        emp4.setName("emp4");


        // Serialization
        try (ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("data.obj"))) {
            out.writeObject(emp4);
        }

        // 5. Using Deserialization
        Employee emp5;
        try (ObjectInputStream in = new ObjectInputStream(new FileInputStream("data.obj"))) {
            emp5 = (Employee) in.readObject();
            emp5.setName("emp5");
        }

        System.out.println(emp1 + ", hashcode : " + emp1.hashCode());
        System.out.println(emp2 + ", hashcode : " + emp2.hashCode());
        System.out.println(emp3 + ", hashcode : " + emp3.hashCode());
        System.out.println(emp4 + ", hashcode : " + emp4.hashCode());
        System.out.println(emp5 + ", hashcode : " + emp5.hashCode());
    }
}

This program will give the following output:

Employee Constructor Called...
Employee Constructor Called...
Employee Constructor Called...
Employee{name='emp1'}, hashcode : 3117192
Employee{name='emp2'}, hashcode : 3117193
Employee{name='emp3'}, hashcode : 3117194
Employee{name='emp4'}, hashcode : 3117195
Employee{name='emp5'}, hashcode : 3117196

You can find the complete source code for this article on this Github Repository and please feel free to provide your valuable feedback.

Topics:
core java, java, reflection api

Published at DZone with permission of Naresh Joshi , DZone MVB. See the original article here.

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