How to create objects in java

How to create an object in java: 1. Use the new keyword; 2. Use the newInstance method of the Class class, which can call the constructor without parameters to create an object; 3. Use the newInstance method of the Constructor class; 4. Use clone method; 5. Use deserialization.

The operating environment of this tutorial: windows7 system, java8 version, DELL G3 computer.

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

Use the new keyword	} → Called the constructor
Use the newInstance method of the Class class	} → Called the constructor
Using the newInstance method of the Constructor class	} → The constructor was called
Using the clone method	} →The constructor was not called
Use deserialization	} → The constructor is not called

If you run the program at the end, you 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();
0: new           #19          // class org/programming/mitra/exercises/Employee
3: dup
4: invokespecial #21          // Method org/programming/mitra/exercises/Employee."":()V

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 way:

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

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

3. Use the newInstance method of the Constructor class

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

These two newInstance methods are what everyone calls reflection. In fact, the newInstance method of Class internally calls the newInstance method of Constructor. This is also the reason why many frameworks, such as Spring, Hibernate, Struts, etc., use the latter. To understand the difference between the two newInstance methods, please read this article Creating objects through Reflection in Java with Example.

4. Use the clone method

Whenever we When the clone method of an object is called, 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 defined by it.

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

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

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

In the following Java program, we will use 5 method to create an Employee object. 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<Employee> 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());
    }
}

The program will output:

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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