Invoke constructor using Reflection in java

In this post, we will see how to invoke constructor using reflection in java.

You can retrieve the constructors of the classes and instantiate object at run time using reflection. java.lang.reflect.Constructor is used to instantiate the objects.
[toc]

Instantiate Constructor with no parameters

In case, you want to create object using no args constructor at runtime, you can do it as below.

Class cl = Class.forName("org.arpit.java2blog.Color");

// Call no-args constructor
Object newInstance = cl.newInstance();
System.out.println(newInstance);

Here,
Class.forName("org.arpit.java2blog.Color") is used to load the class and get an object of type Class and we can retrieve a lot of information such as constructors, methods, and annotations etc. at run time with Class object.

You need to replace org.arpit.java2blog.Color with classname for which you want to instantiate object at run time.

cl.newInstance() can be used to create object with no-args constructor at run time.

Instantiate Constructor with parameters

In case, you want to instantiate constructor with parameters, you need to retrieve constructor with parameters and then pass parameters to instantiate it.

Class cl = Class.forName("org.arpit.java2blog.Color");

// Call parameterized constructor
Class[] type = { String.class,String.class };

// get parameterized constructor which takes 2 Strings as parameters
Constructor cons = cl.getConstructor(type);
// String arguments
Object[] obj = { "Red","#FF0000"};
Object newInstancePC = cons.newInstance(obj);

• You need to pass Class[] to getConstructor() method and retrieve the instance of java.lang.reflect.Constructor from cl.
• Pass Object[] to cons.newInstance to construct object with passed parameters.

You can also get Parameters types with cons.getParameterTypes()

Class[] parameterTypes= cons.getParameterTypes();

Let’s see this with an example.
Consider a class named Color which has two attributes name and htmlCode.

package org.arpit.java2blog;

public class Color {

    String name;
    String htmlCode;

    public Color(String name, String htmlCode) {
        super();
        this.name = name;
        this.htmlCode = htmlCode;
    }

    public Color() {
        name = "black";
        htmlCode = "#000000";
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getHtmlCode() {
        return htmlCode;
    }

    public void setHtmlCode(String htmlCode) {
        this.htmlCode = htmlCode;
    }

    @Override
    public String toString() {
        return "Color [name:" + name + " HtmlCode:" + htmlCode + "]";
    }
}

Create main class named ConstuctorReflectionMain

package org.arpit.java2blog;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class ConstuctorReflectionMain {

    public static void main(String[] args) {

        try {
            Class cl = Class.forName("org.arpit.java2blog.Color");

            System.out.println("====================");
            System.out.println("Calling default constructor");
            System.out.println("====================");
            // Call default constructor
            Object newInstance = cl.newInstance();
            System.out.println(newInstance);

            System.out.println("====================");
            System.out.println("Calling parameterized constructor");
            System.out.println("====================");
            Class[] type = { String.class,String.class };

            // get parameterized constructor which takes 2 Strings as parameters
            Constructor cons = cl.getConstructor(type);
            // String arguments
            Object[] obj = { "Red","#FF0000"};
            Object newInstancePCObj = cons.newInstance(obj);

            // cast it to required type
            Color color=(Color) newInstancePCObj;
            System.out.println(color);

        } catch (ClassNotFoundException | NoSuchMethodException | SecurityException | InstantiationException 
                     | IllegalAccessException | IllegalArgumentException | InvocationTargetException e) {
            e.printStackTrace();
        } 
    }
}

When you run above program, you will get below output:
Output:

As you can see, we are able to instantiate object using java.lang.reflect.Constructor at run time using reflection.

Constructor parameters for primitive types

In case, you have constructor parameters with primitive types, you can use .class suffix with that.
For example:
If the parameter type is of int, then you can use int.class while getting constructor of that type.

Let’s understand with the help of example:
Consider a class Employee with attributes: name and age.

package org.arpit.java2blog;

public class Employee {

    String name;
    int age;

    public Employee(String name, int age) {
        super();
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Employee [name=" + name + ", age=" + age + "]";
    }
}

Create main class named ConstructorParameterPrimMain

package org.arpit.java2blog;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class ConstructorParameterPrimMain {

public static void main(String[] args) {

        try {
            Class cl = Class.forName("org.arpit.java2blog.Employee");

            Class[] type = { String.class,int.class };

            // get parameterized constructor which takes 2 Strings as parameters
            Constructor cons = cl.getConstructor(type);
            // String arguments
            Object[] obj = { "Arpit",28};
            Object newInstanceObj = cons.newInstance(obj);

            // cast it to required type
            Employee emp=(Employee) newInstanceObj;
            System.out.println(emp);

        } catch (ClassNotFoundException | NoSuchMethodException | SecurityException | InstantiationException 
                     | IllegalAccessException | IllegalArgumentException | InvocationTargetException e) {
            e.printStackTrace();
        } 
    }

}

When you run above program, you will get below output:
Output:

You can also pass Integer.Type instead of int.class.

Class[] type = { String.class,Integer.Type };

Conclusion

You have learnt about how to invoke constructor and instantiate objects at runtime using reflection. We have also seen how to handle primitive type parameters while getting constructor from class.
That’s all about how to invoke constructor using Reflection in java.