Java initialize class by name

Create object/instance of class by name using Class.forName in java (examples)

2. Examples: create object/instance of class by name (Class.forName/java)

1. We generally create of the instances of JDBC drivers by proving complete path of driver.
   • e.g. Class sqlDriver = Class.forName(“com.mysql.jdbc.Driver”);
2. Let us look into the very simple example to create the instance of class ArrayList using class Class.
   1. Class loadClass = Class.forName(“java.util.ArrayList”);
   2. ArrayList arrayList = (ArrayList) loadClass.newInstance();
   3. arrayList.add(“Item 0”);
   4. arrayList.add(“Item 1”);
   1. We will create instance of Standard JDK class like ArrayList.
   2. We will create the instance thread in java using class Class.
   3. Create instance of User defined classes.
      1. We will create couple of custom classes.
      2. We will create their instance of custom classes using class Class.
      3. We will create objects USCitizen & SwissCitizen classes using newInstance() method.

      

      Fig 1: User defined objects

      3. Program: create object/instance of class by name (Class.forName/java)

      1. Citizen interface:

      • Citizen interface defines the couple of methods.
      • We will create couple of concrete class like USCitizen & SwissCitizen to implement Citizen interface.

      package org.learn; public interface Citizen

      2. USCitizen class:

      • USCitizen class implements Citizen interface.
      • Complete code of USCitizen class is as follows:

      package org.learn; import java.util.Random; import java.util.UUID; public class USCitizen implements Citizen < public void nationality() < System.out.println("I am US Citizen"); >public void identity() < Random random = new Random(); random.nextInt(); System.out.println("My identification number : "+random.nextInt()); >>

      3. SwissCitizen class:

      • SwissCitizen class implements Citizen interface.
      • Complete code of SwissCitizen class is as follows:

      package org.learn; import java.util.UUID; public class SwissCitizen implements Citizen < public void nationality() < System.out.println("I am Switzerland Citizen"); >public void identity() < System.out.println("My identification number : "+UUID.randomUUID().toString()); >>

      4. MyThread class:

      • We will create concrete thread class extending from JDK Thread class.
      • Complete code of MyThread class is as follows:

      package org.learn; public class MyThread extends Thread < @Override public void run() < System.out.println("Thread started :"); int index = 0; while (index++ < 10) < System.out.println("Iteration "+ index); try < Thread.sleep(300); >catch (InterruptedException e) < break; >> System.out.println("Thread executed successfully"); > >

      5. DemoForClassName class:

      • We will take 3 examples to demonstrate Class.forName concept.
        • Example 1: demoJDKClasses
          • Create the instance of standard JDK class ArrayList
          • We will create objects of User defined classes.
            • USCitizen class implementing Citizen class.
            • SwissCitizen class implementing Citizen class.
            • We will create custom thread class by extending Thread class.

            package org.learn; import java.util.ArrayList; public class DemoForClassName < public static void main(String[] args) throws IllegalAccessException, InstantiationException, ClassNotFoundException, InterruptedException < System.out.println("1. Start Invoking methods of ArrayList class"); //Demo Standard JDK class like Integer demoJDKClasses(); System.out.println("2. End invoke methods of ArrayList class"); System.out.println(); System.out.println("3. Start - Create user defined (Citizen) objects "); //Demo to custom user defined objects demoUserDefinedObjects(); System.out.println("4. End - Create user defined (Citizen) objects "); System.out.println(); System.out.println("5. Create thread in java using class.forName"); //Invoke methods of thread class in java demoCreateThread(); System.out.println("6. End creating thread using class.forName"); >private static void demoJDKClasses() throws ClassNotFoundException, IllegalAccessException, InstantiationException < //Load ArrayList class ClassloadClass = Class.forName("java.util.ArrayList"); //Create instance of ArrayList class ArrayList arrayList = (ArrayList) loadClass.newInstance(); arrayList.add("Item 0"); arrayList.add("Item 1"); //Print attributes of ArrayList class System.out.printf("Items in arrayList : %s, size : %d\n", arrayList.toString(), arrayList.size()); > private static void demoUserDefinedObjects() throws ClassNotFoundException, IllegalAccessException, InstantiationException < System.out.println("3.1 Invoke methods of USCitizen class"); //Load USCitizen class ClassloadClass = Class.forName("org.learn.USCitizen"); //Create instance of USCitizen class Citizen citizen = (Citizen) loadClass.newInstance(); //Invoke method of class citizen.nationality(); citizen.identity(); System.out.println(); System.out.println("3.2 Invoke methods of SwissCitizen class"); //Load Swiss Citizen class loadClass = Class.forName("org.learn.SwissCitizen"); //Create instance of Swiss citizen class citizen = (Citizen) loadClass.newInstance(); //Invoke method of class citizen.nationality(); citizen.identity(); > private static void demoCreateThread() throws ClassNotFoundException, IllegalAccessException, InstantiationException, InterruptedException < //Load MyThread class ClassloadClass = Class.forName("org.learn.MyThread"); //Create instance of Thread class Thread thread = (Thread) loadClass.newInstance(); //Start thread thread.start(); //Wait for completion of thread execution thread.join(); > >

            4. Output: create object/instance of class by name (Class.forName/java)

            1. Start Invoking methods of ArrayList class Items in arrayList : [Item 0, Item 1], size : 2 2. End invoke methods of ArrayList class 3. Start - Create user defined (Citizen) objects 3.1 Invoke methods of USCitizen class I am US Citizen My identification number : -1165991133 3.2 Invoke methods of SwissCitizen class I am Switzerland Citizen My identification number : 86a5b53d-ac52-426f-a494-987d2ad69eac 4. End - Create user defined (Citizen) objects 5. Create thread in java using class.forName Thread started : Iteration 1 Iteration 2 Iteration 3 Iteration 4 Iteration 5 Iteration 6 Iteration 7 Iteration 8 Iteration 9 Iteration 10 Thread executed successfully 6. End creating thread using class.forName

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