Java - java.lang.Class

Introduction

Class class can list everything about a class at runtime.

You can get the reference to the Class object of a class in one of the followings ways:

• Using class literal
• Using the getClass() method of the Object class
• Using the forName() method of the Class class

Example

A class literal is the class name followed by a dot and the word "class."

For example, String class literal is String.class and you can write

Class<String> c = String.class;

You can get the class object for primitive data types and the keyword void using class literals as boolean.class, byte.class, char.class, short.class, int.class, long.class, float.class, double.class, and void.class.

Each wrapper class for these primitive data types has a static field named TYPE, which has the reference to the class object of the primitive data type it represents.

int.class and Integer.TYPE refer to the same class object and the expression int.class == Integer.TYPE evaluates to true.

The following table shows the class literals for all primitive data types and the void keyword.

Data Type	Primitive Class Literal	Wrapper Class Static Field
boolean	boolean.class	Boolean.TYPE
byte	byte.class	Byte.TYPE
char	char.class	Character.TYPE
short	short.class	Short.TYPE
int	int.class	Integer.TYPE
long	long.class	Long.TYPE
float	float.class	Float.TYPE
double	double.class	Double.TYPE
void	void.class	Void.TYPE

getClass() method

Object class getClass() method returns the reference to the Class object.

You can get the reference to the Class object of the Test class as follows:

Test testRef = new Test();
Class<? extends Test> testClass = testRef.getClass();

forName()

Class class forName() static method returns a reference to a Class object.

It is an overloaded method The declarations of the two overloaded versions of this method are

Class<?> forName(String className)
Class<?> forName(String name, boolean initialize, ClassLoader loader)

To load a class named pkg1.Test, you would write

Class testClass = Class.forName("pkg1.Test");

Demo

class Test {
  static {/*www . j a  va  2s  . com*/
    // This will execute when this class is loaded and initialized
    System.out.println("Loading class Bulb...");
  }
}

public class Main {
  public static void main(String[] args) {

    createObject();
    forName();
    forNameVersion2();
    classLiteral();
  }

  public static void classLiteral() {
    Class<Test> c = Test.class;
  }

  public static void forNameVersion2() {
    try {
      String className = "Test";
      boolean initialize = false;
      ClassLoader cLoader = Main.class.getClassLoader();
      Class c = Class.forName(className, initialize, cLoader);
    } catch (ClassNotFoundException e) {
      System.out.println(e.getMessage());
    }
  }

  public static void forName() {
    try {
      String className = "com.book2s.reflection.Bulb";

      Class c = Class.forName(className);
    } catch (ClassNotFoundException e) {
      System.out.println(e.getMessage());
    }
  }
  public static void createObject() {
    new Test();
  }
}

Result

Quiz

• What is the output: class literal?

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