Class getDeclaredConstructor() method in Java with Examples

The getDeclaredConstructor() method of java.lang.Class class is used to get the specified constructor of this class with the specified parameter type. The method returns the specified constructor of this class in the form of Constructor object.

Syntax:

public Constructor<T>
       getDeclaredConstructor(Class[] parameterType)
       throws NoSuchMethodException, SecurityException

Parameter: This constructor accepts a parameters parameterType which is the array of parameter type for the specified constructor.

Return Value: This method returns the specified constructor of this class in the form of Constructor objects.

Exception This method throws:

• NoSuchMethodException if a constructor with the specified name is not found.
• SecurityException if a security manager is present and the security conditions are not met.

  Below programs demonstrate the getDeclaredConstructor() method.

  Example 1:

  // Java program to demonstrate
  // getDeclaredConstructor() method
  
  import java.util.*;
  
  public class Test {
  
      public Test() {}
  
      public static void main(String[] args)
          throws ClassNotFoundException, NoSuchMethodException
      {
  
          // returns the Class object for this class
          Class myClass = Class.forName("Test");
  
          System.out.println("Class represented by myClass: "
                             + myClass.toString());
  
          Class[] parameterType = null;
  
          // Get the constructor of myClass
          // using getDeclaredConstructor() method
          System.out.println(
              "Constructor of myClass: "
              + myClass.getDeclaredConstructor(parameterType));
      }
  }
  Output:

  Class represented by myClass: class Test
  Constructor of myClass: public Test()
  

  Example 2:

  // Java program to demonstrate
  // getDeclaredConstructor() constructor
  
  import java.util.*;
  
  class Main {
  
      private Main() {}
  
      public static void main(String[] args)
          throws ClassNotFoundException, NoSuchMethodException
      {
  
          // returns the Class object for this class
          Class myClass = Class.forName("Main");
  
          System.out.println("Class represented by myClass: "
                             + myClass.toString());
  
          Class[] parameterType = new Class[1];
          parameterType[0] = Long.class;
  
          try {
              // Get the constructor of myClass
              // using getDeclaredConstructor() method
              System.out.println(
                  "Constructor of myClass: "
                  + myClass.getDeclaredConstructor(parameterType));
          }
          catch (Exception e) {
              System.out.println(e);
          }
      }
  }
  Output:

  Class represented by myClass: class Main
  java.lang.NoSuchMethodException: Main.(java.lang.Long)
  

  Reference: https://docs.oracle.com/javase/9/docs/api/java/lang/Class.html#getDeclaredConstructor-java.lang.Class…-