Java Class getDeclaredConstructor() Method

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Description

The Java Class getDeclaredConstructor() method returns a Constructor object that reflects the specified constructor of the class or interface represented by this Class object. The parameterTypes parameter is an array of Class objects that identify the constructor's formal parameter types, in declared order.

Declaration

Following is the declaration for java.lang.Class.getDeclaredConstructor() method

public Constructor<T> getDeclaredConstructor(Class<?>... parameterTypes) throws NoSuchMethodException, SecurityException

Parameters

parameterTypes − This is the parameter array.

Return Value

This method returns the Constructor object for the constructor with the specified parameter list.

Exception

• NoSuchMethodException − If a matching method is not found.

• SecurityException − If a security manager, s, is present.

Getting Declared Constructor of a Class Example

The following example shows the usage of java.lang.Class.getDeclaredConstructor() method. In this program, we've created an instance of ClassDemo and then using getClass() method, the class of the instance is retrieved. In ClassDemo, we've two constructors. Now using getDeclaredConstructor(), we've retrieved a constructor with two arguments and printed it.

package com.tutorialspoint;

import java.lang.reflect.Constructor;

public class ClassDemo {

   public static void main(String[] args) {
    
      try {
         ClassDemo cls = new ClassDemo();
         Class c = cls.getClass();

         // constructor with arguments as Double and Long
         Class[] cArg = new Class[2];
         cArg[0] = Double.class;
         cArg[1] = Long.class;
         Constructor ct = c.getDeclaredConstructor(cArg);
         System.out.println("Constructor = " + ct.toString());
      } catch(NoSuchMethodException e) {
         System.out.println(e.toString());
      } catch(SecurityException e) {
         System.out.println(e.toString());
      }
   }

   private ClassDemo() {
      // System.out.println("no argument constructor");
   }

   public ClassDemo(Double d, Long l) {
      this.d = d;
      this.l = l;
   }

   Double d = new Double(3.9d);
   Long l = new Long(7687);
} 

Output

Let us compile and run the above program, this will produce the following result −

Constructor = public ClassDemo(java.lang.Double,java.lang.Long)

Getting Declared No Arg Constructor of a Class Example

The following example shows the usage of java.lang.Class.getDeclaredConstructor() method. In this program, we've created an instance of ClassDemo and then using getClass() method, the class of the instance is retrieved. In ClassDemo, we've two constructors. Now using getDeclaredConstructor(), we've retrieved a constructor with no arguments and printed it.

package com.tutorialspoint;

import java.lang.reflect.Constructor;

public class ClassDemo {

   public static void main(String[] args) {
    
      try {
         ClassDemo cls = new ClassDemo();
         Class c = cls.getClass();

         // constructor with no argument
         Constructor ct = c.getDeclaredConstructor();
         System.out.println("Constructor = " + ct.toString());
      } catch(NoSuchMethodException e) {
         System.out.println(e.toString());
      } catch(SecurityException e) {
         System.out.println(e.toString());
      }
   }

   private ClassDemo() {
      // System.out.println("no argument constructor");
   }

   public ClassDemo(Double d, Long l) {
      this.d = d;
      this.l = l;
   }

   Double d = new Double(3.9d);
   Long l = new Long(7687);
} 

Output

Let us compile and run the above program, this will produce the following result −

Constructor = private com.tutorialspoint.ClassDemo()

Getting Declared No Arg Constructor of an ArrayList Example

The following example shows the usage of java.lang.Class.getDeclaredConstructor() method. In this program, we've retrieved the class of ArrayList. Now using getDeclaredConstructor(), we've retrieved a constructor with no arguments and printed it.

package com.tutorialspoint;

import java.lang.reflect.Constructor;
import java.util.ArrayList;

public class ClassDemo {

   public static void main(String[] args) {
    
      try {
         Class c = ArrayList.class;
         // constructor with no argument
         Constructor ct = c.getDeclaredConstructor();
         System.out.println("Constructor = " + ct.toString());
      } catch(NoSuchMethodException e) {
         System.out.println(e.toString());
      } catch(SecurityException e) {
         System.out.println(e.toString());
      }
   }
} 

Output

Let us compile and run the above program, this will produce the following result −

Constructor = public java.util.ArrayList()