How to call private method from another class in Java with help of Reflection API?

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We can call the private method of a class from another class in Java (which are defined using the private access modifier in Java).

We can do this by changing the runtime behavior of the class by using some predefined methods of Java. For accessing private method of different class we will use Reflection API.

To call the private method, we will use following methods of Java.lang.class and Java.lang.reflect.Method

Method[] getDeclaredMethods(): This method returns a Method object that reflects the specified declared method of the class or interface represented by this Class object.
setAccessible(): Set the accessible flag for this object to the indicated boolean value. A value of true indicates that the reflected object should suppress Java language access checking when it is used. A value of false indicates that the reflected object should enforce Java language access checks.
invoke():It invokes the underlying method represented by this Method object, on the specified object with the specified parameters.

Below programs demonstrates calling of private method in Java:

Example 1: When the name of private function is known already.

// Java program to call
// private method of a
// class from another class
  
import Java.lang.reflect.Method;
  
// The class containing
// a private method and
// a public method
class Check {
  
    // Private method
    private void private_Method()
    {
        System.out.println("Private Method "
                           + "called from outside");
    }
  
    // Public method
    public void printData()
    {
        System.out.println("Public Method");
    }
}
  
// Driver code
class GFG {
  
    public static void main(String[] args)
        throws Exception
    {
        Check c = new Check();
  
        // Using getDeclareMethod() method
        Method m = Check.class
                       .getDeclaredMethod("private_Method");
  
        // Using setAccessible() method
        m.setAccessible(true);
  
        // Using invoke() method
        m.invoke(c);
    }
}

Output:

Example 2: When the name of private function is not known but class name is known.

// Java program to call private method
// of a class from another class
  
import Java.lang.reflect.Method;
  
// The class contains a private method
class Check {
  
    // Private method
    private void Demo_private_Method()
    {
        System.out.println("Private Method "
                           + "called from outside");
    }
  
    // Public method
    public void printData()
    {
        System.out.println("Public Method");
    }
}
  
// Driver code
class GFG {
  
    public static void main(String[] args)
        throws Exception
    {
        Check c = new Check();
        Method m;
  
        // Using getDeclareMethod() method
        Method method[]
            = Check.class.getDeclaredMethods();
  
        for (int i = 0; i < method.length; i++) {
            String meth
                = new String(method[i].toString());
            if (meth.startsWith("private")) {
  
                String s = method[i].toString();
  
                int a = s.indexOf(".");
                int b = s.indexOf("(");
  
                // Method name retrieved
                String method_name = s.substring(a + 1, b);
  
                // Print method name
                System.out.println("Method Name = "
                                   + method_name);
  
                // Using getDeclareMethod() method
                m = Check.class.getDeclaredMethod(method_name);
  
                // Using setAccessible() method
                m.setAccessible(true);
  
                // Using invoke() method
                m.invoke(c);
            }
        }
    }
}