Reflection in Java

31 May, 2022
Reflection is an API that is used to examine or modify the behavior of methods, classes, and interfaces at runtime. The required classes for reflection are provided under java.lang.reflect package which is essential in order to understand reflection. So we are illustrating the package with visual aids to have a better understanding as follows: 
Reflection gives us information about the class to which an object belongs and also the methods of that class that can be executed by using the object.
Through reflection, we can invoke methods at runtime irrespective of the access specifier used with them.
 Reflection can be used to get information about class, constructors, and methods as depicted below in tabular format as shown:
We can invoke a method through reflection if we know its name and parameter types. We use two methods for this purpose as described below before moving ahead as follows:
getDeclaredMethod()invoke()
getDeclaredMethod()
invoke()
Method 1: getDeclaredMethod(): It creates an object of the method to be invoked. 
Syntax: The syntax for this method
Class.getDeclaredMethod(name, parametertype)
Parameters:
Name of a method whose object is to be created
An array of Class objects
Method 2: invoke(): It invokes a method of the class at runtime we use the following method.
Syntax: 
Method.invoke(Object, parameter)
Tip: If the method of the class doesn’t accept any parameter then null is passed as an argument.
Note: Through reflection, we can access the private variables and methods of a class with the help of its class object and invoke the method by using the object as discussed above. We use below two methods for this purpose.
Method 3: Class.getDeclaredField(FieldName): Used to get the private field. Returns an object of type Field for the specified field name. 
Method 4: Field.setAccessible(true): Allows to access the field irrespective of the access modifier used with the field.
Extensibility Features: An application may make use of external, user-defined classes by creating instances of extensibility objects using their fully-qualified names.
Debugging and testing tools: Debuggers use the property of reflection to examine private members of classes.
Performance Overhead: Reflective operations have slower performance than their non-reflective counterparts, and should be avoided in sections of code that are called frequently in performance-sensitive applications.
Exposure of Internals: Reflective code breaks abstractions and therefore may change behavior with upgrades of the platform.
Example 
Java
// Java Program to demonstrate the Use of Reflection  // Importing required classesimport java.lang.reflect.Constructor;import java.lang.reflect.Field;import java.lang.reflect.Method;  // Class 1// Of Whose object is to be createdclass Test {    // creating a private field    private String s;      // Constructor of this class      // Constructor 1    // Public constructor    public Test() { s = "GeeksforGeeks"; }      // Constructor 2    // no arguments    public void method1()    {        System.out.println("The string is " + s);    }      // Constructor 3    // int as argument    public void method2(int n)    {        System.out.println("The number is " + n);    }      // Constructor 4    // Private method    private void method3()    {        System.out.println("Private method invoked");    }}  // Class 2// Main classclass GFG {      // Main driver method    public static void main(String args[]) throws Exception    {        // Creating object whose property is to be checked          // Creating an object of class 1 inside main()        // method        Test obj = new Test();          // Creating class object from the object using        // getClass() method        Class cls = obj.getClass();          // Printing the name of class        // using getName() method        System.out.println("The name of class is "                           + cls.getName());          // Getting the constructor of the class through the        // object of the class        Constructor constructor = cls.getConstructor();          // Printing the name of constructor        // using getName() method        System.out.println("The name of constructor is "                           + constructor.getName());          // Display message only        System.out.println(            "The public methods of class are : ");          // Getting methods of the class through the object        // of the class by using getMethods        Method[] methods = cls.getMethods();          // Printing method names        for (Method method : methods)            System.out.println(method.getName());          // Creates object of desired method by        // providing the method name and parameter class as        //  arguments to the getDeclaredMethod() method        Method methodcall1            = cls.getDeclaredMethod("method2", int.class);          // Invoking the method at runtime        methodcall1.invoke(obj, 19);          // Creates object of the desired field by        // providing the name of field as argument to the        // getDeclaredField() method        Field field = cls.getDeclaredField("s");          // Allows the object to access the field        // irrespective of the access specifier used with        // the field        field.setAccessible(true);          // Takes object and the new value to be assigned        // to the field as arguments        field.set(obj, "JAVA");          // Creates object of desired method by providing the        // method name as argument to the        // getDeclaredMethod()        Method methodcall2            = cls.getDeclaredMethod("method1");          // Invokes the method at runtime        methodcall2.invoke(obj);          // Creates object of the desired method by providing        // the name of method as argument to the        // getDeclaredMethod() method        Method methodcall3            = cls.getDeclaredMethod("method3");          // Allows the object to access the method        // irrespective of the access specifier used with        // the method        methodcall3.setAccessible(true);          // Invoking the method at runtime        methodcall3.invoke(obj);    }}
Output:
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