Programming Tutorials

What is Reflection? - getConstructors(), getFields(), and getMethods()

By: Lakshmi in Java Tutorials on 2022年09月15日 [フレーム]

Reflection is a mechanism in Java that allows us to inspect or modify the behavior of a program at runtime. We can use reflection to access and manipulate classes, fields, methods, and constructors. It is used to develop various frameworks and tools such as serialization, testing, debugging, and dependency injection.

The following are some examples that demonstrate how to use reflection methods getConstructors(), getFields(), and getMethods().

1. getConstructors() example: This method returns an array of Constructor objects that represent all the public constructors of the class.
   

   import java.lang.reflect.Constructor;
   class MyClass {
    public MyClass(int a, int b) {
    System.out.println("Constructor with two parameters");
    }
    private MyClass(String s) {
    System.out.println("Private constructor with one parameter");
    }
   }
   public class ReflectionExample {
    public static void main(String[] args) {
    Class<?> c = MyClass.class;
    Constructor<?>[] constructors = c.getConstructors();
    for (Constructor<?> constructor : constructors) {
    System.out.println(constructor);
    }
    }
   }
   

   Output

   public MyClass(int,int)
   
   

2. getFields() example: This method returns an array of Field objects that represent all the public fields of the class.

   import java.lang.reflect.Field;
   class MyClass {
    public int a;
    protected String b;
    private double c;
   }
   public class ReflectionExample {
    public static void main(String[] args) {
    Class<?> c = MyClass.class;
    Field[] fields = c.getFields();
    for (Field field : fields) {
    System.out.println(field);
    }
    }
   }
   

   Output:

   public int MyClass.a
   protected java.lang.String MyClass.b
   

3. getMethods() example: This method returns an array of Method objects that represent all the public methods of the class.

   import java.lang.reflect.Method;
   class MyClass {
    public void method1() {
    System.out.println("Method with no parameters");
    }
    public void method2(int a, String b) {
    System.out.println("Method with two parameters");
    }
    private void method3() {
    System.out.println("Private method with no parameters");
    }
   }
   public class ReflectionExample {
    public static void main(String[] args) {
    Class<?> c = MyClass.class;
    Method[] methods = c.getMethods();
    for (Method method : methods) {
    System.out.println(method);
    }
    }
   }
   

   Output:

   public void MyClass.method1()
   public void MyClass.method2(int,java.lang.String)
   public final void java.lang.Object.wait() throws java.lang.InterruptedException
   public final native void java.lang.Object.wait(long) throws java.lang.InterruptedException
   public final void java.lang.Object.wait(long,int) throws java.lang.InterruptedException
   public boolean java.lang.Object.equals(java.lang.Object)
   public java.lang.String java.lang.Object.toString()
   public native int java.lang.Object.hashCode()
   public final native java.lang.Class java.lang.Object.getClass()
   public final native void java.lang.Object.notify()
   public final native void java.lang.Object.notifyAll()
   
   

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