Java Examples

Author: lokeshgupta1981

src/main/java/com/howtodoinjava/reflection/CallingInterfaceMethodWithoutImplementing.java

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+package com.howtodoinjava.reflection;
+
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.MethodHandles.Lookup;
+import java.lang.invoke.MethodType;
+import java.lang.reflect.Constructor;
+import java.lang.reflect.InvocationHandler;
+import java.lang.reflect.Proxy;
+
+public class CallingInterfaceMethodWithoutImplementing {
+
+  public static void main(String[] args) {
+
+    //Since Java 16
+
+    Movable movableProxy = (Movable) Proxy.newProxyInstance(
+        Movable.class.getClassLoader(),
+        new Class<?>[]{Movable.class}, (o, m, p) -> {
+          if (m.isDefault()) {
+            return InvocationHandler.invokeDefault(o, m, p);
+          }
+          return null;
+        });
+
+    // invoke interface methods
+    movableProxy.move();
+    String returnedValue = movableProxy.moveWithParamAndReturn("lion");
+    System.out.println(returnedValue);
+
+    //Java 15 and Before
+
+    Movable movableProxy2 = (Movable) Proxy.newProxyInstance(
+        Movable.class.getClassLoader(),
+        new Class<?>[]{Movable.class}, (o, m, p) -> {
+          if (m.isDefault()) {
+            // Adjust the method type as per the default method's signature
+            MethodType methodType;
+            if (m.getName().equals("move")) {
+              methodType = MethodType.methodType(void.class);
+            } else if (m.getName().equals("moveWithParamAndReturn")) {
+              methodType = MethodType.methodType(String.class, String.class);
+            } else {
+              throw new UnsupportedOperationException("Unknown default method: " + m.getName());
+            }
+            return MethodHandles.lookup()
+                .findSpecial(Movable.class, m.getName(),
+                    methodType,
+                    Movable.class)
+                .bindTo(o)
+                .invokeWithArguments(p);
+          }
+          return null;
+        });
+
+    movableProxy2.move();
+    returnedValue = movableProxy2.moveWithParamAndReturn("lion");
+    System.out.println(returnedValue);
+
+    //JDK 8
+
+    Movable movableProxy3 = (Movable) Proxy.newProxyInstance(
+        Movable.class.getClassLoader(),
+        new Class<?>[]{Movable.class}, (o, m, p) -> {
+          if (m.isDefault()) {
+            Constructor<Lookup> cntr = Lookup.class
+                .getDeclaredConstructor(Class.class);
+            cntr.setAccessible(true);
+            return cntr.newInstance(Movable.class)
+                .in(Movable.class)
+                .unreflectSpecial(m, Movable.class)
+                .bindTo(o)
+                .invokeWithArguments(p);
+          }
+          return null;
+        });
+
+    movableProxy3.move();
+    returnedValue = movableProxy3.moveWithParamAndReturn("lion");
+    System.out.println(returnedValue);
+  }
+
+}
+
+interface Movable {
+
+  default void move() {
+    System.out.println("Inside Movable.move()");
+  }
+
+  default String moveWithParamAndReturn(String param) {
+    System.out.println("Inside Movable.moveWithParamAndReturn()");
+    return STR."\{param}-returned";
+  }
+
+  void moveToImplement();
+}