adds proxy design pattern.

src/main/java/com/designpatterns/structural/proxy/Citizen.java

+package com.designpatterns.structural.proxy;
+
+import com.designpatterns.structural.proxy.president.PresidentSecretary;
+
+public class Citizen {
+
+    public static void main(String[] args) {
+        PresidentSecretary presidentSecretary = new PresidentSecretary();
+        presidentSecretary.leaveValidMessageForPresident("Hello president.");
+    }
+}

src/main/java/com/designpatterns/structural/proxy/president/President.java

+package com.designpatterns.structural.proxy.president;
+
+import com.designpatterns.creational.singleton.Singleton;
+
+public class President {
+
+    private volatile static President instance = null;
+
+    private President() {}
+
+    static President getInstance() {
+        if (instance == null) {
+            synchronized (Singleton.class) {
+                if (instance == null) {
+                    instance = new President();
+                }
+            }
+        }
+        return instance;
+    }
+
+
+
+    void talkToThePresident(String message){
+        System.out.println("I, the President, have received this message:" + message);
+    }
+}

src/main/java/com/designpatterns/structural/proxy/president/PresidentSecretary.java

+package com.designpatterns.structural.proxy.president;
+
+public class PresidentSecretary {
+
+    private President president;
+
+    public PresidentSecretary() {
+        this.president = President.getInstance();
+    }
+
+    public void leaveValidMessageForPresident(String message){
+
+        if(!isMessageValid(message))
+            throw new RuntimeException("invalid message");
+
+        System.out.println("message is being sent to the President...");
+        president.talkToThePresident(message);
+        System.out.println("message is received by the President.");
+
+    }
+
+    private boolean isMessageValid(String message) {
+        return message != null && !message.isEmpty() && message.length() >= 10 && message.length() <= 100;
+    }
+}

src/test/java/com/designpatterns/structural/proxy/Citizen.java

+package com.designpatterns.structural.proxy;
+
+import com.designpatterns.structural.proxy.president.PresidentSecretary;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class Citizen {
+
+
+    private PresidentSecretary presidentSecretary = new PresidentSecretary();
+
+    @Test
+    public void talkToPresident_secretaryShouldRejectTooShortMessage() {
+        String message = "Hi there.";
+
+        Assertions.assertThrows(RuntimeException.class, () -> {
+            presidentSecretary.leaveValidMessageForPresident(message);
+        });
+    }
+
+    @Test
+    public void talkToPresident_secretaryShouldRejectTooLongMessage() {
+        String message = "Hi there. this is a message about some personal issue which I have decided to share with Mr.President.";
+
+        Assertions.assertThrows(RuntimeException.class, () -> {
+            presidentSecretary.leaveValidMessageForPresident(message);
+        });
+    }
+
+    @Test
+    public void talkToPresident_secretaryShouldAcceptTheMessage() {
+        String message = "Hello Mr.President";
+
+        presidentSecretary.leaveValidMessageForPresident(message);
+        Assertions.assertTrue(true);
+    }
+
+
+}