added ex S3

src/exercises/ex_s3/AbstractTweet.java

@@ -0,0 +1,20 @@
+package exercises.ex_s3;
+
+public abstract class AbstractTweet {
+    private String author;
+    private String content;
+
+    public AbstractTweet(String author, String content) {
+        this.author = author;
+        this.content = content;
+    }
+
+    public abstract void addLike();
+    public abstract void removeLike();
+    public abstract long getLikes();
+
+    @Override
+    public String toString() {
+        return "@" + author + " tweeted \"" + content + "\" (" + getLikes() + " likes)";
+    }
+}

src/exercises/ex_s3/AtomicTweet.java

@@ -0,0 +1,27 @@
+package exercises.ex_s3;
+
+import java.util.concurrent.atomic.AtomicLong;
+
+public class AtomicTweet extends AbstractTweet {
+
+    private AtomicLong likes = new AtomicLong(0);
+
+    public AtomicTweet(String author, String content) {
+        super(author, content);
+    }
+
+    @Override
+    public void addLike() {
+        likes.incrementAndGet();
+    }
+
+    @Override
+    public void removeLike() {
+        likes.updateAndGet(i -> i > 0 ? i - 1 : i);
+    }
+
+    @Override
+    public long getLikes() {
+        return likes.get();
+    }
+}

src/exercises/ex_s3/TestingAtomicLong.java

@@ -0,0 +1,32 @@
+package exercises.ex_s3;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class TestingAtomicLong {
+    public static String[] names = {
+        "Alice", "Bob", "Charlie", "Derek", "Emily",
+        "Fionna", "Greg", "Harry", "Isabella", "Julia"
+    };
+    public static void main(String[] args) throws InterruptedException {
+        List<AbstractTweet> tweets = new ArrayList<>();
+        AbstractTweet tweet1 = new Tweet("Alice", "Java is cool !");
+        AbstractTweet tweet2 = new AtomicTweet("Bob", "ISC is the best !");
+        tweets.add(tweet1);
+        tweets.add(tweet2);
+
+        Thread[] users = new Thread[10];
+
+        for (int i = 0; i < 10; i++) {
+            users[i] = new Thread(new User(names[i], tweets));
+            users[i].start();
+        }
+
+        for (int i = 0; i < 10; i++) {
+            users[i].join();
+        }
+
+        System.out.println(tweet1);
+        System.out.println(tweet2);
+    }
+}

src/exercises/ex_s3/Tweet.java

@@ -0,0 +1,21 @@
+package exercises.ex_s3;
+
+public class Tweet extends AbstractTweet {
+    private long likes = 0;
+
+    public Tweet(String author, String content) {
+        super(author, content);
+    }
+
+    public void addLike() {
+        likes += 1;
+    }
+
+    public void removeLike() {
+        likes -= 1;
+    }
+
+    public long getLikes() {
+        return likes;
+    }
+}

src/exercises/ex_s3/User.java

@@ -0,0 +1,24 @@
+package exercises.ex_s3;
+
+import java.util.List;
+
+public class User implements Runnable {
+    private final String name;
+    private final List<AbstractTweet> tweets;
+
+    public User(String name, List<AbstractTweet> tweets) {
+        this.name = name;
+        this.tweets = tweets;
+    }
+
+    @Override
+    public void run() {
+        for (int i = 0; i < 100000; i++) {
+            if (i % 10 == 9) {
+                tweets.forEach(AbstractTweet::removeLike);
+            } else {
+                tweets.forEach(AbstractTweet::addLike);
+            }
+        }
+    }
+}