transfering money tests

Author: mtumilowicz

src/main/java/LockExecutor.java

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+import java.util.concurrent.locks.ReadWriteLock;
+import java.util.function.Supplier;
+
+/**
+ * Created by mtumilowicz on 2019-01-07.
+ */
+public class LockExecutor {
+    private final ReadWriteLock lock;
+
+    LockExecutor(ReadWriteLock lock) {
+        this.lock = lock;
+    }
+
+    public void write(Runnable action) {
+        lock.writeLock().lock();
+        try {
+            action.run();
+        } finally {
+            lock.writeLock().unlock();
+        }
+    }
+
+    public <T> T read(Supplier<T> action) {
+        lock.readLock().lock();
+        try {
+            return action.get();
+        } finally {
+            lock.readLock().unlock();
+        }
+    }
+}

src/main/java/User.java

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+import lombok.Value;
+
+/**
+ * Created by mtumilowicz on 2019-01-26.
+ */
+@Value
+class User {
+    int id;
+    int balance;
+
+    User income(int value) {
+        return new User(id, balance + value);
+    }
+
+    User outcome(int value) {
+        return new User(id, balance - value);
+    }
+}

src/main/java/XXX.java

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+import org.junit.Test;
+
+import java.util.concurrent.locks.ReadWriteLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static org.hamcrest.CoreMatchers.is;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+/**
+ * Created by mtumilowicz on 2019-01-25.
+ */
+public class XXXTest {
+
+    @Test
+    public void transfer() {
+        var userMap = Stream.of(new User(1, 40), new User(2, 33))
+                .collect(Collectors.toMap(User::getId, Function.identity()));
+
+        ReadWriteLock lock = new ReentrantReadWriteLock();
+        LockExecutor executor = new LockExecutor(lock);
+
+        executor.write(() -> {
+            int transfer = 15;
+            userMap.replace(1, userMap.get(1).outcome(transfer));
+            userMap.replace(2, userMap.get(2).income(transfer));
+        });
+
+        assertThat(userMap.get(1).getBalance(), is(25));
+        assertThat(userMap.get(2).getBalance(), is(48));
+    }
+
+    @Test
+    public void read() {
+        var userMap = Stream.of(new User(1, 40), new User(2, 33))
+                .collect(Collectors.toMap(User::getId, Function.identity()));
+
+        ReadWriteLock lock = new ReentrantReadWriteLock();
+        LockExecutor executor = new LockExecutor(lock);
+
+        var balanceAll = executor.read(() -> userMap.values().stream().map(User::getBalance).mapToInt(x -> x).sum());
+
+        assertThat(balanceAll, is(73));
+    }
+}