Synchronized and Locks

public class Counter {
    private int count = 0;

    // Method-level lock (locks on 'this')
    public synchronized void increment() {
        count++;
    }

    // Block-level lock (preferred — smaller critical section)
    public void incrementBlock() {
        synchronized (this) {
            count++;
        }
    }

    // Static synchronized — locks on the Class object
    public static synchronized void staticMethod() { ... }
}

// Two synchronized methods on the same object share the same lock
public class BankAccount {
    private double balance;

    public synchronized void deposit(double amount)  { balance += amount; }
    public synchronized void withdraw(double amount) { balance -= amount; }
    // deposit and withdraw cannot run concurrently on the same BankAccount
}

import java.util.concurrent.locks.*;

public class Counter {
    private final ReentrantLock lock = new ReentrantLock();
    private int count = 0;

    public void increment() {
        lock.lock();
        try {
            count++;
        } finally {
            lock.unlock(); // ALWAYS unlock in finally!
        }
    }

    // Try to acquire lock without blocking
    public boolean tryIncrement() {
        if (lock.tryLock()) {
            try { count++; return true; }
            finally { lock.unlock(); }
        }
        return false;
    }

    // Try with timeout
    public boolean tryIncrementTimeout() throws InterruptedException {
        if (lock.tryLock(100, TimeUnit.MILLISECONDS)) {
            try { count++; return true; }
            finally { lock.unlock(); }
        }
        return false;
    }
}

public class Cache<K, V> {
    private final Map<K, V> map = new HashMap<>();
    private final ReadWriteLock rwLock = new ReentrantReadWriteLock();
    private final Lock readLock  = rwLock.readLock();
    private final Lock writeLock = rwLock.writeLock();

    public V get(K key) {
        readLock.lock();       // multiple threads can read simultaneously
        try { return map.get(key); }
        finally { readLock.unlock(); }
    }

    public void put(K key, V value) {
        writeLock.lock();      // exclusive write access
        try { map.put(key, value); }
        finally { writeLock.unlock(); }
    }
}

public class BoundedQueue<T> {
    private final Queue<T> queue = new LinkedList<>();
    private final int capacity;
    private final Lock lock = new ReentrantLock();
    private final Condition notFull  = lock.newCondition();
    private final Condition notEmpty = lock.newCondition();

    public void put(T item) throws InterruptedException {
        lock.lock();
        try {
            while (queue.size() == capacity) notFull.await();
            queue.add(item);
            notEmpty.signal();
        } finally { lock.unlock(); }
    }

    public T take() throws InterruptedException {
        lock.lock();
        try {
            while (queue.isEmpty()) notEmpty.await();
            T item = queue.poll();
            notFull.signal();
            return item;
        } finally { lock.unlock(); }
    }
}