synchronized

Definition

synchronized lets only one thread at a time run a block of code, using the lock of an object. It protects shared data that several threads change, so updates are not lost.

Example code

A counter that stays correct with two threads

class Counter {
    private int value;

    synchronized void increment() { value++; }       // only one thread at a time
    synchronized int get() { return value; }
}

class Demo {
    public static void main(String[] args) throws InterruptedException {
        Counter counter = new Counter();
        Runnable work = () -> { for (int i = 0; i < 100_000; i++) counter.increment(); };

        Thread a = new Thread(work), b = new Thread(work);
        a.start(); b.start();
        a.join(); b.join();

        System.out.println(counter.get());           // always 200000; without synchronized it would often be less
    }
}

// Output:
// 200000