Summary - Multithreading

Combine Thread, Runnable and synchronized to write a counter.

Summary - Multithreading

In this group you saw Java's core thread tools: the Thread class, the Runnable interface, start and join, mutual exclusion with synchronized, and thread communication with wait/notify.

Now you will combine these into a mini task: building a TaskQueue structure. This summary shows how a concurrent data structure is written.

Exercise Apply synchronized with a TaskQueue

Define a TaskQueue class with a private int size = 0 field. A public synchronized void add() method does size++. A public synchronized int size() method returns size. In a QueueDemo class write main; create a TaskQueue instance, call add() three times and print size().

Show example solution

class TaskQueue {
    private int size = 0;
    public synchronized void add() {
        size++;
    }
    public synchronized int size() {
        return size;
    }
}

class QueueDemo {
    public static void main(String[] args) {
        TaskQueue q = new TaskQueue();
        q.add();
        q.add();
        q.add();
        System.out.println(q.size());
    }
}

Exercise Apply Thread inheritance with WorkerThread

Define a WorkerThread class that extends Thread; in @Override public void run() call System.out.println("Worker ready"). In a WorkerDemo class write main; create a WorkerThread instance and call run() directly.

Show example solution

class WorkerThread extends Thread {
    @Override
    public void run() {
        System.out.println("Worker ready");
    }
}

class WorkerDemo {
    public static void main(String[] args) {
        new WorkerThread().run();
    }
}

Test yourself

What is used to wait for a thread to finish?
- start()
- run()
- yield()
- join()
Show answer

join() — join() waits for the thread to finish.
Match the Multithreading summary

Thread

Thread class
Runnable

Task interface
synchronized

Mutual exclusion
wait/notify

Communication

Next lesson: ExecutorService

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