Constructor Overloading and overriding in JAVA

In Java, constructors are special methods that are used to create objects of a class. Like regular methods, constructors can be overloaded and overridden. Here’s a brief explanation of constructor overloading and overriding in Java:

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Constructor Overloading: Constructor overloading is the process of creating multiple constructors in a class, each with a different set of parameters. It’s useful when you want to create objects of a class with different sets of initial values.

For example, suppose you have a class called Rectangle that represents a rectangle with a given length and width. You could create multiple constructors for the Rectangle class that take different sets of parameters, such as:

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arduino code

public class Rectangle {
    private int length;
    private int width;

    public Rectangle() {
        this.length = 0;
        this.width = 0;
    }

    public Rectangle(int length, int width) {
        this.length = length;
        this.width = width;
    }

    public Rectangle(int length) {
        this.length = length;
        this.width = 0;
    }
}

In this example, we have created three constructors for the Rectangle class. The first constructor initializes the length and width fields to 0. The second constructor takes two parameters and initializes the length and width fields to the values of those parameters. The third constructor takes one parameter and initializes the length field to the value of that parameter and the width field to 0.

Constructor Overriding: Constructor overriding is not possible in Java, as constructors are not inherited like regular methods. However, a subclass can call a constructor of its superclass using the super() keyword.

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For example, suppose you have a subclass called Square that extends the Rectangle class and represents a square with a given side length. You could create a constructor for the Square class that calls the constructor of its superclass, Rectangle, using the super() keyword, like this:

java code

public class Square extends Rectangle {
    public Square(int sideLength) {
        super(sideLength, sideLength);
    }
}

In this example, we have created a constructor for the Square class that takes one parameter, sideLength, and calls the constructor of its superclass, Rectangle, using the super() keyword. The super() call initializes the length and width fields of the Square object to the same value, which creates a square object.

Top 20 Multithreading Java Interview questions and Answers Part-II

14.What is the purpose of the volatile keyword in multithreading?

In Java, the volatile keyword is used to indicate that a variable’s value may be modified by multiple threads and that changes to the variable should be visible to all threads immediately. The volatile keyword ensures that each thread accesses the most up-to-date value of the variable and that changes to the variable are propagated immediately to all threads.

Without the volatile keyword, each thread may cache the value of the variable in its own CPU cache, and changes made by one thread may not be visible to other threads until the cache is flushed to main memory. This can lead to race conditions, where different threads have different versions of the variable’s value, and can cause unpredictable and incorrect behavior.

Here is an example that demonstrates the use of the volatile keyword:

arduinoCopy codeclass MyThread extends Thread {
    volatile boolean stop = false;

    public void run() {
        while (!stop) {
            // Do some work
        }
    }

    public void stopThread() {
        stop = true;
    }
}

In this example, a MyThread class extends the Thread class and has a volatile boolean variable called stop. The run() method of the thread checks the value of the stop variable in a loop and continues to do some work until the variable is set to true. The stopThread() method is used to set the stop variable to true and stop the thread.

The volatile keyword ensures that changes to the stop variable made by the stopThread() method are immediately visible to the run() method, even if the two methods are executed in different threads.

It is important to note that the volatile keyword only ensures visibility of changes to a variable’s value, but it does not provide mutual exclusion or atomicity guarantees. If multiple threads are accessing and modifying the same variable, you may need to use other synchronization mechanisms such as locks or atomic variables to ensure correctness and consistency.

15.What is the Executor framework?

The Executor framework is a high-level abstraction for managing and executing threads in Java. It provides a way to decouple the task submission and execution process, allowing you to focus on the logic of your program instead of managing threads.

The Executor framework consists of three main components:

1. 1. Executor interface: This interface provides a single method called execute(Runnable command) that is used to execute the given command in a thread pool. The Executor interface is a simple interface that provides basic execution facilities.

1. 1. ExecutorService interface: This interface extends the Executor interface and provides additional methods for managing the execution of tasks, such as submitting tasks for execution, waiting for tasks to complete, and shutting down the thread pool.

1. 1. ThreadPoolExecutor class: This class implements the ExecutorService interface and provides a powerful and flexible thread pool implementation. You can customize the behavior of the thread pool by providing parameters such as the number of threads in the pool, the maximum number of tasks that can be queued, and the thread creation policy.

The Executor framework provides several benefits over manually managing threads:

1. 1. It provides a simple and consistent way to execute tasks in a thread pool, without the need to manage threads manually.

1. 1. It improves the performance of your application by reusing threads and avoiding the overhead of creating and destroying threads for each task.

1. 1. It provides a way to manage the lifecycle of the thread pool, including starting and stopping the pool, and waiting for tasks to complete.

Overall, the Executor framework provides a powerful and flexible way to manage threads in Java, making it easier to write high-performance, scalable, and concurrent applications.

16.What is a ReentrantLock?

ReentrantLock is a type of lock provided in Java that allows a thread to acquire and release the lock multiple times. It is called “reentrant” because a thread can re-enter a critical section of code protected by the same lock it has already acquired, without blocking on itself.

The ReentrantLock provides several advantages over the synchronized block for controlling access to a shared resource:

1. 1. A ReentrantLock can be used to implement more complex locking strategies, such as fairness, timed waits, and interruptible locks.

1. 1. The ReentrantLock provides methods to query the state of the lock, such as the number of threads waiting to acquire the lock and the owner of the lock.

1. 1. Unlike the synchronized block, a ReentrantLock can be used to provide non-blocking lock acquisition, where a thread can try to acquire the lock and immediately return if the lock is not available.

Here is an example of how to use a ReentrantLock to protect a shared resource:

csharpCopy codeclass MyResource {
    private final ReentrantLock lock = new ReentrantLock();
    private int count = 0;

    public void increment() {
        lock.lock();
        try {
            count++;
        } finally {
            lock.unlock();
        }
    }

    public int getCount() {
        lock.lock();
        try {
            return count;
        } finally {
            lock.unlock();
        }
    }
}

In this example, the MyResource class has a private ReentrantLock object called lock that is used to protect the count variable. The increment() method acquires the lock, increments the count, and releases the lock. The getCount() method acquires the lock, reads the value of the count, and releases the lock.

Using a ReentrantLock can provide better control over the synchronization of shared resources and can lead to better performance and scalability compared to using synchronized blocks. However, it requires more code and can be more complex to use correctly.

17.What is the difference between ReentrantLock and synchronized block?

Both ReentrantLock and synchronized blocks are used to synchronize access to shared resources in Java. However, there are some differences between the two approaches:

1. 1. Locking mechanism: The synchronized block is built into the Java language and provides a simple and easy-to-use mechanism for synchronizing access to shared resources. In contrast, ReentrantLock is a lock implementation provided by the Java API that provides more advanced locking features.

1. 1. Lock acquisition: The synchronized block acquires and releases the lock automatically when entering and exiting the block. In contrast, with ReentrantLock, you need to acquire and release the lock manually by calling the lock() and unlock() methods.

1. 1. Reentrancy: The synchronized block is reentrant, which means that a thread can re-enter a synchronized block it has already entered. Similarly, ReentrantLock is also reentrant, but it provides more control over the number of times a thread can enter a critical section of code.

1. 1. Fairness: The synchronized block does not provide any guarantees about which thread will acquire the lock when it becomes available. In contrast, ReentrantLock provides fairness by allowing threads to acquire the lock in the order in which they requested it.

1. 1. Advanced features: ReentrantLock provides more advanced locking features, such as timed waits, interruptible locks, and lock conditions. These features are not available with synchronized blocks.

In general, synchronized blocks are simpler and easier to use, and they are appropriate for most synchronization needs. However, if you need more advanced locking features or greater control over the locking mechanism, ReentrantLock may be a better choice.

18.What is the Callable interface?

The Callable interface in Java is similar to the Runnable interface, but it allows a thread to return a result and throw a checked exception. The Callable interface is defined in the java.util.concurrent package and has a single method:

csharpCopy codepublic interface Callable<V> {
    V call() throws Exception;
}

The call() method returns a result of type V and can throw a checked exception of type Exception. This makes it easier to write concurrent programs that need to return a value from a thread and handle exceptions.

To use a Callable in a thread, you need to submit it to an ExecutorService using the submit() method. The submit() method returns a Future object that can be used to retrieve the result of the computation and handle any exceptions that occurred.

Here is an example of how to use a Callable to compute the factorial of a number:

javaCopy codeimport java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;

public class Factorial implements Callable<Long> {
    private final int number;

    public Factorial(int number) {
        this.number = number;
    }

    public Long call() throws Exception {
        long result = 1;
        for (int i = 1; i <= number; i++) {
            result *= i;
        }
        return result;
    }

    public static void main(String[] args) throws InterruptedException, ExecutionException {
        Factorial factorial = new Factorial(5);
        FutureTask<Long> futureTask = new FutureTask<>(factorial);
        Thread thread = new Thread(futureTask);
        thread.start();
        Long result = futureTask.get();
        System.out.println("Factorial of " + factorial.number + " is " + result);
    }
}

In this example, the Factorial class implements the Callable<Long> interface and overrides the call() method to compute the factorial of the given number. The main() method creates a FutureTask object with the Factorial instance and submits it to a new thread using the Thread class. Finally, it waits for the result using the get() method of the FutureTask object.

Using Callable can make it easier to write concurrent programs that need to return a value from a thread and handle exceptions.

19.What is a CyclicBarrier?

A CyclicBarrier is a synchronization aid that allows a set of threads to wait for each other to reach a common barrier point before proceeding. It is similar to a CountDownLatch, but it can be reused multiple times.

A CyclicBarrier is initialized with a count and a barrier action, which is a Runnable that is executed when the barrier is reached. Each thread that reaches the barrier point calls the await() method, which blocks until all other threads have also called await(). Once the count reaches zero, the barrier action is executed, and the count is reset to its initial value.

Here is an example of how to use a CyclicBarrier to synchronize three threads:

arduinoCopy codeimport java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;

public class Worker implements Runnable {
private final CyclicBarrier barrier;
private final int id;

public Worker(CyclicBarrier barrier, int id) {
this.barrier = barrier;
this.id = id;
}

public void run() {
try {
System.out.println(“Worker ” + id + ” started”);
Thread.sleep(1000); // do some work
System.out.println(“Worker ” + id + ” reached barrier”);
barrier.await(); // wait for all workers to reach the barrier
System.out.println(“Worker ” + id + ” finished”);
} catch (InterruptedException | BrokenBarrierException e) {
Thread.currentThread().interrupt();
}
}

public static void main(String[] args) {
int nWorkers = 3;
CyclicBarrier barrier = new CyclicBarrier(nWorkers, () -> System.out.println(“All workers reached barrier”));
for (int i = 0; i < nWorkers; i++) {
new Thread(new Worker(barrier, i)).start();
}
}
}

In this example, the Worker class implements the Runnable interface and waits for the barrier signal before doing some work and signaling that it has finished. The main() method creates three Worker instances and starts them in separate threads. Once all three workers have reached the barrier point, the barrier action is executed and all workers are released.

Using CyclicBarrier can make it easier to synchronize multiple threads that need to perform a set of operations in a specific order and want to wait for each other to reach a common barrier point.

20.What is a CountDownLatch?

A CountDownLatch is a synchronization aid that allows one or more threads to wait for a set of operations to complete before proceeding. It is similar to a barrier, but it can be used only once, whereas a barrier can be reused.

A CountDownLatch is initialized with a count, and each await() call blocks until the count reaches zero. The count can be decremented by calling the countDown() method, which signals that an operation has completed. Once the count reaches zero, all waiting threads are released.

Here is an example of how to use a CountDownLatch to synchronize two threads:

//java code
//
//import java.util.concurrent.CountDownLatch;
//
//public class Worker implements Runnable {
//    private final CountDownLatch startLatch;
//    private final CountDownLatch endLatch;
//    private final int id;
//
//    public Worker(CountDownLatch startLatch, CountDownLatch endLatch, int id) {
//        this.startLatch = startLatch;
//        this.endLatch = endLatch;
//        this.id = id;
//    }
//
//    public void run() {
//        try {
//            startLatch.await(); // wait for the start signal
//            System.out.println("Worker " + id + " started");
//            Thread.sleep(1000); // do some work
//            System.out.println("Worker " + id + " finished");
//        } catch (InterruptedException e) {
//            Thread.currentThread().interrupt();
//        } finally {
//            endLatch.countDown(); // signal that this worker has finished
//        }
//    }
//
//    public static void main(String[] args) throws InterruptedException {
//        int nWorkers = 2;
//        CountDownLatch startLatch = new CountDownLatch(1);
//        CountDownLatch endLatch = new CountDownLatch(nWorkers);
//        for (int i = 0; i < nWorkers; i++) {
//            new Thread(new Worker(startLatch, endLatch, i)).start();
//        }
//        Thread.sleep(5000); // wait for some time
//        startLatch.countDown(); // start the workers
//        endLatch.await(); // wait for all workers to finish
//        System.out.println("All workers finished");
//    }
//}
//
//
//In this example, the Worker class implements the Runnable interface and waits for the startLatch signal before doing some work and signaling that it has finished using the endLatch. The main() method creates two Worker instances and starts them in separate threads. It waits for some time and then signals the startLatch to start the workers. Finally, it waits for all workers to finish using the endLatch.

Top 20 Multithreading Java Questions and Answers -Part 1

1. What is multithreading in Java?

Multithreading in Java is the ability of a program to have multiple threads of execution running concurrently within the same process. A thread is a lightweight sub-process that can perform a task independently of other threads, and allows for the simultaneous execution of multiple parts of a program. In Java, multithreading is implemented through the java.lang.Thread class and the java.lang.Runnable interface. Multithreading can be used to improve the performance of an application, allowing for more efficient use of system resources, as well as providing a means of asynchronous execution of code.

2.What are the benefits of multithreading?

There are several benefits of multithreading in Java, including:

1. Improved performance: Multithreading allows for the simultaneous execution of multiple parts of a program, which can improve the overall performance of the program.
2. Efficient use of system resources: Multithreading allows for more efficient use of system resources, such as CPU time and memory, by allowing multiple threads to share resources.
3. Responsiveness: Multithreading can improve the responsiveness of a program, as it allows the user interface to remain active while other tasks are being performed in the background.
4. Asynchronous processing: Multithreading allows for asynchronous processing of code, which can be useful for long-running tasks, such as network operations or file I/O.
5. Simplified programming: Multithreading can simplify programming by allowing for the separation of different tasks into separate threads, making it easier to understand and maintain the code.

Overall, multithreading provides a way to optimize the performance of Java programs and take advantage of modern multi-core CPUs to achieve faster and more efficient processing.

3.What is a thread in Java?

In Java, a thread is a lightweight sub-process that can be executed independently of other threads within the same process. A thread consists of a sequence of instructions and has its own stack and program counter, allowing it to execute code independently of other threads.

Every Java program has at least one thread, which is called the “main” thread. Additional threads can be created by either extending the java.lang.Thread class or implementing the java.lang.Runnable interface. When a thread is created, it is started by calling the start() method, which causes the run() method to be executed in a separate thread of execution.

Threads can be used to perform tasks concurrently, allowing for more efficient use of system resources, as well as providing a means of asynchronous execution of code. However, when multiple threads access shared resources, such as memory or files, it can result in synchronization issues, such as race conditions or deadlocks. Proper synchronization mechanisms, such as locks or semaphores, must be used to ensure thread safety and avoid these issues.

4.How can you create a thread in Java?

In Java, there are two ways to create a thread:

1. Extending the Thread class: You can create a new class that extends the java.lang.Thread class and override the run() method to define the thread’s behavior. For example:

java code

public class MyThread extends Thread {
    public void run() {
        // thread behavior here
    }
}

// create and start a new thread
MyThread myThread = new MyThread();
myThread.start();

2. Implementing the Runnable interface: You can also create a new class that implements the java.lang.Runnable interface and define the thread’s behavior in the run() method. Then, you can create a new instance of the class and pass it as an argument to a new Thread object. For example:

javaCopy codepublic class MyRunnable implements Runnable {
    public void run() {
        // thread behavior here
    }
}

// create a new thread and start it
MyRunnable myRunnable = new MyRunnable();
Thread myThread = new Thread(myRunnable);
myThread.start();

Both approaches will create a new thread and start its execution. Note that the second approach is generally preferred as it allows for more flexibility in the implementation of the thread behavior, as the same runnable instance can be used to create multiple threads, if desired.

5.What is the difference between thread and process?

In Java, a process is an instance of a program that is executed by the operating system, while a thread is a lightweight sub-process that is executed within the context of a process. Here are some key differences between threads and processes:

1. Resource sharing: Threads share resources with other threads within the same process, while processes do not share resources with other processes.
2. Communication: Threads can communicate with each other directly through shared memory, while communication between processes requires more complex mechanisms, such as inter-process communication (IPC).
3. Overhead: Creating a new thread has less overhead than creating a new process, as creating a new process requires duplicating the parent process and its resources.
4. Scheduling: Scheduling threads is generally faster and more efficient than scheduling processes, as threads share the same memory space and can be easily switched between.
5. Protection: Processes are isolated from each other and cannot interfere with each other’s memory, while threads within the same process share memory and can potentially interfere with each other’s execution.

Overall, threads and processes are both used for concurrent execution of code, but they have different characteristics and are used in different situations. Processes are generally used for more heavyweight tasks that require isolation and protection, while threads are used for lighter-weight tasks that require more efficient communication and sharing of resources within the same process.

6.What is a runnable interface?

The Runnable interface in Java is a functional interface that represents a task or block of code that can be executed by a thread. It is used to define the behavior of a thread when it is executed and is an alternative way of creating a thread, compared to extending the Thread class.

The Runnable interface has only one method, called “run()”, which contains the code that will be executed when the thread is started. This method has no arguments and returns no values.

Here is an example of how to use the Runnable interface to create and start a thread:

java code

public class MyRunnable implements Runnable {
    public void run() {
        // code to be executed by the thread
    }
}

// create a new instance of the MyRunnable class
MyRunnable myRunnable = new MyRunnable();

// create a new thread and start it, passing in the MyRunnable instance as an argument
Thread thread = new Thread(myRunnable);
thread.start();

Note that implementing the Runnable interface is a more flexible way to define the behavior of a thread, as it allows you to separate the thread logic from the thread’s execution environment, and can be used to pass data or parameters to the thread, if needed.

7.What is a thread pool?

A thread pool in Java is a collection of pre-initialized, reusable threads that are used to execute a large number of small tasks, instead of creating a new thread for each task. The thread pool provides a way to manage and limit the number of threads that are created and used, which can improve the performance and scalability of the application.

When a new task is submitted to the thread pool, it is assigned to an available thread from the pool, which executes the task and then becomes available again for another task. If all the threads in the pool are busy, the task is added to a queue and waits for a thread to become available.

Thread pools are typically used in applications that need to handle a large number of short-lived tasks, such as web servers or network servers, as creating and destroying threads for each request can be very expensive in terms of memory and CPU usage. By using a thread pool, the overhead of creating and destroying threads is reduced, and the overall performance of the application can be improved.

In Java, thread pools can be created using the java.util.concurrent.ExecutorService interface and its implementations, such as the ThreadPoolExecutor class. The ExecutorService provides a way to submit tasks to the pool, manage the threads in the pool, and control the size of the pool.

8.What is synchronization in multithreading?

Synchronization in multithreading refers to the coordination of multiple threads to ensure that they access shared resources in a safe and consistent manner. In Java, synchronization is achieved through the use of monitors, which are objects that provide mutual exclusion and coordination between threads.

The main purpose of synchronization is to prevent multiple threads from accessing the same shared resource at the same time, which can lead to inconsistent or incorrect results, also known as a race condition. By synchronizing access to shared resources, only one thread can access the resource at a time, ensuring that the data is consistent and avoiding race conditions.

In Java, synchronization is implemented using the synchronized keyword. When a method or block of code is declared as synchronized, only one thread at a time can execute that code, and all other threads that try to access the same code block will be blocked until the executing thread completes.

For example, suppose we have a shared counter variable that is accessed by multiple threads:

csharp code

public class Counter {
    private int count = 0;

    public void increment() {
        count++;
    }

    public int getCount() {
        return count;
    }
}

To ensure that the increment() method is synchronized and only one thread can access it at a time, we can add the synchronized keyword to the method declaration:

public synchronized void increment() {
    count++;
}

This will ensure that only one thread can execute the increment() method at a time, preventing race conditions and ensuring consistent updates to the count variable.

9.What is deadlock in multithreading?

Deadlock in multithreading is a situation where two or more threads are blocked, waiting for each other to release a resource that they hold, resulting in a deadlock state where none of the threads can proceed. In other words, each thread is waiting for the other thread to release a resource, while holding a resource that the other thread needs to proceed.

Deadlock is a serious problem in multithreaded applications and can cause the application to hang or become unresponsive. Deadlock can occur when two or more threads compete for the same set of resources, and each thread holds a resource that is needed by the other thread to proceed.

Here is an example of a simple deadlock scenario involving two threads and two resources:

csharp code

Thread 1:
- Acquires lock A
- Waits for lock B

Thread 2:
- Acquires lock B
- Waits for lock A

In this scenario, both threads acquire one lock each but then wait for the other lock to be released before proceeding. Since neither thread can release the lock it holds, both threads are stuck in a deadlock state and cannot make progress.

To prevent deadlocks in multithreaded applications, it is important to ensure that threads do not hold onto resources for longer than necessary and that they release resources as soon as they are no longer needed. This can be achieved by careful design of the application, using appropriate synchronization mechanisms, and avoiding unnecessary locking or blocking operations.

10.What is the purpose of the wait() and notify() methods?

The wait() and notify() methods are synchronization primitives in Java that are used to coordinate the execution of threads that share the same resource. The purpose of these methods is to allow a thread to wait for a specific condition to be met before proceeding, while allowing other threads to continue executing.

The wait() method is called on an object when a thread needs to wait for a specific condition to be met. When the wait() method is called, the thread releases the lock on the object and waits until another thread notifies it that the condition has been met. The wait() method must be called from within a synchronized block or method, and it can only be called on an object that the thread owns the monitor of.

The notify() method is called on the same object by another thread to wake up the waiting thread and signal that the condition has been met. When the notify() method is called, it wakes up one of the threads that are waiting on the object, and that thread then re-acquires the lock and continues executing. The notify() method does not release the lock on the object, so the calling thread must release the lock by exiting the synchronized block or method.

Together, the wait() and notify() methods provide a way for threads to synchronize their access to shared resources and coordinate their execution. They are often used in conjunction with a shared condition variable, which is used to indicate the state of the resource being shared between threads.

For example, suppose we have a shared queue that is accessed by multiple threads:

arduino code

public class SharedQueue {
    private List<String> queue = new ArrayList<>();

    public synchronized void enqueue(String item) {
        queue.add(item);
        notify();
    }

    public synchronized String dequeue() throws InterruptedException {
        while (queue.isEmpty()) {
            wait();
        }
        return queue.remove(0);
    }
}

In this example, the enqueue() method adds an item to the queue and notifies any waiting threads that the queue is no longer empty. The dequeue() method waits until the queue is not empty before removing and returning the first item in the queue. If the queue is empty, the thread calls the wait() method, which releases the lock on the object and waits until another thread notifies it that the queue is no longer empty. When an item is added to the queue by another thread, the notify() method is called to wake up the waiting thread and allow it to continue executing.

11.What is the join() method?

The join() method in Java is a synchronization mechanism that allows one thread to wait for the completion of another thread. When a thread calls the join() method on another thread, it suspends its own execution and waits for the other thread to complete before continuing.

The join() method takes no arguments or a timeout value and returns only when the thread being joined has completed or when the timeout period has elapsed. If the timeout period elapses before the joined thread completes, the join() method returns and the calling thread can continue executing.

Here is an example that illustrates how to use the join() method to wait for another thread to complete:

csharp code

public class JoinExample {
    public static void main(String[] args) throws InterruptedException {
        Thread t1 = new Thread(() -> {
            System.out.println("Thread 1 started");
            try {
                Thread.sleep(2000); // Sleep for 2 seconds
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("Thread 1 finished");
        });

        Thread t2 = new Thread(() -> {
            System.out.println("Thread 2 started");
            try {
                Thread.sleep(1000); // Sleep for 1 second
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("Thread 2 finished");
        });

        t1.start();
        t2.start();

        t1.join(); // Wait for t1 to complete
        System.out.println("Main thread resumed after t1 completed");

        t2.join(); // Wait for t2 to complete
        System.out.println("Main thread resumed after t2 completed");
    }
}

In this example, two threads (t1 and t2) are started and then the main thread waits for them to complete by calling the join() method on each thread. The output of the program shows that the main thread waits for t1 to complete before continuing, and then waits for t2 to complete before exiting.

The join() method is useful in situations where the result of a computation in one thread is needed before another thread can proceed, or where the order of execution of threads needs to be controlled.

12.What is a daemon thread?

In Java, a daemon thread is a thread that runs in the background and provides support to other non-daemon threads. Daemon threads are commonly used for tasks such as garbage collection, memory management, or I/O event handling, where they perform work that is not critical to the application’s main functionality.

When a Java application is started, it creates one or more threads to execute its main code. These threads are called non-daemon threads because they run in the foreground and the application will not exit until all of them have completed. However, the JVM also creates some additional threads that are marked as daemon threads, such as the garbage collector and finalizer threads.

Daemon threads have a lower priority than non-daemon threads, which means that they are less likely to interfere with the application’s main functionality. If all non-daemon threads have completed, the JVM will exit immediately, even if there are still daemon threads running. This can be useful in cases where an application needs to perform some background tasks that do not need to be completed before the application can exit.

To create a daemon thread in Java, you can call the setDaemon() method on the Thread object before starting the thread. For example:

java code

Thread myThread = new Thread(new MyRunnable());
myThread.setDaemon(true); // Mark the thread as a daemon thread
myThread.start();

In this example, the MyRunnable class implements the Runnable interface and defines the code to be executed by the thread. The setDaemon() method is called to mark the thread as a daemon thread before it is started with the start() method.

It is important to note that once a thread is started, you cannot change its daemon status. Also, if a daemon thread creates another thread, the new thread is not automatically a daemon thread. You must explicitly set the new thread’s daemon status if necessary.

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In Java, a race condition occurs when two or more threads access a shared resource or variable simultaneously and try to modify its value at the same time, resulting in unpredictable and erroneous behavior. Race conditions can occur when the ordering of events between threads is not controlled or guaranteed, and can lead to incorrect results or even program crashes.

For example, consider a situation where two threads are trying to increment a shared variable called counter at the same time:

class Counter {
    int counter = 0;
    
    void increment() {
        counter++;
    }
}

class MyThread implements Runnable {
    Counter counter;

    public MyThread(Counter counter) {
        this.counter = counter;
    }

    public void run() {
        for (int i = 0; i < 100000; i++) {
            counter.increment();
        }
    }
}

public class Main {
    public static void main(String[] args) throws InterruptedException {
        Counter counter = new Counter();
        Thread t1 = new Thread(new MyThread(counter));
        Thread t2 = new Thread(new MyThread(counter));
        t1.start();
        t2.start();
        t1.join();
        t2.join();
        System.out.println("Counter value: " + counter.counter);
    }
}

In this example, two threads are created and each thread calls the increment() method on a shared Counter object. If the threads execute concurrently, the counter value may not be incremented correctly, leading to incorrect results.

To avoid race conditions in Java, you can use synchronization mechanisms such as locks, semaphores, or atomic variables to ensure that only one thread can access a shared resource at a time. Synchronization ensures that the code block or method is executed atomically, which means that no other thread can access the shared resource while the first thread is using it.

Here is an example of using synchronization to avoid race conditions:

class Counter {
    int counter = 0;
    
    synchronized void increment() {
        counter++;
    }
}

In this example, the increment() method is marked as synchronized, which ensures that only one thread can execute the method at a time. This guarantees that the counter value will be incremented correctly and avoids race conditions.