Multithreading in Java refers to the ability of a program to perform multiple tasks concurrently, allowing it to take full advantage of modern computer hardware that typically has multiple CPU cores. Multithreading is useful for applications that need to perform multiple tasks simultaneously, such as user interfaces that need to remain responsive while performing background tasks, or servers that need to handle multiple client connections at the same time.

In Java, multithreading can be implemented using the Thread class, which allows you to create and manage threads. Here is a simple example that shows how to create and start a new thread:

class MyThread extends Thread {
    public void run() {
        // Code to be executed in this thread
    }
}

MyThread thread = new MyThread();
thread.start();

When the start method is called, a new thread is created and the run method of the MyThread class is executed in that thread. You can use the Thread.sleep method to introduce a delay in the thread, allowing other threads to execute in the meantime.

Another way to implement multithreading in Java is by using the Runnable interface. This allows you to define the task to be executed in the thread separately from the thread itself, which can make the code more modular and easier to read. Here is an example:

class MyRunnable implements Runnable {
    public void run() {
        // Code to be executed in this thread
    }
}

Thread thread = new Thread(new MyRunnable());
thread.start();

In addition to creating and managing threads, Java provides a number of thread synchronization mechanisms to help prevent conflicts between threads that are accessing shared resources. These include the synchronized keyword, which can be used to create synchronized blocks of code that only one thread can execute at a time, and the wait and notify methods, which can be used to coordinate the actions of multiple threads.

Java also provides a number of higher-level abstractions for multithreading, such as the Executor and ExecutorService interfaces, which allow you to manage a pool of threads that can execute tasks concurrently. The java.util.concurrent package provides a range of other tools for managing threads, such as thread-safe collections and atomic variables.