Multithreading is a feature of Java that allows two or more parts of a program to be executed at the same time to maximize the use of the CPU. Each part of this kind of program is called a thread. Therefore, the thread is a lightweight process in process.

Threads can be created by using two mechanisms : 

1. Extending the Thread class 
2. Implementing the Runnable Interface

Thread creation by extending the Thread class

We create a class that extends the java.lang.Thread class. This class overrides the run() method available in the Thread class. The thread starts its life cycle in the run() method. We create an object of a new class and call the start() method to start the execution of the thread. Start() calls the run() method on the Thread object.

Multithreading in Java is a technique that allows multiple threads of execution to run concurrently within a single program. A thread is a lightweight unit of execution that can be scheduled by the operating system to run concurrently with other threads. In Java, the Thread class is used to create and manage threads.

Multithreading is used in Java to improve the performance of applications that perform tasks that can be parallelized, such as performing complex calculations, downloading files, or handling user input. By dividing the work among multiple threads, the application can perform tasks more quickly and efficiently.

To create a thread in Java, you can extend the Thread class or implement the Runnable interface. Once a thread is created, it can be started using the start() method, which will call the run() method of the thread. The run() method contains the code that will be executed by the thread.

Java provides a number of methods for controlling the behavior of threads, such as the sleep() method, which pauses the execution of a thread for a specified period of time, and the join() method, which waits for a thread to complete its execution before continuing with the rest of the program.

However, multithreading can also introduce synchronization and concurrency issues, such as race conditions and deadlocks, which can cause unexpected behavior in the program. To avoid these issues, Java provides synchronization mechanisms such as locks and semaphores, which can be used to coordinate access to shared resources among multiple threads.