In Java, the synchronized keyword is used to control the access of multiple threads to a shared resource. When a method or block of code is declared as synchronized, only one thread can execute it at a time. This prevents race conditions, which can occur when multiple threads try to access the same resource at the same time.

The synchronized keyword can be used to:

• Lock a method: This prevents other threads from executing the method until the current thread has finished.
• Lock a block of code: This prevents other threads from executing the code in the block until the current thread has finished.
• Lock an object: This prevents other threads from calling any synchronized methods or blocks of code on the object until the current thread has finished.

The synchronized keyword is a powerful tool for preventing race conditions in multithreaded applications. However, it should be used sparingly, as it can also introduce performance overhead.

Here is an example of how the synchronized keyword can be used to lock a method:

Java

public synchronized void incrementCounter() {
  counter++;
}

In this example, the incrementCounter() method is declared as synchronized. This means that only one thread can execute the method at a time. This prevents race conditions from occurring when multiple threads try to increment the counter variable.

Here is an example of how the synchronized keyword can be used to lock a block of code:

Java

public void doSomething() {
  synchronized (this) {
    // Code that needs to be synchronized goes here.
  }
}

In this example, the doSomething() method has a synchronized block. This block of code will only be executed by one thread at a time. This prevents race conditions from occurring when multiple threads try to access the code in the block.