Applicability and examples

The applicability and examples of the command pattern are as follows:

• Undo/redo operations: The command pattern allows us to store the command object in a queue. This way, we can implement undo and redo operations.
• Composite commands: Complex commands can be composed of simple commands using the composite pattern, and are run in a sequential order. In this way, we can build macros in an object-oriented-design manner.
• The asynchronous method invocation: The command pattern is used in multithreading applications. Command objects can be executed in the background in separate threads. The java.lang.Runnable is a command interface.

In the following code, the runnable interface acts as a command interface, and is implemented by RunnableThread:

class RunnableThread implements Runnable
{
  public void run() 
  {
    // the command implementation code
  }
}

The client invokes the command to start a new thread:

public class ClientThread 
{
  public static void main(String a[])
  {
    RunnableThread mrt = new RunnableThread();
    Thread t = new Thread(mrt);
    t.start();
  }
}