Strategy Pattern

Intent
Define a family of algorithms(algo1, algo2, ...);
Encapsulate each one, and make them interchangeable.
Strategy lets algorithms vary independently from clients that use it.

Steps
First, you need to define an interface of IStrategy;
Secondly, you need to create a concrete ClientContext Class to execute the operations in the IStrategy.
Thirdly, concrete implementations class about IStrategy.

// 1. IStrategy interface
public interface IStrategy {
   public void operate();
}

//2. ClientContext to execute strategy.
public class ClientContext {
    private IStrategy strategy;
    public ClientContext(IStrategy strategy){
        this.strategy = strategy;
    }

    public void doOperation(){
        this.strategy.operate();        // execute IStrategy method.
    }
}

//3. Concrete strategy class.
public class Strategy1 implements IStrategy{
    public void operate(){
        System.out.println("Strategy 1 is executed.");
    }
}

//4. Concrete strategy class. Of course, you can have many strategy classes.
public class Strategy2 implements IStrategy {
    public void operate(){
        System.out.println("Strategy 2 is executed.");
    }
}

// main 
public class Test {
    public static void main(String[] args){
        ClientContext context;
        context = new ClientContext(new Strategy1());   // execute strategy1
        context.doOperation();  // output: Strategy 1 is executed.

        context = new ClientContext(new Strategy2());  // execute strategy2.
        context.doOperation();  // output: Strategy 2 is executed.
    }
}