Java policy pattern

In strategy pattern, the behavior of a class or its algorithm can be changed at run time. This type of design pattern belongs to behavioral pattern.

In the policy pattern, we create objects representing various policies and a context object whose behavior changes with the change of policy objects. The policy object changes the execution algorithm of the context object.

introduce

Intent: define a series of algorithms, encapsulate them one by one, and make them replaceable.

Main solution: when there are many similar algorithms, it is complex and difficult to maintain when using if...else.

When to use: a system has many classes, and what distinguishes them is their direct behavior.

How to solve it: encapsulate these algorithms into classes one by one and replace them arbitrarily.

Key code: implement the same interface.

Application example:   ① Zhuge Liang's clever plan, each bag is a strategy. ②   The way of travel, choose to ride a bike or take a car. Each way of travel is a strategy. ③ LayoutManager in JAVA AWT.

advantage:   ①   The algorithm can be switched freely.   ② Avoid using multiple conditional judgments. ③ Good scalability.

Disadvantages:   ① Policy classes will increase. ② All policy classes need to be exposed.

Usage scenario:   ① If there are many classes in a system, and the only difference between them is their behavior, then using the policy pattern can dynamically let an object choose one behavior among many behaviors. ② A system needs to dynamically choose one of several algorithms. ③ If an object has a lot of behaviors, without appropriate patterns, these behaviors have to be implemented by multiple conditional selection statements.

Note: if a system has more than four policies, you need to consider using mixed mode to solve the problem of policy class expansion.

realization

We will create a Strategy interface that defines the activity and an entity policy class that implements the Strategy interface. Context is a class that uses a certain policy.

StrategyPatternDemo, our presentation class uses Context and policy objects to demonstrate the behavior change of Context when the policy it configures or uses changes.

Step 1

Create an interface.

public interface Strategy {
   public int doOperation(int num1, int num2);
}
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Step 2

Create an entity class that implements the interface.

public class OperationAdd implements Strategy{
   @Override
   public int doOperation(int num1, int num2) {
      return num1 + num2;
   }
}
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public class OperationSubtract implements Strategy{
   @Override
   public int doOperation(int num1, int num2) {
      return num1 - num2;
   }
}
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public class OperationMultiply implements Strategy{
   @Override
   public int doOperation(int num1, int num2) {
      return num1 * num2;
   }
}
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Step 3

Create a Context class.

public class Context {
   private Strategy strategy;
 
   public Context(Strategy strategy){
      this.strategy = strategy;
   }
 
   public int executeStrategy(int num1, int num2){
      return strategy.doOperation(num1, num2);
   }
}
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Step 4

Use Context to see the behavior changes when it changes the Strategy.

public class StrategyPatternDemo {
   public static void main(String[] args) {
      Context context = new Context(new OperationAdd());    
      System.out.println("10 + 5 = " + context.executeStrategy(10, 5));
 
      context = new Context(new OperationSubtract());      
      System.out.println("10 - 5 = " + context.executeStrategy(10, 5));
 
      context = new Context(new OperationMultiply());    
      System.out.println("10 * 5 = " + context.executeStrategy(10, 5));
   }
}
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Step 5

Execute the program and output the results:

10 + 5 = 15
10 - 5 = 5
10 * 5 = 50