Strategy Design Pattern in Java Tutorial with Example


Strategy mode

In the Strategy Pattern, the behavior of a class or its algorithm can be changed at runtime. This type of design pattern is a behavioral model.

In policy mode, we create objects that represent various policies and a context object whose behavior changes as the policy object changes. The policy object changes the execution algorithm of the context object.

Introduction

Intent: Define a series of algorithms, wrap them one by one, and make them interchangeable.

Main solution: In the case of multiple algorithms, the use of if...else is complex and difficult to maintain.

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

How to solve: Wrap these algorithms into a single class and replace them arbitrarily.

Key code: implements the same interface.

Application example: 1, Zhuge Liang's tricks, each kit is a strategy. 2, the way of travel, choose to ride a bicycle, take a car, each type of travel is a strategy. 3. LayoutManager in JAVA AWT.

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

Disadvantages: 1. The strategy category will increase. 2. All strategy classes need to be exposed.

Usage scenarios: 1. If there are many classes in a system, the only difference between them is their behavior, then using a policy pattern can dynamically let an object choose a behavior in many behaviors. 2. A system needs to dynamically select one of several algorithms. 3. If an object has a lot of behavior, if you don't use the proper mode, these behaviors have to be implemented using multiple conditional selection statements.

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

Implementation

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

StrategyPatternDemo, our demo class uses Context and a policy object to demonstrate how the Context changes behavior when the policy it configures or uses changes.

strategy_pattern_uml_diagram

Step 1

Create an interface.

Strategy.java

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

Step 2

Create an entity class that implements the interface.

OperationAdd.java

public class OperationAdd implements Strategy{ @Override public int doOperation(int num1, int num2) { return num1 + num2; } }

OperationSubstract.java

public class OperationSubstract implements Strategy{ @Override public int doOperation(int num1, int num2) { return num1 - num2; } }

OperationMultiply.java

public class OperationMultiply implements Strategy{ @Override public int doOperation(int num1, int num2) { return num1 * num2; } }

Step 3

Create the Context class.

Context.java

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); } }

Step 4

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

StrategyPatternDemo.java

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 OperationSubstract()); System.out.println("10 - 5 = " + context.executeStrategy(10, 5)); context = new Context(new OperationMultiply()); System.out.println("10 * 5 = " + context.executeStrategy(10, 5)); } }

Step 5

Executing the program, outputting results:

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





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