Design Pattern: Strategy Pattern in Java

Strategy pattern is used when there are multiple algorithms available for the same purpose, and we need to decide between those algorithms. Strategy pattern helps to decide the algorithm and return the generated object to be used.

Check Details of Strategy Pattern

This article demonstrates Strategy pattern implementations in Java. Check the following examples.

Examples

Here are some examples of Strategy pattern implementation-

Example #1: Transport

Let’s take the example of a transport system. Take a look at class diagram.

Transport Interface

// Transport.java

package com.bigboxcode;

public interface Transport {
    void start();

    void stop();

    void repair();

    void getInfo();

    void operate();
}

Transport Implementation for Bike

// Bike.java

package com.bigboxcode;

public class Bike implements Transport{

    @Override
    public void start() {
        System.out.println("Bike started");
    }

    @Override
    public void stop() {
        System.out.println("Bike stopped");
    }

    @Override
    public void repair() {
        System.out.println("Bike repair");
    }

    @Override
    public void getInfo() {
        System.out.println("Transport type: Bike");
        System.out.println("Number of wheels: 2");
        System.out.println("Average Weight: 700 Pounds");
    }

    @Override
    public void operate() {
        System.out.println("Riding Bike ............");
    }
}

Transport Implementation for Car

// Car.java

package com.bigboxcode;

public class Car implements Transport{
    @Override
    public void start() {
        System.out.println("Car started");
    }

    @Override
    public void stop() {
        System.out.println("Car stopped");
    }

    @Override
    public void repair() {
        System.out.println("Car repair");
    }

    @Override
    public void getInfo() {
        System.out.println("Transport type: Car");
        System.out.println("Number of wheels: 4");
        System.out.println("Average Weight: 4,000 Pounds");
    }

    @Override
    public void operate() {
        System.out.println("Driving car ............");
    }
}

Transport Implementation for Plane

// Plane.java

package com.bigboxcode;

public class Plane implements Transport{

    @Override
    public void start() {
        System.out.println("Plane started");
    }

    @Override
    public void stop() {
        System.out.println("Plane stopped");
    }

    @Override
    public void repair() {
        System.out.println("Plane repair");
    }

    @Override
    public void getInfo() {
        System.out.println("Transport type: Plane");
        System.out.println("Number of wheels: 3");
        System.out.println("Average Weight: 50,000 Pounds");
    }

    @Override
    public void operate() {
        System.out.println("Flying plane ............");
    }
}

Transport Strategy Class

// TransportStrategy.java

package com.bigboxcode;

public class TransportStrategy {
    private final Transport transport;

    public TransportStrategy(Transport transport) {
        this.transport = transport;
    }

    public void execute() {
        this.transport.start();

        this.transport.getInfo();

        this.transport.operate();
    }

    public void repair() {
        this.transport.repair();
    }

    public void stop() {
        this.transport.stop();
    }
}

Demo

// Main.java

package com.bigboxcode;

public class Main {
    public static void main(String[] args) {
        // Use bike
        System.out.println("Operating Bike:");

        TransportStrategy myTransport = new TransportStrategy(new Bike());
        myTransport.execute();
        myTransport.stop();

        // Use car
        System.out.println("\n\nOperating Car:");

        myTransport = new TransportStrategy(new Car());
        myTransport.execute();
        myTransport.stop();
        myTransport.repair();

        // Use plane
        System.out.println("\n\nOperating plane:");

        myTransport = new TransportStrategy(new Plane());
        myTransport.execute();
        myTransport.stop();


    }
}

Output

Operating Bike:
Bike started
Transport type: Bike
Number of wheels: 2
Average Weight: 700 Pounds
Riding Bike ............
Bike stopped


Operating Car:
Car started
Transport type: Car
Number of wheels: 4
Average Weight: 4,000 Pounds
Driving car ............
Car stopped
Car repair


Operating plane:
Plane started
Transport type: Plane
Number of wheels: 3
Average Weight: 50,000 Pounds
Flying plane ............
Plane stopped

Example #2: Mathematical Calculations

Let’s take a look at another example. In this example, we will demonstrate how Strategy pattern can be used while developing functionality for a calculator. we are just considering basic calculations here, the same steps can be used to implement more complex calculations.

Calculation Interface

// Calculation.java

package com.BigBoxCode;

public interface Calculation {
    float execute(float num1, float num2);
}

Calculation Implementation for Add

// Add.java

package com.BigBoxCode;

public class Add implements Calculation {
    @Override
    public float execute(float num1, float num2) {
        return num1 + num2;
    }
}

Calculation Implementation for Subtract

// Subtract.java

package com.BigBoxCode;

public class Subtract implements Calculation {
    @Override
    public float execute(float num1, float num2) {
        return num1 - num2;
    }
}

Calculation Implementation for Multiply

// Multiply.java

package com.BigBoxCode;

public class Multiply implements Calculation {
    @Override
    public float execute(float num1, float num2) {
        return num1 * num2;
    }
}

Calculation Implementation for Divide

// Divide.java

package com.BigBoxCode;

public class Divide implements Calculation {
    @Override
    public float execute(float num1, float num2) {
        return num1 / num2;
    }
}

Calculation Strategy Class

// CalculationStrategy.java

package com.BigBoxCode;

public class CalculationStrategy {
    private final Calculation calculation;

    public CalculationStrategy(Calculation calculation) {
        this.calculation = calculation;
    }

    public float execute(float num1, float num2) {
        return this.calculation.execute(num1, num2);
    }
}

Demo

// Main.java

package com.BigBoxCode;

public class Main {
    public static void main(String[] args) {
        // Perform Addition
        System.out.println("Performing operation: ADD");

        CalculationStrategy myCalculation = new CalculationStrategy(new Add());
        float result = myCalculation.execute(10, 5);
        System.out.println("10 + 5 = " + result);

        // Perform Subtraction
        System.out.println("\n\nPerforming operation: SUBTRACT");

        myCalculation = new CalculationStrategy(new Subtract());
        result = myCalculation.execute(10, 5);
        System.out.println("10 - 5 = " + result);

        // Perform Multiplication
        System.out.println("\n\nPerforming operation: MULTIPLY");

         myCalculation = new CalculationStrategy(new Multiply());
        result = myCalculation.execute(10, 5);
        System.out.println("10 * 5 = " + result);

        // Perform Division
        System.out.println("\n\nPerforming operation: DIVIDE");

        myCalculation = new CalculationStrategy(new Divide());
        result = myCalculation.execute(10, 5);

        System.out.println("10 / 5 = " + result);
    }
}

Result

Performing operation: ADD
10 + 5 = 15.0


Performing operation: SUBTRACT
10 - 5 = 5.0


Performing operation: MULTIPLY
10 * 5 = 50.0


Performing operation: DIVIDE
10 / 5 = 2.0

Example #3: File Storage

Define Storage Interface

// Storage.java

package com.bigboxcode;

public interface Storage {
    int storeFile(String tempPath);

    String retrieveFile(int fileId);

    void printFileInfo(int fileId);
}

Implement Storage Interface for Local Storage

// LocalStorage.java

package com.bigboxcode;

import java.util.Random;

public class LocalStorage implements Storage {
    @Override
    public int storeFile(String tempPath) {
        // Code to store file here

        // return the ID that is obtained from the database record or any other unique identifier.

        // For demo purpose a random number is returned here
        return (new Random()).nextInt(100);
    }

    @Override
    public String retrieveFile(int fileId) {
        // Retrieve the url and return back

        // Some dummy url is returned for demo purpose
        return "https://bigboxcode.com/files/local/" + fileId;
    }

    @Override
    public void printFileInfo(int fileId) {
        System.out.println("Storage type: Local Storage");
        System.out.println("File ID: " + fileId);
        System.out.println("File URL: " + this.retrieveFile(fileId));
    }
}

Implement Storage Interface for Google Drive

// GoogleDriveStorage.java

package com.bigboxcode;

import java.util.Random;

public class GoogleDriveStorage implements Storage {
    @Override
    public int storeFile(String tempPath) {
        // Code to store file here

        // return the ID that is obtained from the database record or any other unique identifier.

        // For demo purpose a random number is returned here
        return (new Random()).nextInt(1000);
    }

    @Override
    public String retrieveFile(int fileId) {
        // Retrieve the url and return back

        // Some dummy url is returned for demo purpose
        return "https://drive.google.com/file/d/1234_9K7654hu6RT_9j7JKY3fK/view";
    }
    @Override
    public void printFileInfo(int fileId) {
        System.out.println("Storage type: Google Drive");
        System.out.println("File ID: " + fileId);
        System.out.println("File URL: " + this.retrieveFile(fileId));
    }
}

Implement Storage Interface for AWS S3

// S3Storage.java

package com.bigboxcode;

import java.util.Random;

public class S3Storage implements Storage {
    @Override
    public int storeFile(String tempPath) {
        // Code to store file here

        // return the ID that is obtained from the database record or any other unique identifier.

        // For demo purpose a random number is returned here
        return (new Random()).nextInt(10000);
    }

    @Override
    public String retrieveFile(int fileId) {
        // Retrieve the url and return back

        // Some dummy url is returned for demo purpose
        return "https://bigboxcode.s3.amazonaws.com/pdf/UC-0e7654338-5697-4f99-b33-d89h87g5gf4gwfg.pdf";
    }

    @Override
    public void printFileInfo(int fileId) {
        System.out.println("Storage type: AWS S3");
        System.out.println("File ID: " + fileId);
        System.out.println("File URL: " + this.retrieveFile(fileId));
    }
}

Storage Strategy Class

// StorageStrategy.java

package com.bigboxcode;

public class StorageStrategy {
    private Storage storage;

    public StorageStrategy(Storage storage) {
        this.storage = storage;
    }

    public int uploadFile(String tempPath) {
        int fileId = this.storage.storeFile(tempPath);
        this.storage.printFileInfo(fileId);

        return fileId;
    }

    public String getFileUrl(int fileId) {
        return this.storage.retrieveFile(fileId);
    }
}

Demo

// Main.java

package com.bigboxcode;

public class Main {
    public static void main(String[] args) {
        // Use Local storage
        System.out.println("Using local storage:");

        StorageStrategy fileStorage = new StorageStrategy(new LocalStorage());
        fileStorage.uploadFile("/some-temp-path");

        // Use car
        System.out.println("\n\nUsing AWS S3:");

        fileStorage = new StorageStrategy(new S3Storage());
        fileStorage.uploadFile("/some-temp-path");

        // Use plane
        System.out.println("\n\nUsing Google Drive:");

        fileStorage = new StorageStrategy(new GoogleDriveStorage());
        fileStorage.uploadFile("/some-temp-path");

    }
}

Output

Using local storage:
Storage type: Local Storage
File ID: 98
File URL: https://bigboxcode.com/files/local/98


Using AWS S3:
Storage type: AWS S3
File ID: 4905
File URL: https://bigboxcode.s3.amazonaws.com/pdf/UC-0e7654338-5697-4f99-b33-d89h87g5gf4gwfg.pdf


Using Google Drive:
Storage type: Google Drive
File ID: 736
File URL: https://drive.google.com/file/d/1234_9K7654hu6RT_9j7JKY3fK/view