JUnit Java Command Line 

Following example shows how to unit test command line input in Java.

Example

A command line app to test

package com.logicbig.example;

import java.io.InputStream;
import java.io.PrintStream;
import java.util.Scanner;

public class UserInputExample {
    private Scanner scanner;
    private PrintStream printStream;

    public UserInputExample(InputStream inputStream, PrintStream printStream) {
        this.scanner = new Scanner(inputStream);
        this.printStream = printStream;
    }

    public void start() {
        printStream.println("Enter number 1");
        int x = scanner.nextInt();
        printStream.println("Enter number 2");
        int y = scanner.nextInt();
        printStream.println("output: " + (x + y));
    }

    public static void main(String[] args) {
        UserInputExample userInputExample = new UserInputExample(System.in, System.out);
        userInputExample.start();
    }
}

Unit testing by using System.setIn() and System.setOut()

package com.logicbig.example;

import org.junit.Assert;
import org.junit.Test;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.InputStream;
import java.io.PrintStream;

public class UserInputExampleTest {

    @Test
     public void mainTest(){
        InputStream stdin = System.in;
        System.setIn(new ByteArrayInputStream("3\n4\n".getBytes()));

        ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
        PrintStream ps = new PrintStream(byteArrayOutputStream);
        PrintStream stdout = System.out;
        System.setOut(ps);

        UserInputExample.main(new String[0]);

        System.setIn(stdin);
        System.setOut(stdout);

        String outputText = byteArrayOutputStream.toString();
        String key = "output:";
        String output = outputText.substring(outputText.indexOf(key) + key.length()).trim();
        Assert.assertEquals(output, "7");
     }
}

D:\java-test-user-command-line-input>mvn -q test -Dtest=UserInputExampleTest.java

-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running com.logicbig.example.UserInputExampleTest
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.058 sec

Results :

Tests run: 1, Failures: 0, Errors: 0, Skipped: 0

Unit testing by providing InputStream and PrintStream

package com.logicbig.example;

import org.junit.Assert;
import org.junit.Test;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.PrintStream;

public class UserInputExampleTest2 {

    @Test
    public void userInputTest() {
        ByteArrayInputStream inputStream = new ByteArrayInputStream("3\n4\n".getBytes());
        ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
        PrintStream ps = new PrintStream(byteArrayOutputStream);
        UserInputExample userInputExample = new UserInputExample(inputStream, ps);
        userInputExample.start();
        String outputText = byteArrayOutputStream.toString();
        String key = "output:";
        String output = outputText.substring(outputText.indexOf(key) + key.length()).trim();
        Assert.assertEquals(output, "7");
    }
}

D:\java-test-user-command-line-input>mvn -q test -Dtest=UserInputExampleTest2.java

-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running com.logicbig.example.UserInputExampleTest2
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.051 sec

Results :

Tests run: 1, Failures: 0, Errors: 0, Skipped: 0

Example Project

Dependencies and Technologies Used:

• junit 4.12: JUnit is a unit testing framework for Java, created by Erich Gamma and Kent Beck.
• JDK 1.8
• Maven 3.6.3