Stdin/Stdout Helper for JUnit

Useful for testing java/kotlin programs interaction with the command line.

How to install

Include the following dependency on your pom file:

<dependency>
    <groupId>org.dropproject</groupId>
    <artifactId>stdin-stdout-junit-helper</artifactId>
    <version>0.4.1</version>
</dependency>

How to use

public class Main {
    public static void main(String args[]) {
        Scanner scanner = new Scanner(System.in);
        
        System.out.print("Enter your name");
        String name = scanner.nextLine();
        System.out.println("Your name is " + name);
        
        scanner.close();
    }
}


class TestStdinStdout {

    @Test
    public void testAskName() {
        StdinStdoutHelper helper = new StdinStdoutHelper()
                .expectOutput("Enter your name")
                .simulateInput("Pedro")
                .expectOutput("Your name is Pedro");

        helper.start();
        main(null);
        helper.stop();
    }
}

Contextual information on failure

In highly interactive programs, it can be difficult to understand what happened on certain test failures. You can pass a showDetailedErrors and a òutputBufferSize to the StdinStdoutHelper constructor, as in this example:

StdinStdoutHelper helper = new StdinStdoutHelper(true, 20)
                .expectOutput("Enter your name")
                .simulateInput("Pedro")
                .expectOutput("Enter your age")
                .simulateInput("45")
                .expectOutput("Pedro, you're 45 years old!");

This will show the last 20 (or less) lines of the interaction before the failure.

Ignoring output lines

If the program writes several lines to the stdout that you don't want to test, you can use the function expectNNumberOfLines:

StdinStdoutHelper helper = new StdinStdoutHelper()
                .expectOutput("Enter the file name")
                .simulateInput("data.txt")
                .expectOutput("The data contained in the file:")
                .expectNNumberOfLines(57)  // suppose the file has 57 lines
                .expectOutput("Read another file? [Y|N]")
                .simulateInput("N");

Dynamic behaviour

Suppose you have a program that prints a random number to the stdout, and you want to test that the printed number is within a certain range. You can use the matchOutput which receives a predicate function. This function receives the line that was printed to the screen and returns true if it matches the expected value. This is evaluated during the program's execution.

Example in Kotlin

val helper = StdinStdoutHelper()
    .expectOutput("Press enter to generate a random number between 1 and 100")
    .simulateInput("")  // press enter
    .matchOutput { it in 1..100 }