How Can You Simulate User Input in JUnit Tests for Command-Line Programs?

Simulating System.in Testing with JUnit

In the realm of software testing, one often faces the challenge of simulating user input when dealing with command-line programs. When a program prompts for input via System.in, how does one automate this behavior in JUnit tests?

Solution

To bypass System.in and inject simulated user input, follow these steps:

1. Establish an Abstraction Layer:
   Avoid directly calling System.in within your code. Introduce an abstraction layer to manage the input source. You can achieve this through dependency injection or by passing an I/O context.

2. Switch System.in Dynamically:
   Use Java 8 streams to manipulate the System.in stream. For instance:

   <code class="java">String data = "Hello, World!\r\n";
   InputStream stdin = System.in;
   try {
       System.setIn(new ByteArrayInputStream(data.getBytes()));
       Scanner scanner = new Scanner(System.in);
       System.out.println(scanner.nextLine());
   } finally {
       System.setIn(stdin);
   }</code>

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