How to Test System Output with New Lines in Java Using assertEquals

How can I test the output of System.out, including new lines, in Java using assertEquals?

OutputCapture outputCapture = new OutputCapture();
outputCapture.start();
System.out.println("Hello, World!\nThis is a new line.");
String capturedOutput = outputCapture.getOutput();
assertEquals("Hello, World!\nThis is a new line.\n", capturedOutput);

Testing the console output in Java can be challenging, especially when dealing with new lines. In this guide, we will explore how to capture and test output from System.out.println using assertEquals from the JUnit framework, making sure to handle new lines correctly.

import org.junit.Test;
import static org.junit.Assert.assertEquals;

public class OutputTest {
    @Test
    public void testPrintOutput() {
        OutputCapture outputCapture = new OutputCapture();
        outputCapture.start();
        System.out.println("Hello, World!\nThis is a new line.");
        String capturedOutput = outputCapture.getOutput();
        assertEquals("Hello, World!\nThis is a new line.\n", capturedOutput);
    }
}

Causes

• New line characters not being processed correctly in assertions.
• Output being captured before the println statement executes.

Solutions

• Use a custom OutputStream to capture System.out output.
• Ensure the output is captured after all print statements.

Mistake: Not accounting for the new line character in the expected output.

Solution: Ensure the expected string in assertEquals includes the '\n' for lines that are expected to break.

Mistake: Capturing output before any print statements execute.

Solution: Always retrieve captured output after your print statements are executed.