How to Mock System.in and System.out for Java Unit Tests

Mocking System.in and System.out in Java Unit Tests

When a unit test depends on user input via Scanner scanner = new Scanner(System.in), you can replace System.in and System.out with in‑memory streams to simulate input and capture output without manual interaction.

public MockInOut(String input) {
    orig = System.out;
    irig = System.in;

    os = new ByteArrayOutputStream();
    try {
        System.setOut(new PrintStream(os, false, charset.name()));
    } catch (UnsupportedEncodingException ex) {
        throw new RuntimeException(ex);
    }

    is = new ByteArrayInputStream(input.getBytes());
    System.setIn(is);
}

The constructor saves the original System.out and System.in, redirects System.out to a ByteArrayOutputStream, and feeds the supplied input string to a ByteArrayInputStream. After this setup, calls such as scanner.nextLine() read the prepared input instantly, and any calls to System.out.println write to the buffer instead of the console.

public void close() {
    os = null;
    is = null;
    System.setOut(orig);
    System.setIn(irig);
}

Calling close() restores the original streams, allowing the test environment to return to its normal state.

import java.io.*;
import java.util.*;

class HelloWorld {
    public static void main(String[] args) throws IOException {
        PrintStream orig = System.out;
        ByteArrayOutputStream os = new ByteArrayOutputStream();
        System.setOut(new PrintStream(os, false, "UTF-8"));
        // No console output here
        for (int i = 0; i < 2; i++) {
            System.out.println("FunTester" + i);
        }
        System.setOut(orig);
        // Print the captured output
        output("Collected output: " + os.toString("UTF-8"));

        InputStream is = System.in;
        System.setIn(new ByteArrayInputStream("FunTester
FunTester
".getBytes()));
        Scanner scanner = new Scanner(System.in);
        // Output all captured input lines
        output(scanner.nextLine());
        output(scanner.nextLine());
        try {
            // This will throw because there is no more input
            System.out.println(scanner.nextLine());
        } catch (NoSuchElementException e) {
            e.printStackTrace();
        }

        System.setIn(is);
        scanner = new Scanner(System.in);
        // This will wait for user input
        System.out.println("Input: " + scanner.nextLine());
    }
}

This simple approach demonstrates how injecting and swapping streams can isolate I/O dependencies in unit tests, allowing you to focus on the logic under test. More sophisticated mocking frameworks exist, but the presented technique is lightweight and easy to integrate.