Standard Output redirection in Java for a characterization test

Last weekend I did the UglyTrivia legacy code refactoring kata in Java.

Before starting to refactor, I added a characterization test to describe (characterize) the actual behavior of the original code. Since the only "visible effects" of the code were the lines it was writing to the standard output, I had to use that output to create the characterization test.

This is the code of the characterization test:

	package com.adaptionsoft.games.trivia;
	import static org.junit.Assert.assertEquals;
	import java.io.ByteArrayOutputStream;
	import java.util.Random;
	import org.junit.After;
	import org.junit.Before;
	import org.junit.Test;
	import com.adaptionsoft.games.uglytrivia.Board;
	import com.adaptionsoft.games.uglytrivia.Dice;
	import com.adaptionsoft.games.uglytrivia.Game;
	import com.adaptionsoft.games.uglytrivia.Players;
	import com.adaptionsoft.games.uglytrivia.Questions;
	import com.adaptionsoft.games.uglytrivia.Rules;
	public class RunCharacterizationTest {
	StandardOutputRedirection outputRedirection;
	@Before
	public void setUp() {
	outputRedirection = new StandardOutputRedirection(new ByteArrayOutputStream(), System.out);
	}
	@After
	public void tearDown() {
	outputRedirection.reset();
	}
	@Test
	public void gameCharacterizationForSeed1234() {
	Game game = createGame(1234);
	game.run();
	assertEquals(expectedOutput(), outputRedirection.getRedirectedOutput());
	}
	private Game createGame(int seed) {
	Random rand = new Random(seed);
	Game game = new Game(rand,
	new Players("Chet", "Pat", "Sue"),
	new Rules(), new Questions(),
	new Board(), new Dice(rand));
	return game;
	}
	private String expectedOutput() {
	String expectedOutput = "Chet was added\n";
	expectedOutput += "They are player number 1\n";
	expectedOutput += "Pat was added\n";
	expectedOutput += "They are player number 2\n";
	expectedOutput += "Sue was added\n";
	expectedOutput += "They are player number 3\n";
	expectedOutput += "Chet is the current player\n";
	expectedOutput += "They have rolled a 4\n";
	expectedOutput += "Chet's new location is 4\n";
	expectedOutput += "The category is Pop\n";
	expectedOutput += "Pop Question 0\n";
	expectedOutput += "Answer was correct!!!!\n";
	expectedOutput += "Chet now has 1 Gold Coins.\n";
	expectedOutput += "Pat is the current player\n";
	expectedOutput += "They have rolled a 4\n";
	expectedOutput += "Pat's new location is 4\n";
	expectedOutput += "The category is Pop\n";
	expectedOutput += "Pop Question 1\n";
	expectedOutput += "Answer was correct!!!!\n";
	expectedOutput += "Pat now has 1 Gold Coins.\n";
	expectedOutput += "Sue is the current player\n";
	expectedOutput += "They have rolled a 1\n";
	expectedOutput += "Sue's new location is 1\n";
	expectedOutput += "The category is Science\n";
	expectedOutput += "Science Question 0\n";
	expectedOutput += "Answer was correct!!!!\n";
	expectedOutput += "Sue now has 1 Gold Coins.\n";
	expectedOutput += "Chet is the current player\n";
	expectedOutput += "They have rolled a 5\n";
	expectedOutput += "Chet's new location is 9\n";
	expectedOutput += "The category is Science\n";
	expectedOutput += "Science Question 1\n";
	expectedOutput += "Answer was correct!!!!\n";
	expectedOutput += "Chet now has 2 Gold Coins.\n";
	expectedOutput += "Pat is the current player\n";
	expectedOutput += "They have rolled a 3\n";
	expectedOutput += "Pat's new location is 7\n";
	expectedOutput += "The category is Rock\n";
	expectedOutput += "Rock Question 0\n";
	expectedOutput += "Question was incorrectly answered\n";
	expectedOutput += "Pat was sent to the penalty box\n";
	expectedOutput += "Sue is the current player\n";
	expectedOutput += "They have rolled a 1\n";
	expectedOutput += "Sue's new location is 2\n";
	expectedOutput += "The category is Sports\n";
	expectedOutput += "Sports Question 0\n";
	expectedOutput += "Answer was correct!!!!\n";
	expectedOutput += "Sue now has 2 Gold Coins.\n";
	expectedOutput += "Chet is the current player\n";
	expectedOutput += "They have rolled a 3\n";
	expectedOutput += "Chet's new location is 0\n";
	expectedOutput += "The category is Pop\n";
	expectedOutput += "Pop Question 2\n";
	expectedOutput += "Answer was correct!!!!\n";
	expectedOutput += "Chet now has 3 Gold Coins.\n";
	expectedOutput += "Pat is the current player\n";
	expectedOutput += "They have rolled a 5\n";
	expectedOutput += "Pat is getting out of the penalty box\n";
	expectedOutput += "Pat's new location is 0\n";
	expectedOutput += "The category is Pop\n";
	expectedOutput += "Pop Question 3\n";
	expectedOutput += "Answer was correct!!!!\n";
	expectedOutput += "Pat now has 2 Gold Coins.\n";
	expectedOutput += "Sue is the current player\n";
	expectedOutput += "They have rolled a 1\n";
	expectedOutput += "Sue's new location is 3\n";
	expectedOutput += "The category is Rock\n";
	expectedOutput += "Rock Question 1\n";
	expectedOutput += "Answer was correct!!!!\n";
	expectedOutput += "Sue now has 3 Gold Coins.\n";
	expectedOutput += "Chet is the current player\n";
	expectedOutput += "They have rolled a 3\n";
	expectedOutput += "Chet's new location is 3\n";
	expectedOutput += "The category is Rock\n";
	expectedOutput += "Rock Question 2\n";
	expectedOutput += "Answer was correct!!!!\n";
	expectedOutput += "Chet now has 4 Gold Coins.\n";
	expectedOutput += "Pat is the current player\n";
	expectedOutput += "They have rolled a 5\n";
	expectedOutput += "Pat's new location is 5\n";
	expectedOutput += "The category is Science\n";
	expectedOutput += "Science Question 2\n";
	expectedOutput += "Question was incorrectly answered\n";
	expectedOutput += "Pat was sent to the penalty box\n";
	expectedOutput += "Sue is the current player\n";
	expectedOutput += "They have rolled a 3\n";
	expectedOutput += "Sue's new location is 6\n";
	expectedOutput += "The category is Sports\n";
	expectedOutput += "Sports Question 1\n";
	expectedOutput += "Answer was correct!!!!\n";
	expectedOutput += "Sue now has 4 Gold Coins.\n";
	expectedOutput += "Chet is the current player\n";
	expectedOutput += "They have rolled a 4\n";
	expectedOutput += "Chet's new location is 7\n";
	expectedOutput += "The category is Rock\n";
	expectedOutput += "Rock Question 3\n";
	expectedOutput += "Answer was correct!!!!\n";
	expectedOutput += "Chet now has 5 Gold Coins.\n";
	expectedOutput += "Pat is the current player\n";
	expectedOutput += "They have rolled a 3\n";
	expectedOutput += "Pat is getting out of the penalty box\n";
	expectedOutput += "Pat's new location is 8\n";
	expectedOutput += "The category is Pop\n";
	expectedOutput += "Pop Question 4\n";
	expectedOutput += "Answer was correct!!!!\n";
	expectedOutput += "Pat now has 3 Gold Coins.\n";
	expectedOutput += "Sue is the current player\n";
	expectedOutput += "They have rolled a 5\n";
	expectedOutput += "Sue's new location is 11\n";
	expectedOutput += "The category is Rock\n";
	expectedOutput += "Rock Question 4\n";
	expectedOutput += "Answer was correct!!!!\n";
	expectedOutput += "Sue now has 5 Gold Coins.\n";
	expectedOutput += "Chet is the current player\n";
	expectedOutput += "They have rolled a 4\n";
	expectedOutput += "Chet's new location is 11\n";
	expectedOutput += "The category is Rock\n";
	expectedOutput += "Rock Question 5\n";
	expectedOutput += "Answer was correct!!!!\n";
	expectedOutput += "Chet now has 6 Gold Coins.\n";
	return expectedOutput;
	}
	}

view raw RunCharacterizationTest.java hosted with ❤ by GitHub

Although in this final version, the expected output reflects the fixing of two bugs in the original code, at the beginning it was just what the original code was writing to the standard output given a fixed seed for the random number generator.

This article describes this technique very well: Working Effectively With Characterization Tests.

To be able to redirect the standard output I used the StandardOutputRedirection class:

	package com.adaptionsoft.games.trivia;
	import java.io.ByteArrayOutputStream;
	import java.io.PrintStream;
	public class StandardOutputRedirection {
	private ByteArrayOutputStream newOut;
	private PrintStream old_out;
	public StandardOutputRedirection(
	ByteArrayOutputStream newOut,
	PrintStream old_out) {
	super();
	this.newOut = newOut;
	this.old_out = old_out;
	System.setOut(new PrintStream(this.newOut));
	}
	public void reset() {
	System.setOut(old_out);
	}
	public String getRedirectedOutput() {
	return new String(newOut.toByteArray());
	}
	}

view raw StandardOutputRedirection.java hosted with ❤ by GitHub

The characterization test shown before yielded a code coverage of 93% which gave me confidence enough to start refactoring the code.