Some new JUnit tricks...

JUnit.md

How Many Assertions Per Unit Test

There is a really good exchange on Programmers Stack Exchange on the number of assertions to have in a single unit test: Is it OK to have multiple asserts in a single unit test?

Proper unit tests should fail for exactly one reason, that's why you should be using one assert per unit test.

More Assertions for JUnit

The "match at least one" example below relies on Hamcrest matchers. There is an alternative to Hamcrest, see AssertJ documentation.

  @Test
  public void testMatchThisOrThat() {
    // What if the result is either one or two?
    // Gotcha' covered! :)
    // Remember to import static org.hamcrest.CoreMatchers.* so the matchers come into scope.
    assertThat(dataType, anyOf(is("one"), is("two")));
  }

In this next case, I had an array of values where one or more elements should be matched. The Hamcrest IsIn matcher seems like a good fit, e.g.

@Test
public void testMatchIsIn() {
  // The result should be one of an array value.
  assertThat(dataType, isIn(new String[]{"one","two","three"}));
}

Note: This requires the Hamcrest library.

<dependency>
	<groupId>org.hamcrest</groupId>
	<artifactId>hamcrest-library</artifactId>
	<version>1.3</version>
</dependency>

Exception Testing

@Test(expected = IndexOutOfBoundsException.class) 
public void empty() { 
     new ArrayList<Object>().get(0); 
}

Deeper Exception Testing

@Test
public void testExceptionMessage() {
    try {
        new ArrayList<Object>().get(0);
        fail("Expected an IndexOutOfBoundsException to be thrown");
    } catch (IndexOutOfBoundsException anIndexOutOfBoundsException) {
        assertThat(anIndexOutOfBoundsException.getMessage(), is("Index: 0, Size: 0"));
    }
}

See Expected Exceptions.

MatchAtLeastOneOf.java

import static org.hamcrest.collection.IsIn.*;
import static org.hamcrest.CoreMatchers.*;
import static org.junit.Assert.*;

import org.junit.After;
import org.junit.Before;
import org.junit.Ignore;
import org.junit.Test;

/**
 * Some new assertions for JUnit.
 */
public class MatchAtLeastOneOf {
  @Before
  public void setUp() {
    
  }
  
  @After
  public void tearDown() {
    
  }
  
  // https://stackoverflow.com/questions/4667903/checking-for-2-expected-values-in-junit
  @Test(expected=NullPointerException.class)
  public void testCatchExpectedException() {
    theMethod();
  }
  
  @Test
  public void testCatchWithTryCatch() {
    try {
      theMethod();
      fail("I was expecting a different exception");
    } catch (NullPointerException e) {
      // Success!
    }
  }
  
  // Ignore this test for now.
  @Ignore("Why?")
  @Test
  public void testIgnoringThisOne() {
    
  }
  
  // Uses the Hamcrest CoreMatcher.
  // https://stackoverflow.com/questions/6028750/how-to-assert-an-actual-value-against-2-or-more-expected-values
  // https://github.com/junit-team/junit/wiki/Matchers-and-assertthat
  // https://code.google.com/archive/p/hamcrest/wikis/Tutorial.wiki
  @Test
  public void testMatchThisOrThat() {
    // What if the result is either one or two?
    // Gotcha' covered! :)
    // Remember to import static org.hamcrest.CoreMatchers.* so the matchers come into scope.
    assertThat(dataType, anyOf(is("one"), is("two")));
  }
  
  @Test
  public void testIsInTheArray() {
    assertThat(dataType, isIn(new String[]{"one","two","three"}));
  }
  
  // TODO - I came across a question where the poster wanted to test System.out.
  // JUnit test for System.out.println()
  // https://stackoverflow.com/questions/1119385/junit-test-for-system-out-println
  private final ByteArrayOutputStream outContent = new ByteArrayOutputStream();
  private final ByteArrayOutputStream errContent = new ByteArrayOutputStream();

  @Before
  public void setUpStreams() {
      System.setOut(new PrintStream(outContent));
      System.setErr(new PrintStream(errContent));
  }
  
  @After
  public void cleanUpStreams() {
      System.setOut(null);
      System.setErr(null);
  }
  
  @Test
  public void out() {
      System.out.print("hello");
      assertEquals("hello", outContent.toString());
  }
  
  @Test
  public void err() {
      System.err.print("hello again");
      assertEquals("hello again", errContent.toString());
  }
  
  // Additional References:
  // https://stackoverflow.com/questions/12729280/junit-how-to-check-if-string-is-equal-to-one-of-two-strings
  // https://stackoverflow.com/questions/18766235/how-to-assert-greater-than-using-junit-assert
}