Assert

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layout: default title: assert Method

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assert methods are methods JUnit executes to test whether the actual result of a method meets an expectation.

Most come from the org.junit.Assert class, and we import them using static import statements.

Practice Exercise

When an assert method fails, the actual result is an AssertionError, a Java class that is not a JUnit class.

assertEquals(expected, actual)

This overloaded method allows us to test if two objects or primitives are equal.

import static org.junit.Assert.assertEquals;

import org.junit.Test;

public class StringManipulatorTests {
  @Test
  public void test_addCharacters_add_correct_number_of_character() {
    StringManipulator sm = new StringManipulator();
    String input = "hello";
    String expected = "hello!!!";
    String output = sm.addCharacters(input, '!', 3);
    assertEquals(expected, output);
  }
  // ...
}

This method can handle null values without issue.

Practice Exercise

Order of method arguments is important. Be sure to put the value you expected, followed by the actual value the application method returns, because this affects JUnit's output. The test will fail either way, but your output would be misleading.

assertEquals(expected, actual, delta)

assertEquals for float and double values has an additional parameter, delta.

• delta is how much difference there can be between the expected and actual values, to account for inaccuracy in the data types.
• delta can usually be a very small value, like 0.0001.

assertTrue(boolean) and assertFalse(boolean)

These statements test boolean conditions, like the output of a method or a logical statement.

assertNull(Object) and assertNotNull(Object)

These test whether a null value was returned from a method call.

assertNotNull is useful when we expect to use an object's methods or fields, like an array's length.

@Test
public void test_convertToArray_returns_non_null_array_of_length_four() {
  StringManipulator sm = new StringManipulator();
  String input = "I am a String.";
  String[] outArr = sm.convertToArray(input, " ");
  assertNotNull(outArr);
  assertEquals(4, outArr.length);
}

Practice Exercise

Many assert methods are overloaded to add a custom String message as the first parameter.

Drill

UnitTesting/test/com.example.unittesting.drills.TextConverterTests

Copy code from TextConverterTests2 if you did not complete the previous drill. * Complete the toCaps methods in the test class. (toCaps is the only method in TextConverter with full functionality.) * Note that you need an instance of TextConverter in your test methods. You cannot just call methods.

(Solution: TextConverterTests3.java)

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