How to Effectively Assert Exceptions in JUnit Tests?

Executing Exception Assertions in JUnit Tests

Asserting that a specific exception is thrown during code execution is a common test scenario in JUnit. To do this effectively, JUnit provides several methods that enhance readability and simplify the testing process.

JUnit 5 and 4.13 Assertions

In JUnit 5 and 4.13, the @Test annotation with the expected attribute can be used directly:

@Test
public void testIndexOutOfBoundsException() {
    ArrayList<Object> emptyList = new ArrayList<>();
    assertThrows(IndexOutOfBoundsException.class, () -> emptyList.get(0));
}

AssertJ and Google Truth Assertions

External assertion libraries like AssertJ and Google Truth offer additional methods for exception assertions:

AssertJ:

import static org.assertj.core.api.Assertions.*;

@Test
public void testIndexOutOfBoundsException() {
    ArrayList<Object> emptyList = new ArrayList<>();
    assertThatThrownBy(() -> emptyList.get(0))
        .isInstanceOf(IndexOutOfBoundsException.class);
}

Google Truth:

import static com.google.common.truth.Truth.*;

@Test
public void testIndexOutOfBoundsException() {
    ArrayList<Object> emptyList = new ArrayList<>();
    assertThatCode(() -> emptyList.get(0)).willThrow(IndexOutOfBoundsException.class);
}

JUnit Pre-4.13 Assertions (Deprecated)

For JUnit versions prior to 4.13, a more cumbersome approach was to manually check for the exception within a try-catch block:

@Test
public void testIndexOutOfBoundsException() {
    boolean thrown = false;
    ArrayList<Object> emptyList = new ArrayList<>();

    try {
        emptyList.get(0);  // Intentionally triggers an exception
    } catch (IndexOutOfBoundsException e) {
        thrown = true;
    }

    assertTrue(thrown);
}

This approach is deprecated and less idiomatic than the aforementioned methods. Refer to the [JUnit Test-FAQ](https://junit.org/junit4/faq.html) for more details.

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