Test Exceptions

If a method is expected to throw an exception, we need to test that it does.

The Account class below throws the unchecked exception IllegalArgumentException when a negative withdrawal is attempted.

public class Account {
  private double balance;

  public void withdraw(double amount) {
    if(amount < 0.0) {
      throw new IllegalArgumentException("Amount cannot be less than zero.");
    }
    balance -= amount;
  }
  // ...
}

We can test that a method throws an Exception with the expected attribute.

Add expected to the @Test annotation, with the type of exception class.

@Test(expected=IllegalArgumentException.class)
public void test_withdraw_throws_IllegalArgumentException_for_negative_input() {
  Account a = new Account(100.0);
  a.withdraw(-0.01);
}

If a method throws a checked exception, we must declare it in the test method. * The point of expected is that an exception is thrown from the method for JUnit to examine.

This method throws Exception.

public class Account {
  private double balance;
  // ...
  public void withdrawException(double amount) throws Exception {
    if(amount < 0.0) {
      throw new Exception("Amount cannot be less than zero.");
    }
    balance -= amount;
  }
  // ...
}

This method must declare it.

@Test(expected=Exception.class)
public void test_withdrawException_throws_Exception_for_negative_input() throws Exception {
  Account a = new Account(100.0);
  a.withdrawException(-0.01);
}

Drill

ExceptionsJUnit/test/com.example.exceptionsjunit.examples.AccountTests * Run the example to see that the tests pass. * Change the withdraw amount to a positive number. What happens when you run the program? Change it back. * Change the expected exception to NullPointerException and run the program. What happens? Change it back.

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