How to cover try and catch block in test apex class

Question

How to write test conditions for try and catch blocks in apex. My code coverage is not 100% by below code. Please help.

Apex class:

public static List<String> getUserName(List<String> currentOwnerValues) {
    List<String> ownerNames = new List<String>();
    for (String currentOwnerValue : currentOwnerValues) {
        String ownerName = currentOwnerValue;
        try {
            User u = [SELECT Id, Name FROM User WHERE FederationIdentifier = :currentOwnerValue];
            if (u != null) {
                ownerName = String.escapeSingleQuotes(u.Name);
            }
        } catch (Exception ex) {
            ownerName = String.escapeSingleQuotes(currentOwnerValue);
        }
        ownerNames.add(ownerName);
    }
    return ownerNames;
}

Apex Test class:

public static void getUserName() {
    List<User> usersList = new List<User>();
    List<String> ownerList = new List<String>();
    controller.getUserName(ownerList);
    User u = new User();
    u.FederationIdentifier = '123456';
    usersList.add(u);
    for (User u1 : usersList) {
        ownerList.add(u.FederationIdentifier);
    }
    controller.getUserName(ownerList);
}

Answer 1

Will the exception ever actually occur? For conditions you don't plan on happening best not to add exception handling and allow the exception to pass up and be reported.

If the exception handling is part of the required logic, add to the code under test:

@TestVisible private static Boolean GENERATE_EXCEPTION_FOR_TEST = FALSE;

...

      if (GENERATE_EXCEPTION_FOR_TEST) {
          Integer i = 1 / 0;
      }

and set that boolean from your test:

ApexClassName.GENERATE_EXCEPTION_FOR_TEST = true;

The idea is that this is fairly self documenting. Do assert that the values returned for the exception case are correct.

Note that this:

User u = [SELECT Id, Name From User WHERE FederationIdentifier =:currentOwnerValue];

will generate an exception when there are no rows so better:

User[] users = [...];
if (users.size() > 0) {
    ... users[0].Name ...

or my preference of:

for (User u : [...]) {
    ... u.Name ...

Answer 2

Try to execute your method wrapped in the try-catch block in your unit test method. It will catch the exception and you can assert if it was really caught.

@IsTest
static void testMethodException() {
    Boolean exceptionCaught = false;

    Test.startTest();
    try {
        // this should trigger null pointer exception
        YourClass.getUserName(null);
    } catch (Exception ex) {
        exceptionCaught = true;
    }
    Test.stopTest();

    System.assertEquals(true, exceptionCaught, 'exception was not caught');
}

Update

As Keith suggested in his answer, the exception won't be thrown here. Therefore, if you want to catch an exception, then update your original method like this:

  public static List<String> getUserName(List<String> currentOwnerValues) {
    List<String> ownerNames = new List<String>();

    try {
      List<User> users = [
        SELECT Id, Name
        FROM User
        WHERE FederationIdentifier IN :currentOwnerValues
        WITH SECURITY_ENFORCED
        LIMIT 100
      ];

      if (!users.isEmpty()) {
        for (User user : users) {
          ownerNames.add(user.Name);
        }
      }
    } catch (Exception ex) {
      throw new AuraHandledException(ex.getMessage());
    }

    return ownerNames;
  }

1. In this case, you will see an exception if the value you pass to the method is empty.
2. Executing SOQL inside a for loop is a not recommended
3. In your variant it's very unlikely you will have the exception you expect. I think it can be thrown if you have users without access to the records or fields.