java.lang.NullPointerException for null input in null safe static method

Question

@Test
fun isEmpty_null_true(){
    Assert.assertEquals(StringUtils.isEmpty(null),true)
}

public static boolean isEmpty(@Nullable String value) {
    return (TextUtils.isEmpty(value) || value.trim().isEmpty());
}

All test methods except above one are working and passed. I am getting java.lang.NullPointerException for this one. The implementation of StringUtils.isEmpty() is also mentioned above. StringUtils class is written in Java whereas test case file is written in Kotlin.

Answer 1

The complete solution, I have got-
Due to default values set in app/build.gradle, I was getting false returned from TextUtils.isEmpty() which was not expected by me.

testOptions {
    unitTests.returnDefaultValues = true
}

Reference - TextUtils.isEmpty(null) returns false
I cannot change the value in build.gradle file, so I needed the solution for my unit test method only. There is a way to provide implementation to TextUtils.isEmpty() method and get the real returned value.

    @Before
    public void setup() {
        PowerMockito.mockStatic(TextUtils.class);
        PowerMockito.when(TextUtils.isEmpty(any(CharSequence.class))).thenAnswer(new Answer<Boolean>() {
            @Override
            public Boolean answer(InvocationOnMock invocation) throws Throwable {
                CharSequence a = (CharSequence) invocation.getArguments()[0];
                return !(a != null && a.length() > 0);
            }
        });
   }

Reference - Need help to write a unit test using Mockito and JUnit4

I got the same problem with android.graphics.Color.parseColor() so above solution applies to all classes lies in android package.

Answer 2

Maybe instead of: value.trim().isEmpty()
try this: value.trim().equals("")

Answer 3

The problem lies in this StringUtils.isEmpty() method itself because sometime TextUtils.isEmpty(null) returns false at that time you are passing value as null then you are invoking trim() method on the null value that is causing the java.lang.NullPointerException modify the method like this to avoid java.lang.NullPointerException

public static boolean isEmpty(@Nullable String value) {
        return (TextUtils.isEmpty(value) || (value != null && value.trim().isEmpty()));
    }

Answer 4

As you know, || operator will invoke the code on both sides (to get one true value) and null.trim().. will cause the exception.

Since the expected behavior of StringUtils.isEmpty method is to throw an exception when a null value is being passed so you should write the unit test to expect an exception with null value as:

@Test(expected = NullPointerException::class)
fun isEmpty_null_exception(){
    StringUtils.isEmpty(null)
}

here excepted is used to assert that this test method throws a NullPointerException exception.

Note: Using mock for StringUtils.isEmpty static method is done with PowerMock framework but this will violate the purpose of testing to test the actual behavior of code so use the above-recommended approach to verify exception. You may want to create a mocked input variable and mock trim method but then you can simply use a non-null input string, won't be any difference.