AssertNull should be used or AssertNotNull

assertNotNull asserts that the object is not null. If it is null the test fails, so you want that.

The assertNotNull() method means "a passed parameter must not be null": if it is null then the test case fails.
The assertNull() method means "a passed parameter must be null": if it is not null then the test case fails.

String str1 = null;
String str2 = "hello";              

// Success.
assertNotNull(str2);

// Fail.
assertNotNull(str1);

// Success.
assertNull(str1);

// Fail.
assertNull(str2);

Use assertNotNull(obj). assert means must be.