AssertNull should be used or AssertNotNull

Use assertNotNull(obj). assert means must be.

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);

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

I just want to add that if you want to write special text if It null than you make it like that

  Assert.assertNotNull("The object you enter return null", str1)