FAQ - why does my code throw a null pointer exception - common reason #1 Redeclaration

A common reason for Null Pointer Exceptions in Java is a variable redeclaration instead of instantiation. Learn what that means, how to avoid it, and how to spot it, in this presentation. Read the full blog post: http://testerhq.com/post/blogs/javafortesters/2017-08-29-faq-null-pointer-exception/ Visit my Java Web Site: http://javafortesters.com --- # FAQ - why does my code throw a null pointer exception - common reason #1 Redeclaration - Using `@BeforeClass` or `@Before` can setup data for use in tests - Any 'variables' we instantiate need to be 'fields' rather than variables - We want to instantiate them in the setup method rather than redeclare them --- # Example of the Problem I know I will use an `Adder` in my test so I create it as a field: ~~~~~~~~ public class WhyCodeThrowsNullPointerExceptionTest { Adder adder; ~~~~~~~~ I don't want to re-instantiate it each time so I make an `@BeforeClass` method to instantiate it: ~~~~~~~~ @BeforeClass public static void setupAdder(){ Adder adder = new Adder(); } ~~~~~~~~ **Warning: Error in the above code** --- # Semantic Error I just made a Semantic coding error. This won't be caught by a compiler, but it will cause my `@Test` to fail with a Null Pointer Exception. In the setup method I really wanted to assign a value to the field, instead I created an new variable with the same name. # In General - Try to write one test at a time so that if you have a problem it is easier to identify where the problem is - Try to write working isolated tests and then refactor to a more general solution when you need it - that way, you know it was working, so you just have to work backwards to find out what went wrong - Try to use automated IDE refactoring rather than move code around manually - Use the IDE syntax highlighting to help spot any issues Read less