Creating assertions

Question

So I want to create an assertion class like how AssertJ works. I'm having trouble getting started.


    public class Assertion {
    
        static object assertThis(Object o){}
        static Integer assertThis(int i){}
        static String assertThis(String s){}
        static Object isNotNull(){}
    
    }

My question is how does JUNIT take in a particular object/string/int and store it? Let's say I pass in a Assertion.assertThis("hello").isNotNull() I should be getting a string object back. Do I need a field to store the object file? And how is that changed by the different objects being passed through the assertThis method?

junit is open source. You can just look at what they do instead of asking SO to go look at it and paste the JUnit source in an answer, no? Seems a bit lazy. — rzwitserloot, Mar 22 '22 at 18:02
As far as I know, [JUnit](https://junit.org/junit5/docs/current/api/) does not have API such as `assertThis(...).isNotNull()`. Are you sure you're not thinking of [AssertJ](https://assertj.github.io/doc/)? If so, then take a look at [§2.6.2 Custom Assertions](https://assertj.github.io/doc/#assertj-core-custom-assertions). — Slaw, Mar 22 '22 at 18:08
see also: [hamcrest](http://hamcrest.org/JavaHamcrest/tutorial) — Andrew S, Mar 22 '22 at 18:11
@GhostCat so that's what I tried for the assertThis(Object o) {return o;) but I'm stuck on the isNotNull(). My code for IsNotNull() is public Object isNotNull(){ if (this instanceof Object){if(this == null) throw new UnsupportedOperationException();} return this; } But when I am trying run it through my main class with Assertion.assertThis(new Object()).isNotNull() Intellij is giving me a 'cannot resolve method isnotnull in object' — rd1346, Mar 22 '22 at 18:13
I would think that `assertThis()` should return an *helper* (or intermediate) object that has the `isNotNull()` method... — user16320675, Mar 22 '22 at 18:15
@GhostCat not quite. `x instanceof Object` will return false for x equal null. — knittl, Mar 22 '22 at 19:23
I ended up doing what @OscarRyz suggested below with some tweaks and it works! The problem I was having was the return type I needed as an assertion class not an object — rd1346, Mar 22 '22 at 19:37

score 2 · Accepted Answer · answered Mar 22 '22 at 18:21

2

I don't think that's how JUnit works (but AssertJ does).

But yes, you create an instance with a static method and hold the value, and then perform an assertion against that value.

New invocations to the static method (also know as factory method) will create different instances.

Here's a very simple example:


class Assert {

   // Thing we're going to evaluate
   private String subject; 

   // Factory method. Creates an instance of `Assert` holding the value.
   public static Assert assertThat(String actual) {
      Assert a = new Assert();
      a.subject = actual;
      return a;
   }

   // Instance method to check if subject is not null
   public void isNotNull() {
     assert subject != null;
   }
}

// Used somewhere else...
import static Assert.assertThat;

class Main {
  public static void main( String ... args ) {
      assertThat("hello").isNotNull();
  }
}

answered Mar 22 '22 at 18:21

OscarRyz

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Bear in mind that using a library like AssertJ is way better than rolling your own (they already solved a bunch of things you would find) But for learning purposes this is what you could try. – OscarRyz Mar 22 '22 at 18:26
user16320675, you are correct, but then again, you shouldn't roll your own assertion framework either and if you do I think it would be expected you know how to enable assertions ( `-ea` flag btw) – OscarRyz Mar 22 '22 at 18:46
This helped! It's for learning purposes. I tweaked it a little because i also want isNotNull to return back the same object so it can be chained again to something else. – rd1346 Mar 22 '22 at 19:38

Creating assertions

1 Answers1