Thread: Help with JUnit testing

So I am brand spanking new to JUnit testing and code testing in general. I found a helpful guide on YouTube but it only got me so far. I wrote a simple Change Making problem and now I want to do some JUnit testing on it. Here is the method I am attempting to test:

public <String> List<String> minCoins(int[] coins, int amount){
 
        List list = new ArrayList();
 
        int num;
 
        if(coins.length == 0){
            throw new IllegalArgumentException("Array of size 0 is not allowed.");
        }
 
        for(int i = 0; i < coins.length; i++){
 
            if(coins[i]<=amount){
                num = amount/coins[i];
 
                for(int j = 0; j < num; j++){
 
                    list.add(coins[i]);
                }
 
                amount -= num*coins[i];
            }
        }
 
        return list;
    }

First test:

@Test
    public void testMinCoins1() {
        System.out.println("minCoins");
        int[] coins = {25,10,5,1};
        int amount = 50;
        Lab7 instance = new Lab7();
        List expResult = ??;
        List result = instance.minCoins(coins, amount);
        assertEquals(expResult, result);
    }

Not sure what to put in the expResult section. I tried putting in (25,25) but it says:

')' expected
<identifier> expected
incompatible types int cannot be converted to List

Not sure what's going on there. I would expect since my list is filled with int's that int's would be what is expected on the output. I know the program runs because I created a driver for it and ran it.

Second thing I want to test is if I have an empty array I want it to throw an exception but I have no idea how to do that.

Any help would be appreciated!

Okay well I figured out how to test the contents of my list properly if anyone is wondering here is my test case:

@Test
    public void testMinCoins() {
        System.out.println("minCoins");
        int[] coins = {25,10,5,1};
        int amount = 10;
        Lab7 instance = new Lab7();
        List<Integer> expected = new ArrayList<>();
        expected.add(25);
        List result = instance.minCoins(coins, amount);
        assertEquals(expected, result);
        // TODO review the generated test code and remove the default call to fail.
        fail("The test case is a prototype.");
    }

So that is good. However, I still don't understand how to test my NullPointerException. I have tried a few different things to no avail. The problem I am having is that I can not seem to find the correct method for testing for my specific IDE which is NetBeans. Either that or the IDE doesn't matter and my JUnit version is the problem. I can't seem to figure out which version of JUnit I am running.

If someone could show me how to test my NullPointerException that would be great!