How to remove errors from the ouput of a program?

Question

So I have written a program tat outputs as I want it to correctly but for some reason I am still getting some errors at the end of the program which I'd prefer not to be shown. I've tried using Systematize(out); but all this does is change the color of the errors from red to the same color as the rest of the program. Here is the output of the program:

************Wolf"************
Name = Bob
Age = 2
Noise = Woof!
************Parrot************
Parrot's name = Polly
Age = 6
Noise = Argh!
************Exception examples************
java.lang.Exception: Carnivores only eat meat!
Exception caught above
Exception caught below
java.lang.Exception: Herbivores only eat plants!
************Herbivore non-caught Exception example************
Herbivores eat Vegetables
************Carnivore non-caught Exception example************
Carnivores eat Steak
************Omnivore eating habbit************
Omniivores eat meat or fruit and vegertables
************New herbivore animal************
Parrot's name = Haryy
Age = 2
Noise = Squeek!
************Animal being fed************
Wolf has eaten 10 times
************New wolf creation************
Name = newborn
Age = 0
************ArrayList before sorting************
Sam,5
************ArrayList after sorting************
Pat,10
Wesley,7
Sam,5
George,3
Wesley,7
************ArrayList after sorting************
Pat,10
Wesley,7
Sam,5
George,3
Sam,5
************ArrayList after sorting************
Pat,10
Wesley,7
Sam,5
George,3
Exception in thread "main" java.util.ConcurrentModificationException
George,3
************ArrayList after sorting************
Pat,10
Wesley,7
Sam,5
George,3
    at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1380)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
    at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174)
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.util.stream.ReferencePipeline.forEachOrdered(ReferencePipeline.java:423)
    at QuestionEditor.Main.main(Main.java:124)
C:\Users\lee-pc\AppData\Local\NetBeans\Cache\8.2\executor-snippets\run.xml:53: Java returned: 1
BUILD FAILED (total time: 0 seconds)

Here is the main method that outputs the part that contains errors:

   Demo.main(args); // populate the ArrayList in Demo Class main method 2.
    ArrayList<Animal> animalGroup = Demo.animalGroup; // Retrieve the ArrayList in Demo class.
    animalGroup.stream().map((animal) -> {
        System.out.println(animal.getName()+","+animal.getAge());
            return animal;
        }).map((Animal _item) -> {
            System.out.println("************ArrayList after sorting************");
            return _item;
        }).map((Animal _item) -> {
            Collections.sort(animalGroup,new AgeComparator()); // Sort highest to lowest
            return _item;
        }).forEachOrdered((Animal _item) -> {
            animalGroup.forEach((animal2) -> {
                System.out.println(animal2.getName() + "," + animal2.getAge());
            });
        });

Does anybody know why this is happening? And what is the best way of resolving this issue? Thanks for any feedback, much appreciated.

Answer 1

Collections.sort modifies the content of animalGroup if there are any elements not in sorted order. You are doing this in the middle of iterating over animalGroup. If while iterating over the list some elements get swapped, the outcome would be a mess. For this reason it is forbidden to modify a list while iterating over it, hence the ConcurrentModificationException. This is known as the fail-fast policy of collection implementations in Java, you can read more about it here.

What you're doing with the streams doesn't make a lot of sense. It seems you're looking for this:

List<Animal> animalGroup = Demo.animalGroup; 

animalGroup.forEach(animal -> System.out.println(animal.getName() + "," + animal.getAge()));

System.out.println("************ArrayList after sorting************");

Collections.sort(animalGroup, new AgeComparator()); // Sort highest to lowest

animalGroup.forEach(animal -> System.out.println(animal.getName() + "," + animal.getAge()));