JAVA COLLECTIONS SORT HOW TO
JAVA COLLECTIONS SORT CODE

What I'm doing here is I'm avoiding having to create a whole new Object, then have that Object implement the Comparator interface, then the appropriate compare(String o1, String o2) method, then instantiate that Object and pass it into the Collections.sort() method. But we're doing something neat, we're declaring an anonymous inner type for this Comparator. || o1.equalsIgnoreCase('o') || o1.equalsIgnoreCase('u') Īlright, so you see here that we are using a Comparator inside of the Collections.sort() method. Return o1.equalsIgnoreCase('a') || o1.equalsIgnoreCase('e') || o1.equalsIgnoreCase('i') * true if the letter is a vowel, false otherwise * o1 the String that may or may not be a vowel * This method determine if the String being passed in is a vowel or not. Return o1.toLowerCase().compareTo(o2.toLowerCase()) It has a special notation, so if you don't quite get it, no worries: Collections.sort(theArrayList, new int compare(String o1, String o2) It's kind of like creating a whole class that can be used within an existing method. I'm going to be making use of something called an Anonymous Inner Type. If you study the Collections.sort() methods, you'll see that there's one that takes two parameters, a List and a Comparator, so this is the method we're interested in using.

JAVA COLLECTIONS SORT CODE

So let's get on with the good code where we make use of our Comparator. The resulting ArrayList would look like this:Īs you can see, that's not what we're looking for we want all the vowels to appear first. So you see here that we're populating an ArrayList with our “test” sentence and then we invoke the Collections.sort() method. TheArrayList.add('R') theArrayList.add('A') theArrayList.add('M') theArrayList.add('S')

TheArrayList.add('P') theArrayList.add('R') theArrayList.add('O') theArrayList.add('G') TheArrayList.add('J') theArrayList.add('A') theArrayList.add('V') theArrayList.add('A') theArrayList.add(' ') TheArrayList.add('L') theArrayList.add('O') theArrayList.add('V') theArrayList.add('E') theArrayList.add(' ') TheArrayList.add('I') theArrayList.add(' ') construct the ArrayList with our letters

JAVA COLLECTIONS SORT HOW TO

So let's have a look at the code and how this can be made possible, here's the example of how to do a regular old sort of an ArrayList: public class SortingExample Well, since we are trying to sort an ArrayList of Strings then we will need to use the Comparator.

Don't read anything else until you have come up with a guess in your mind. Okay, so given the information above, and knowing that we want to sort an ArrayList do you know which one of the interfaces you would use? It uses the compareTo(Object anotherObjectOfSameType) method to perform its magic.Compare this instance of HumanBeing to another HumanBeing) This interface is used to compare the current instance of your Object to another one (i.e.Use this interface if you want to custom sort an Object that you have created (i.e.It uses the compare(Object object1, Object object2) method to perform its magic.This interface is used to compare two different instances of an Object.Use this interface if you want to custom sort on an Object type that you didn't create (i.e.They will both allow you to define a custom sorting algorithm for your Objects. Well just keep in mind that they both do the same thing in the end. So how in the world do you pick between them? So you'll need to use a customized sorting algorithm, which means picking between either the Comparable or Comparator interfaces. The static sort method of the Collections Class that takes a List will just arrange the letters alphabetically and it won't put all the vowels first. Well, if you just tried to use the standard Collections.sort(theArrayList), it wouldn't get you very far. The resulting ArrayList should look like this: Then then you need to list all the consonants after the vowels (also in alphabetical order).You first need to show all of the vowels first (in alphabetical order).With this ArrayList, let's say you're given the requirements to sort all of the letters in a unique way. Let's assume you're presented with the following ArrayList: So let's start off with a simple example so that you have a clear understanding of what I'm talking about. That's a great question, and I'll answer it soon. You might ask “Why are there two different interfaces used for sorting?”. So how can this be done? And moreover, how can this be done in a way that's fully customizable? Comparator/ Comparable InterfacesĮnter the Comparator and Comparable interfaces… these are what we use to accomplish the sorting of Collections and Arrays in a customizable way. Have you run into a situation where you had a Collection of Objects (or an Array), and you needed to have them ordered in a certain way?Ĭhances are that if you've been programming with Java and running through the assignments available on this blog, then you have.

Java collections sort