Collection Overview
Below is a table that represents various data structures in Java and reasons to use them. These can be used in your project by importing java.util.*. If you do not add the util library, you will be given an error: cannot find symbol when compiling.
import java.util.*; // add to top of file
| Interface | Classes | Reasons to use it |
|---|---|---|
| List | ArrayList, LinkedList, Vector, Stack | collection of Objects(most similar to simple array). Data is accessed by index |
| Sets | HashSet, TreeSet | unique collection of Objects(like a list, but no duplicates). Data is accessed by value |
| Maps | HashMap, TreeMap | set of key/value pairs(similar to a dictionary). Keys must be unique; data is accessed by using the key |
ArrayList
Like an [] but without a set length.
Example
ArrayList<String> favoriteThings = new ArrayList<String>();
favoriteThings.add("jaws");
favoriteThings.add("popcorn");
for (int i = 0; i < favoriteThings.size(); i++) {
System.out.println("where i=" +i+ " : " + favoriteThings.get(i));
}
HashMap
Can be thought of as a dictionary of values. They are stored as a key with an associated value.
key : value
age : 32
likesPizza: true
Example
HashMap<String, Integer> ages = new HashMap<String, Integer>();
ages.put("James", 32);
ages.put("Spoorthi", 22);
ages.put("Lichard", 29);
System.out.println("James is " + ages.get("James") + " years old");
System.out.println("There are " + ages.size() + " items stored as key:value in this HashMap");