Intro to ArrayLists

Intro to ArrayLists: 7.1.0

• Arrays have Limitations
  • The size of an array can’t be changed after declaration
• ArrayLists are resizable
  • The underlying array grows and shrinks as needed
  • This makes an arraylist mutable
  • Often called a List on the CS A exam

Import package: 7.1.1

• You do not need to worry about import statements on the AP exam!
• ArrayList class is housed in the java.util package
  • A package is a set/library of related classes
  • java.lang is the package of main java classes; it is automatically imported
    • You need to manually import everything else
      • You can also use the full name of the class
        • java.util.ArrayList
• Import statements are the first bits of code in a Java source file
  • This tells java what you mean when you use a short name like ArrayList, and where to find the underlying class
    • This adds the class to your namespace!
    • This is useful because there can be many classes with the same short name, and it specifies which one you actually mean
• There are a few ways to import
  • Importing just the class you want
    • import java.util.ArrayList;
  • Importing all classes in a given package
    • import java.util.*;

Declaring and Creating ArrayLists: 7.1.2

• ArrayList<Type> name
  • Type is whatever object you want to store
  • You don’t have to specify a type because it’ll default to Object
    • All classes extend java.lang.Object
    • It is good practice to specify one anyway
      • You can control what is actually allowed in the ArrayList
  • The java compiler will warn you if you don’t specify a type
  • it breaks type safety

• To Create an ArrayList, use new ArrayList<Type>()

Using Primitive values

• Arraylists can only hold Objects, so primitive types must be wrapped in objects
  • int becomes Integer
  • double becomes Double
  • etc.
• All kinds of objects can be used in an ArrayList
  • Even ones you write yourself!

Converting arrays to lists

This isn’t covered on the exam!

String divisions = {"Carver", "Galileo", "Hopper", "Newton", "Roebling", "Turing"};
ArrayList<String> frcDivisions = new ArrayList<String>(Arrays.asList(divisions));
System.out.println(frcDivisions);

Other Arraylist functions

• The number of items in a list can be found using .size()
  • An empty arraylist has a size of 0
  • you get a NullPointerException when calling .size() on a null arraylist
    • You can’t do something on an object which doesn’t exist!
• You can add values to a list using .add()

Summary: 7.1.4

• ArrayLists are resizable arrays
• The ArrayList class is found in java.util
  • It must be imported to be used
• new ArrayList() constructs an ArrayList with a size of 0
• a type can be specified when constructing an arraylist
  • This is preferred so the compiler can find errors ahead of time
• Arraylists can only hold objects - not primitives
  • Primitives must first be wrapped