ArrayLists & Lists

• A huge limitation of arrays is that it has a fixed length, and can only store, one specific type of data.

  • Ex: You have a group of action figures, an array would require that all the action figures are of the same type. So they would all need to have the same features, ex: flying or protective body armor. However, the number of action figures is represented using an array set to a fixed number. Extra spaces will simply be ignored, and once you reach one it will result in a NullPointerException.

• An ArrayList is a solution for this issue.

• An ArrayList object is dynamically sized, it can become smaller or larger as elements are added or removed.

• It can store multiple types of data without specific limits.

• It’s best to choose between an Array and ArrayList based on your needs and the functionality of the array.

• ArrayLists are very adaptable, so if you need to make an ArrayList set to a specific type, you set it up as typed ArrayList.

  • Ex: Say you want to make an ArrayList called lunchBag.

ArrayList lunchBag = new ArrayList();

• Unlike establishing an array, the number of objects or length isn’t set.

• The way you access data within an ArrayList is different from that of Arrays.

• Note: Bracket notation can’t be used, unlike in Array’s.

  • For example, say you want to return the second object, an Apple Object, from the ArrayList and store it in a variable.

Apple red = lunchBag.get(1);

• Other useful methods include add, set, remove, and size.

• Arraylists are unique because only objects can be strained in them. The primitive data types of int, and double can’t be stored in ArrayLists.

• Instead, you have to use the Integer or Double wrapper classes. Integer and Double objects can be created with integers and doubles, as parameters.

  • Ex:

Integer n = new Integer (5);
Double x = new Double(6.1);

• To call these values from the example above you can use the intValue(), and doubleValue() methods.

  • Ex:

int a = n.intValue();
int y = x.doubleValue();

• Other variables that the AP Computer Science Java Subset includes are the static variables of MIN_VALUE, and MAX_VALUE found in the Integer class.

• These static variables store the minimum and maximum values of an integer.

  • Ex: Consider the following code segment. What is printed as a result of executing the code segment?

ArrayList list = new ArrayList();

list.add(“A”);

list.add(“B”);

list.add(0,”C”);

list.add(“D”);

list.set(2,”E”);

list.remove(1);

System.out.println(list);

Answer: It should print out “C E D”. This is because our list at first will be A B. Since we ask to add C to the index of 0 the array will look like this- C A B. Then D gets added to become C A B D. The B gets replaced with E to become C A E D. Then we remove A, because it’s at index 1. It becomes C E D.

Differences between Array’s and ArrayLists: