Arrays versus ArrayList

1. Arrays versus ArrayList Barb Ericson Georgia Institute of Technology June 2006 Georgia Institute of Technology

2. Learning Goals • Understand when you might want to use an array • Understand when you might want to use an ArrayList • Understand the major differences between Arrays and ArrayLists • Understand how to use an Iterator Georgia Institute of Technology

3. Arrays • We use arrays to hold many things of the same type • Like sample values in a sound • Like pixels in a picture • We can create arrays of primitive types or objects • double[] gradeArray = new double[10]; • Picture[] pictureArray; // declare the pictureArray • pictureArray = new Picture[5]; // create the array • We don't have to name each element • We name the array • We use indices to access each element (starting at 0) • pictureArray[3]; Georgia Institute of Technology

4. Exercise • Look at SlideShow.java in examples/SlideShow-Array • Change the main to create an array of 7 pictures instead of 5 • Add pictures created from files in mediasources • Add two new pictures to the slide show at indices 5 and 6 • Run the main method Georgia Institute of Technology

5. Benefits of Arrays • Holds many items that are related and are of the same type • Quick access to any item based on the index of that item • Calculates address based on the size of items in the array and distance from the beginning of the array • Easy to loop through all items in the array • Can use a for-each loop Georgia Institute of Technology

6. Adding to a Full Array • To add items into a full array • You would need to create a bigger array • Copy all the old items before the insert point (if any) • Add the new items • Copy any remaining old items (ones past the insertion point) 3 | 5 | 7 | 9 3 | 5 | 1 | 2 | 3| 4 | 7 | 9 Add 1, 2, 3, 4 at index 2 Georgia Institute of Technology

7. Removing From a Full Array • You could just copy the items to the left • But then you have values at the end that aren't really in the array any more • You can zero out primitive types • You can set object references to null • But what if these are in the array as normal values? • You can keep track of the number of valid positions in the array 3 | 1 | 2 | 0 | 4 | 7 | 9 | 0 3 | 5 | 1 | 2 | 0| 4 | 7 | 9 3 | 1 | 2 | 0 | 4| 7 | 9 | 9 Size = 7 Remove 5 Georgia Institute of Technology

8. ArrayList • Is a implementation of the List interface • Using an array • The array will grow or shrink as needed to fit the data • You can add items to the List • They will be added at the end • You can add items at a specified index • Other items will be moved to the right to make room • You can remove items • Items will be moved to the left Georgia Institute of Technology

9. Differences between Array and ArrayList • Get the number of items in • Array: arrayName.length (public field) • ArrayList: size() (method call) • How you declare them • Array: Type[] arrayName (use square brackets) • ArrayList: List (use interface name as type) • How you create them • new int[5]; • new ArrayList(); • Can store • Arrays: primitive types or objects • ArrayList: only objects (can wrap primitive types) Georgia Institute of Technology

10. Interface Name as Type • Any class that implements an interface can be referred to by a variable that is declared with the interface type • ArrayList implements the List interface • List pictureList = new ArrayList(); • This allows you to change your mind in the future about which class to use and only change the code in one place • Where you create the list • Instead of everywhere you specify the type Georgia Institute of Technology

11. Exercise • Modify the SlideShow class in exercises/SlideShow-Array to use an ArrayList • Change the field from pictArray to pictList • Change the type from an array of pictures (Picture[] to a List) • Don't forget to change the documentation, too • Change from creating an array to creating an ArrayList • Add two new methods that add new Pictures to the slide show • One that takes an index to insert at and one that adds it to the end of the list Georgia Institute of Technology

12. Processing a List • If you just want to loop through all items in a list • Use a for-each loop • If you want to remove an item from the list during the loop • Use an Iterator and call remove on the Iterator • If you want to add an item to the list during the loop • Use a ListIterator Georgia Institute of Technology

13. Processing a List using a While Loop • You can also process a list using a while loop • But be careful if you remove items from the list during the loop • Only increment the index if you didn't remove an item from the list int index = 0; while (index < theList.size()) { obj = theList.get(index); if (obj.test()) theList.remove(index); else index ++; } • It is easier to use an iterator Georgia Institute of Technology

14. Iterators • Really an interface with these methods: • hasNext() returns a boolean value • next() returns the next item in the list • remove() returns the last returned item from the list Iterator iterator = theList.iterator(); while (iterator.hasNext()) { obj = iterator.next(); if (obj.test()) iterator.remove(); } Georgia Institute of Technology

15. Exercise • Modify the SlideShow class in SlideShow-Generic to remove all pictures from the slide show that have a passed string as part of the file name • Use an iterator to loop through the pictures in the list • And use indexOf method to look for the passed search string • If you find the search string remove the last item from the iterator Georgia Institute of Technology

16. ListIterator • An interface • Inherits from the List interface • Allows you to process a list from the front or back • hasNext() or hasPrevious() to test for elements • next() or previous() to get an element • Can add or set items in the list • add(E o) before next or set(E o) to replace last Georgia Institute of Technology

17. Summary • Use arrays to hold groups of data of the same type • Especially when the size of the data is known and doesn't change • Use ArrayList to hold objects in order • Especially when you don't know how much data to expect • And there may be some additions or deletions • There isn't too much data • Use iterators to process a list • Especially if you want to remove or add items during the loop Georgia Institute of Technology