Intro to Programming II ArrayLists Chris Brooks Department of Computer Science University of San Francisco Department of Computer Science University of San Francisco p 1 8 2 The ArrayList class Last time we learned about how to use arrays in Java Java also provides an ArrayList class that can help manage arrays ArrayList is a generic container That means that we can use the same container to store different kinds of elements Department of Computer Science University of San Francisco p 2 8 3 The ArrayList class An example ArrayList band new ArrayList 3 band add john band add paul band add george band add ringo Department of Computer Science University of San Francisco p 3 8 4 The ArrayList class Notice that we declared an initial array size The array is then able to grow dynamically beyond that It s better to allocate in advance if we can We can also remove elements and access them We can also add in the middle of a list band add 1 ringo Department of Computer Science University of San Francisco p 4 8 5 Comparison with Arrays With arrays we d need to allocate a new array and copy everything over yuck int array1 new int 5 for i 0 i 5 i array1 i int array2 new int 10 for i 0 i 5 i array2 i array1 i array1 array2 Department of Computer Science University of San Francisco p 5 8 6 Accessing elements get index lets us access the element at a particular index Elements in an ArrayList are stored as Objects This means that we need to cast them back to Strings Can t store primitives String name String band get 2 Department of Computer Science University of San Francisco p 6 8 7 Exercise 1 Redo the Student exercise from Monday using an ArrayList Prompt the user for a number of students Store the students in an ArrayList Print them out in order Department of Computer Science University of San Francisco p 7 8 8 Iterators It can be cumbersome to iterate through a list using integers What if the size of the list changes while we re iterating over it Why do we need to know the size of the array to do something to each element Java provides an abstraction for us known as an iterator Department of Computer Science University of San Francisco p 8 8 9 Example List myList ArrayList 10 fill in the list ListIterator i myList ListIterator while i hasNext System out println i next Department of Computer Science University of San Francisco p 9 8 10 Exercise 2 Modify the previous code to use a ListIterator Department of Computer Science University of San Francisco p 10 8 11 Finding elements We can use indexOf to find where an element is located remove lets us remove things int index band indexOf ringo band remove index Department of Computer Science University of San Francisco p 11 8 12 Exercise 3 Add a method that allows the user to enter the name of a student Loop through the array to find that student and remove them Now try doing it with indexOf Department of Computer Science University of San Francisco p 12 8 13 Other methods also size tells us how many elements are in the array isEmpty tells us whether there s anything in the list contains Object o tells us whether an object is in the list clear removes everything Department of Computer Science University of San Francisco p 13 8 14 Specifying element types If we want to avoid casting elements we can also define the ArrayList to accept particular sorts of objects Java 1 5 only ArrayList String al new ArrayList String 3 al add john al add paul al add george al add ringo String name al get 2 System out println name Department of Computer Science University of San Francisco p 14 8 15 Exercise 4 Change your code to use the template notation so that we don t have to cast everything anymore Department of Computer Science University of San Francisco p 15 8 16 ArrayList vs arrays ArrayLists manage inserting searching and removing for you Can grow dynamically Only work with objects not primitives Provides some nice convenience methods Department of Computer Science University of San Francisco p 16
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