COM S 207 ArrayList Instructor: Ying Cai Department of Computer Science Iowa State University

Disadvantages of Array  When writing a program that collects values, we don’t always know how many values we will have  We will have to manage the size of an array, and the valid length (i.e., lastIndex) ourselves  We need to implement operations such as insert and remove ourselves

ArrayList  Array lists can grow and shrink as needed  The ArrayList class supplies methods for common tasks, such as inserting and removing elements  import java.util.ArrayList

Declare and Use an ArrayList ArrayList friends = new ArrayList (); friends.add(“Cindy”); friends.add(“Scott”); String name = friends.get(1); friends.set(0, “Harry”); Variable type Variable name An array list object of size 0 The add method appends an element to the array list Use get and set methods to access an element Note: For now, use ArrayList only to store String. You cannot add an primitive type of data (etc., int, double)

Some Key Points  ArrayList must be initialized before use  When the ArrayList is initialized, the size of the array is 0  The size increases when you add an object, decreases when you remove an object  The last valid index is names.size()-1  As with arrays, it is an error to access a nonexistent element ArrayList names; names.add(“Harry”); // Error! int i = names.size(); String name = names.get (i); // Error!

Some Key Points  To set an array list element to a new value, use the set method  You can insert an element in the middle of an array list // overwrite the element at position 2 names.set(2, “John”); // add a new element at position 1 // and moves all elements with index // 1 or larger by one position names.add(1, “Scott”);

Some Algorithms // visit all elements of an array list for (int i=0; i<names.size(); i++) { String name = names.get(i); System.out.println(name); } // copy an array list ArrayList newNames = newArrayList (names); // reverse the names list in reverse order, and store // starting with the last element and store ArrayList reversed = new ArrayList (); for (int i=names.size()-1; i>=0; i--) { reversed.add(names.get(i)); }

Exercise What does the array list names contain after the following statements? ArrayList names = new ArrayList ; names.add(“Bob”); names.add(0, “Ann); names.remove(1); names.add(“Cal”);