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ArrayList Array’s are limited because we need to know the size before we use them. An ArrayList is an extension of an array that grows and shrinks as needed. The ArrayList class is part of the java.util package of the Java standard class library. 1 ArrayLists contain Objects Numbers are not objects. Unlike an array, you can not have an ArrayList of ints or doubles or booleans or chars. 2 ArrayList Each element in the sequence can be accessed separately. We can explicitly overwrite an object at a specified position in the sequence, thus changing its value. We can inspect the object at a specified location in the sequence. We can add an object into a specified position of the sequence. We can add an object to the end of the sequence. We can remove an object from a specified location in the sequence. 3 ArrayLists for AP CS A Notice that the AP CS A Subset requires the knowledge that ArrayList implements List. 4 java.util.ArrayList<E> for AP CS A int size() // returns the number of elements in this list boolean add(E x) // appends x to the end of list; returns true E get(int index) // returns the element at the specified position in this list. E set(int index, E x) // replaces the element at index with x // returns the element formerly at the specified position 5 class java.util.ArrayList<E> for AP CS A void add(int index, E x) // inserts x at position index, sliding elements // at position index and higher to the right // (adds 1 to their indices) and adjusts size E remove(int index) // removes element from position index, sliding // subsequent elements to the left (subtracts 1 from their // indices) and adjusts size // returns the element at the specified position in this list. 6 An example: import java.util.ArrayList; public class ArrayListTest { public ArrayListTest { System.out.println("ArrayListTest"); ArrayList<String> aList = new ArrayList <String>(); aList.add("Dan"); aList.add("George"); aList.add("Mary"); System.out.println("aList contains:"); for(int x = 0; x < aList.size(); x++) { System.out.println(aList.get(x)); } } } 7 Using enhanced FOR loop: import java.util.ArrayList; public class ArrayList_01_ForEach { public static void main (String [] arg) { System.out.println("ArrayListTest"); ArrayList<String> aList = new ArrayList <String>(); aList.add(new String("Dan")); aList.add("George"); aList.add("Mary"); System.out.println("aList contains:"); for(String e:aList) { System.out.println(e); } } } 8 Other ArrayList methods // ArrayListTest2 ArrayList<String> students = new ArrayList<String>(); students.add("Mary" ); students.add("James"); students.add("Kevin"); students.add(1, "Tanya"); String temp = students.get(3); System.out.println(temp); students.remove(2); students.set(1, "John"); System.out.println(students.size()); 9 What happens? ArrayList_02 Mary ArrayList<String> students = new ArrayList<String>(); students.add("Mary"); students.add("James"); students.add("Kevin"); Mary James Tanya Kevin James Kevin students.add(1, "Tanya"); String temp = students.get(3); System.out.println(temp); temp Mary Tanya Kevin students.remove(2); students.set(1, "John"); System.out.println(students.size()); Mary John Kevin 10 An ArrayList is a sequence of objects. Array lists can hold any kind of object. For the generic ArrayList, you must include the type of object the ArrayList will hold. • ArrayList<Athlete> athletes = new ArrayList<Athlete>(); • ArrayList<Student> csci6 = new ArrayList<Student>(); • ArrayList<BankAccount> accounts = new ArrayList<BankAccount>(); The ArrayList method add adds an Object of the type specified in the array list declaration. The ArrayList method get returns an Object of the type specified in the array list declaration. 11 What happens? import java.util.ArrayList; public class StringTest { public StringTest() { ArrayList<String> stringList = new ArrayList<String>(); stringList.add("Fran"); stringList.add("Marie"); stringList.add("Joe"); System.out.println(stringList); } } 12