AP CS Workshop ArrayList It is very common for applications to require us to store a large amount of data. Array lists store large amounts of data in a single collection that can be referred to with a single variable.

AP CS Workshop ArrayList An ArrayList is a sequence of objects that grows and shrinks as needed. The ArrayList class is part of the java.util package of the Java standard class library. –must import java.util.ArrayList

AP CS Workshop 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.

AP CS Workshop java.util.ArrayList (implements List) ArrayList is generic ArrayList

AP CS Workshop ArrayList s for AP CS A Notice that the AP CS A Subset does not require the knowledge that ArrayList implements List.

AP CS Workshop java.util.ArrayList 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

AP CS Workshop class java.util.ArrayList 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.

AP CS Workshop Course Description AP CS AB Notice that the AB Subset DOES require the knowledge that ArrayList implements List. iterators SHOULD BE TAUGHT in A

AP CS Workshop An example: import java.util.ArrayList; public class ArrayList_01 { public static void main (String [] arg) { System.out.println("ArrayListTest"); ArrayList aList = new ArrayList (); aList.add(new String("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)); }

AP CS Workshop Using enhanced FOR loop: import java.util.ArrayList; public class ArrayList_01_ForEach { public static void main (String [] arg) { System.out.println("ArrayListTest"); ArrayList aList = new ArrayList (); aList.add(new String("Dan")); aList.add("George"); aList.add("Mary"); System.out.println("aList contains:"); for(String e:aList) { System.out.println(e); }

AP CS Workshop Other ArrayList methods // ArrayList_02 ArrayList students = new ArrayList (); 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());

AP CS Workshop What happens? ArrayList_02 ArrayList students = new ArrayList (); 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()); MaryJamesKevin MaryTanyaJamesKevin temp MaryTanyaKevin MaryJohnKevin

AP CS Workshop 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 athletes = new ArrayList (); –ArrayList csci6 = new ArrayList (); –ArrayList accounts = new ArrayList (); 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.

AP CS Workshop A bit more about toString()

AP CS Workshop What happens? public class StringTest { public static void main(String[] args) { ArrayList stringList = new ArrayList (); stringList.add("Fran"); stringList.add("Marie"); stringList.add("Joe"); System.out.println(stringList); }

AP CS Workshop WHY???? public class StringTest { public static void main(String[] args) { ArrayList stringList = new ArrayList (); stringList.add("Fran"); stringList.add("Marie"); stringList.add("Joe"); System.out.println(stringList); } [Fran, Marie, Joe] Let's look at the API!

AP CS Workshop The ArrayList is defined using generics. A generic is a method that is recompiled with different types as the need arises. List employeeList = new ArrayList (); Use of generics provides compile-time checking to make sure you are using the correct type Employee emp = employeeList.get(i).getName();

AP CS Workshop ArrayLists contain Objects Numbers are not objects. You can not have an ArrayList of int s or double s or boolean s or char s.

AP CS Workshop Again: An ArrayList is a sequence of objects. Array lists can hold any kind of object. The ArrayList method add adds an Object of specified type. The ArrayList method get returns an Object of specified type. What if we want our ArrayList to hold int s or double s?

AP CS Workshop Filling the ArrayList for (int k = 1; k <= 20; k++) { nbrs.add(new Integer(k)); }

AP CS Workshop Printing the ArrayList for(Integer e : nbrs) { System.out.println(e); } Integer has a toString method that does the right thing.

AP CS Workshop Printing the sum of the integer values in the ArrayList int sum = 0; Integer temp; for (int k = 0; k < nbrs.size(); k++) { temp = nbrs.get(k); sum += temp.intValue(); } System.out.println("sum equals: " + sum);

AP CS Workshop Printing the sum of the integer values in the ArrayList //alternate method using enhanced for and auto //boxing/unboxing sum = 0; for (Integer e:nbrs) { sum += e; } System.out.println("sum equals: " + sum);

AP CS Workshop Java 5.0 introduced Auto-boxing and Auto-unboxing (not tested) –The idea of auto-boxing and auto-unboxing is to make it easier to convert between primitive data types like int, double, and boolean and their equivalent classes, like Integer, Double, and Boolean. It is sometimes frustrating and tedious to have to do such a conversion, especially if results must be converted back to their original form.

AP CS Workshop Autoboxing/unboxing ArrayList numbers = new ArrayList (); numbers.add(14.5); numbers.add(3.4); double sum = numbers.get(0) + numbers.get(1);

AP CS Workshop Auto-boxing and Auto-unboxing Integer/int equality tests int == intOK Integer.equals(Integer)OK Integer == intOK Integer.equals(int)OK Integer.compareTo(int)==0OK

AP CS Workshop Auto-boxing and Auto-unboxing Integer/int equality tests –int.equals(Integer)NO –int.equals(int)NO –Integer == IntegerNO (as test for value equality) –Integer.equals(null)NO

AP CS Workshop Auto-boxing and Auto-unboxing NOT part of the AP CS Testable Subset. Students need to understand –intValue() –doubleValue()

AP CS Workshop ArrayList Applications Finding values, counting, max and min, average, median, mode….. Your students should be able to manipulate all sorts of algorithms for ArrayList s.

AP CS Workshop AP CS A 2007 Question 3a

AP CS Workshop AP CS A 2007 Question 3b

AP CS Workshop AP CS A 2007 Question 3b