© A+ Computer Science - www.apluscompsci.com ArrayList © A+ Computer Science - www.apluscompsci.com Lab 16.

© A+ Computer Science - www.apluscompsci.com
ArrayList © A+ Computer Science - Lab 16

ArrayList Arraylist is a class that houses an array. It is internally implemented in Java as an array. An ArrayList can store any type. It can store a combination of types. All ArrayLists store the first reference at index/position/subscript 0. ArrayList can store a reference to any type of Object. ArrayList was built using an array[] of object references. © A+ Computer Science -

What is an array? int[] nums = new int[10]; //Java int array nums An array is a group of items all of the same type which are accessed through a single identifier. © A+ Computer Science -

ArrayList ArrayList is a descendant of List (which extends Collection), but because List and Collection are interfaces, you cannot instantiate them. Collection bad = new Collection(); //illegal List ray = new ArrayList(); //legal ArrayList list = new ArrayList(); //legal In the example above, ray is an ArrayList that stores Object references. In order to call non-Object methods on a spot in ray, casting would be required. ray.add(0,"hello"); out.println(((String)ray.get(0)).charAt(0)); ray and list store Object references. © A+ Computer Science -

ArrayList References ArrayList list; list null null nothing A reference variable is used to store the location of an Object. In most situations, a reference stores the actual memory address of an Object. list stores the location / memory address of an ArrayList. list is a reference to an ArrayList. © A+ Computer Science -

ArrayList Instantiation © A+ Computer Science - www.apluscompsci.com
new ArrayList(); 0x213 [] ArrayLists are Objects. © A+ Computer Science -

ArrayList ArrayList list = new ArrayList(); list 0x213 0x213 [] A reference variable is used to store the location of an Object. In most situations, a reference stores the actual memory address of an Object. list stores the location / memory address of an ArrayList. list is a reference to an ArrayList. © A+ Computer Science -

ArrayList Example of an ArrayList without the type <T> identifier: List ray = new ArrayList(); ray.add("hello"); ray.add(“school"); ray.add("contests"); System.out.println(( (String) ray.get(0)).substring(0,1)); System.out.println(( (String) ray.get(2)). substring(0,1)); OUTPUT h c In the example above, ray is an ArrayList that stores Object references. In order to call non-Object methods on a spot in ray, casting would be required. ray.add(0,"hello"); out.println(((String)ray.get(0)).charAt(0)); casting (must cast in order to send messages to the object) ray stores Object references. © A+ Computer Science -

Open arraylistobjects.java © A+ Computer Science -

Generic ArrayLists Generic ArrayLists are array lists that take a type parameter. In other words you identify the type of all the elements in the array list. © A+ Computer Science -

ArrayList With Java 5, you can now specify which type of reference you want to store in the ArrayList. ArrayList<String> words; words = new ArrayList<String>(); ArrayList<Double> decNums; decNums = new ArrayList<Double>(); In the example above, words can only store String references. decNums can only store Double references. Java knows the exact type of reference in both ArrayLists; thus, there is no need for casting when accessing class specific methods. words.add("Hello"); out.println(words.get(0).charAt(0)); © A+ Computer Science -

ArrayList With Java 5, you can now specify which type of reference you want to store in the ArrayList. ArrayList<Integer> bigStuff; bigStuff = new ArrayList<Integer>(); ArrayList<Student> studentList; studentList = new ArrayList<Student>(); In the example above, words can only store String references. decNums can only store Double references. Java knows the exact type of reference in both ArrayLists; thus, there is no need for casting when accessing class specific methods. itList.add(new It(34.21)); out.println(itList.get(0).getIt()); © A+ Computer Science -

ArrayList List<String> ray; ray = new ArrayList<String>(); ray.add("hello"); ray.add(“school"); ray.add("contests"); System.out.println(ray.get(0).substring(0,1)); System.out.println(ray.get(2).substring(0,1)); OUTPUT h c In the example above, ray is an ArrayList that stores String references. Casting would not be required to call non-Object methods on ray. ray.add(0,"hello"); ray.add(1,"chicken"); out.println(ray.get(0).charAt(0)); out.println(ray.get(1).charAt(5)); ray stores String references. © A+ Computer Science -

Open arraylistgenerics.java © A+ Computer Science -

ArrayList Methods © A+ Computer Science -

frequently used methods © A+ Computer Science - www.apluscompsci.com
ArrayList frequently used methods Name Use add(item) adds item to the end of the list add(index,item) adds item at index – shifts items up-> set(index,item) replaces item at index (returns old element) (similar to z[index]=item) get(index) returns the item at index (similar to z[index] ) size() returns the # of items in the list (similar to z.length) remove(index) removes the item at index (returns element that was removed) remove(value) removes first item that matches this value (returns true if item was found, else false) clear() removes all items from the list import java.util.ArrayList; © A+ Computer Science -

add() ArrayList<String> words; words = new ArrayList<String>(); words.add(0,"it"); words.add("is"); words.add("a"); words.add("lie"); System.out.println(words); OUTPUT [it, is, a, lie] The add(item) method adds item to the end of the ArrayList. The add(spot, item) method adds the item at the spot specified. All other items are shifted toward the end of the ArrayList. © A+ Computer Science -

add() ArrayList<Integer> nums; nums = new ArrayList<Integer>(); nums.add(34); nums.add(0,99); nums.add(21); nums.add(11); System.out.println(nums); OUTPUT [99, 34, 21, 11] The add(item) method adds item to the end of the ArrayList. The add(spot, item) method adds the item at the spot specified. All other items are shifted toward the end of the ArrayList. © A+ Computer Science -

Open arraylistaddone.java arraylistaddtwo.java © A+ Computer Science -

set() ArrayList<Integer> ray; ray = new ArrayList<Integer>(); ray.add(23); ray.add(11); ray.set(0,66); ray.add(53); ray.set(1,93); ray.add(22); System.out.println(ray); OUTPUT [66, 93, 53, 22] The add(item) method adds item to the end of the ArrayList. The set(spot, item) method replaces the reference at spot with item. The location / address of item is placed in spot. © A+ Computer Science -

size() for (int i=0; i<ray.size(); i++) { System.out.println(ray.get(i)); } The size() method returns the number of items in the ArrayList. If the ArrayList is storing seven references, size() would return a 7. .size( ) returns the number of elements (logical/physical size) in the array list. © A+ Computer Science -

get() OUTPUT23.23 65.6 ArrayList<Double> ray;
ray = new ArrayList<Double>(); ray.add(23.23); ray.add(11.11); ray.add(12.1); ray.add(65.6); System.out.println(ray.get(0)); System.out.println(ray.get(3)); OUTPUT The get(spot) method returns the reference stored at spot. .get(index) returns the reference stored at the index! © A+ Computer Science -

get() OUTPUT23 11 12 65 ArrayList<Integer> ray;
ray = new ArrayList<Integer>(); ray.add(23); ray.add(11); ray.add(12); ray.add(65); for(int i=0; i<ray.size(); i++) System.out.println(ray.get(i)); OUTPUT The get(spot) method returns the reference stored at spot. .get(index) returns the reference stored at the index! © A+ Computer Science -

enhanced for loop ArrayList<Integer> ray; ray = new ArrayList<Integer>(); ray.add(23); ray.add(11); ray.add(12); ray.add(65); for( int val : ray ) System.out.println(val); OUTPUT The ehanced for loop pulls one value at a time from ray and puts each value in val. val receives one value from ray each iteration. © A+ Computer Science -

Open arraylistset.java arraylistget.java © A+ Computer Science -

remove methods © A+ Computer Science -

remove() ArrayList<String> ray; ray = new ArrayList<String>(); ray.add("a"); ray.add("b"); ray.remove(0); ray.add("c"); ray.add("d"); System.out.println(ray); OUTPUT [c, d] The remove method will remove the item at the specified spot / location or the specified value. When an item is removed, all items above the removed item are shifted down toward the front of the ArrayList. All items are shifted to the left. [a, b] becomes [b] [b, c, d] becomes [c, d] © A+ Computer Science -

remove() ArrayList<String> ray; ray = new ArrayList<String>(); ray.add("a"); ray.add("b"); ray.remove("a"); ray.add("c"); ray.add("d"); ray.remove("d"); System.out.println(ray); OUTPUT [b, c] The remove method will remove the item at the specified spot / location or the specified value. When an item is removed, all items above the removed item are shifted down toward the front of the ArrayList. All items are shifted to the left. [a, b] becomes [b] [b, c, d] becomes [b, c] © A+ Computer Science -

Open arraylistremoveone.java arraylistremovetwo.java © A+ Computer Science -

clear() ArrayList<String> ray; ray = new ArrayList<String>(); ray.add("a"); ray.add("x"); ray.clear(); ray.add("t"); ray.add("w"); System.out.println(ray); OUTPUT [t, w] The clear() method removes all items from the ArrayList. The ArrayList becomes an [] empty ArrayList. © A+ Computer Science -

Open arraylistclear.java © A+ Computer Science -

What, no initializer list?
How can you initialize an ArrayList? ArrayList<String> names = new ArrayList<String>(); names.add(“Smith”); names.add(“Jones”); Or similar to an initializer list for an array: ArrayList<String> names = new ArrayList<String> (Arrays.asList("Smith", "Jones")); import java.util.ArrayList; import java.util.Arrays; © A+ Computer Science -

EmployeeArrayList Purpose: To get familiar with ArrayLists Make a copy of your EmployeeNames and EmployeeNamesTester and name them EmployeeArrayList and EmployeeArrayListTester. Change all occurrences of arrays to ArrayLists. © A+ Computer Science -

enhanced for loop ArrayList<Integer> ray; ray = new ArrayList<Integer>(); ray.add(23); ray.add(11); ray.add(53); for(int num : ray) { System.out.println(num); } OUTPUT The new for loop is great to print out Arrays and Collections. The new for loop extracts an item from ray each time it iterates. The new for loop is an iterator based loop. Once the loop reaches the end of ray, it stops iterating. © A+ Computer Science -

Open newforloopone.java © A+ Computer Science -

AutoBoxing AutoUnboxing © A+ Computer Science -

Objects (wrapper class) © A+ Computer Science - www.apluscompsci.com
Box/Unbox Primitive types Objects (wrapper class) byte Byte short Short int Integer long Long float Float double Double char Character boolean Boolean == .equals() For each of the primitive types, there is a corresponding wrapper class. int has a corresponding wrapper class named Integer. int stores a non-decimal value. Integer stores the location / memory address of an Integer Object which stores an int value. double has a corresponding wrapper class named Double. double stores decimal and non-decimal values. Double stores the location / memory address of a Double Object which stores a double value. © A+ Computer Science -

Box/Unbox Before Java 5 added in autoboxing and autounboxing, you had to manually wrap primitives. Integer x = new Integer(98); or int y = 56; x= new Integer(y); Before autoboxing and autounboxing, a reference could only refer to a primitive if a Wrapper class was instantiated and the primitive passed to the constructor. © A+ Computer Science -

Box/Unbox Java now wraps automatically. Integer num1 = 99; and Integer num1 = new Integer(99); These two lines are equivalent. Integer numOne = 99; is equivalent to Integer numOne = new Integer(99); With the introduction of Java 5, the new Integer() Object instantiation code happens in the background. Java takes care of these details, but does not show the work. © A+ Computer Science -

Box/Unbox Java now wraps automatically. Double num1 = 99.1; and Double num1 = new Double(99.1); These two lines are equivalent. Double numOne = 99; is equivalent to Double numOne = new Double(99); With the introduction of Java 5, the new Double() Object instantiation code happens in the background. Java takes care of these details, but does not show the work. © A+ Computer Science -

Box/Unbox Before Java 5 added in autoboxing and autounboxing, you had to manually unwrap references. If you boxed 98 as an Integer like this: Integer ref = new Integer(98); Then you had to unbox it like this: int y = ref.intValue(); Before autoboxing and autounboxing, a reference value could only be stored in a primitive if the corresponding reference get method was called to retrieve the primitive value from the reference. © A+ Computer Science -

Box/Unbox Java now unwraps automatically. int prim; Integer num = new Integer(3); OLD WAY: prim=num.intValue(); System.out.println(prim); NEW WAY: prim = num; These two lines are equivalent. OUTPUT 3 3 Integer numOne = new Integer(99); int primInt = numOne; is equivalent to int primInt = numOne.intValue(); With the introduction of Java 5, the intValue() method call happens in the background. Java takes care of these details, but does not show the work. © A+ Computer Science -

Box/Unbox OUTPUT Double dub = 9.3; double prim = dub; System.out.println(prim); int num = 12; Integer big = num; System.out.println(big.compareTo(12)); System.out.println(big.compareTo(17)); System.out.println(big.compareTo(10)); Before autoboxing and autounboxing, a reference could only refer to a primitive if a Wrapper class was instantiated and the primitive passed to the constructor. Before autoboxing and autounboxing, a reference value could only be stored in a primitive if the corresponding reference get method was called to retrieve the primitive value from the reference. With the introduction of Java 5, the wrapping and unwrapping / boxing and unboxing happens in the background. Java takes care of these details, but does not show the work. © A+ Computer Science -

Open autoboxunbox.java © A+ Computer Science -

enhanced for loop ArrayList<Integer> ray; ray = new ArrayList<Integer>(); //add some values to ray int total = 0; for(Integer num : ray) { //this line shows the AP preferred way //it shows the manual retrieval of the primitive total = total + num.intValue(); // total += num.intValue(); //the line below accomplishes the same as the line above //but it uses autounboxing to get the primitive value total = total + num; // total += num; } System.out.println(total); OUTPUT153 The new for loop is great to print out Arrays and Collections. The new for loop extracts an item from ray each time it iterates. The new for loop is an iterator based loop. Once the loop reaches the end of ray, it stops iterating. © A+ Computer Science -

Open newforloopone.java newforlooptwo.java © A+ Computer Science -

Start work on Lab 16a (Factor List) © A+ Computer Science -

ArrayList with User-defined classes © A+ Computer Science -

ArrayList w/User Classes
public class Creature implements Comparable { private int size; public Creature(int girth) size=girth; } public void setSize(int girth) public String toString() return “” + size; //Other methods not shown here Instance variable Constructor initializes size of Creature Creature is a class used to store information about Creatures. toString() returns the String representation of Creature © A+ Computer Science -

ArrayList w/User Classes © A+ Computer Science - www.apluscompsci.com
ArrayList<Creature> creatureList; creatureList = new ArrayList<Creature>(); creatureList.add(new Creature(4)); creatureList.add(new Creature(9)); creatureList.add(new Creature(1)); 0x12 0x32 0xD2 creatureList Creature Creature Creature © A+ Computer Science -

ArrayList<Creature> creatureList; creatureList = new ArrayList<Creature>(); creatureList.add(new Creature(4)); creatureList.add(new Creature(9)); creatureList.add(new Creature(1)); System.out.println(creatureList.get(0)); creatureList.get(0).setSize(7); System.out.println(creatureList.get(2)); OUTPUT 4 7 1 Creatures is an ArrayList that stores reference to Creature objects. In this example, creatures stores the addresses / locations of 3 Creature objects. © A+ Computer Science -

creatureList.get(0) . setSize(7); 0x242 0x242 Creature What does this return? What does the . dot do? Creatures.get(0) returns the address / location of a Creature object. Applying a . dot to a reference grants you access to the object referred to by the reference. .setSize() is a Creature method that changes the size property of the Creature. The . dot grants access to the Object at the stored address. © A+ Computer Science -

Collections frequently used methods Name Use sort(x) Puts all items in array list x in ascending order binarySearch(x,y) Prerequisite: array list x must be sorted. Checks x for the location of y. This method returns the index where y is found. If y is not found, it returns -1 minus (the position in the array list where x would be added). fill(x,y) Fills all elements in x with value y rotate(x,y) Shifts items in x left or right y locations reverse(x) Reverses the order of the items in x import java.util.Collections; © A+ Computer Science -

Collections ArrayList<Integer> ray; ray = new ArrayList<Integer>(); ray.add(23); ray.add(11); ray.add(66); ray.add(53); Collections.sort(ray); System.out.println(ray); System.out.println(Collections.binarySearch(ray,677)); System.out.println(Collections.binarySearch(ray,66)); OUTPUT [11, 23, 53, 66] -5 3 Collections.sort() will put all items in natural ascending order. Collectoins.binarySearch() will locate an item. If the item does not exist, binarySearch() will return -1+ -(where the value would be if it was there). -3 is (2 is the spot where the item would be) © A+ Computer Science -

Collections ArrayList<Integer> ray = ArrayList<Integer>(); ray.add(23); ray.add(11); ray.add(53); ray.add(100); System.out.println(ray); Collections.rotate(ray,2); Collections.rotate(ray,2); // back to [23, 11, 53, 100] Collections.reverse(ray); OUTPUT [23, 11, 53, 100] [53, 100, 23, 11] [100, 53, 11, 23] Collections.rotate() rotates items to the right or to the left a specified number of spots / positions. A negative number rotates to the left and a positive number rotates to the right. Collections. reverse() reverses the order of all items. © A+ Computer Science -

Collections ArrayList<Integer> ray; ray = new ArrayList<Integer>(); ray.add(0); ray.add(20); ray.add(50); System.out.println(ray); Collections.fill(ray,33); OUTPUT [0, 20, 50] [33, 33, 33] Collections.fill() will fill in all spots with a specified value. © A+ Computer Science -

ArrayList frequently used methods Name Use contains(y) Checks to see if the list contains y. Returns either true or false. indexOf(y) Checks the array list for the location of y. Returns the index where y is found. If not found, it returns -1. isEmpty() Returns true if the array list is empty (no elements) or false if it contains elements. © A+ Computer Science -

search methods ArrayList<Integer> ray; ray = new ArrayList<Integer>(); ray.add(23); ray.add(11); ray.add(66); ray.add(53); System.out.println(ray); System.out.println(ray.indexOf(21)); System.out.println(ray.indexOf(66)); System.out.println(ray.contains(21)); System.out.println(ray.contains(66)); OUTPUT [23, 11, 66, 53] -1 2 false true © A+ Computer Science -

Java Interfaces The following are important interfaces included in the Java language: Collection List © A+ Computer Science -

Map Java Collection Sub Interfaces -extends Collection SortedMap Implementing Classes Sub Interfaces -extends HashMap HashTable Implementing Classes List Set Sub Interfaces -extends TreeMap Implementing Classes SortedSet This Collections hierarchy chart is very important. It is a must to know which classes implement which interfaces and which interfaces extend which interfaces. Implementing Classes ArrayList LinkedList Vector Stack TreeSet AbstractSet HashSet LinkedHashSet © A+ Computer Science -

The Collection Interface © A+ Computer Science - www.apluscompsci.com
The Collection interface is the parent of List and Set. The Collection interface has many methods including add(), clear(), remove(), and size(). Collection List Set others not shown © A+ Computer Science -

The List Interface The List interface extends the Collection interface. Additionally, the List interface also has the get() method as well as several other methods. List ArrayList LinkedList others not shown © A+ Computer Science -

ArrayList ArrayList is a descendant of List and Collection, but because List and Collection are interfaces, you cannot instantiate them. Collection bad = new Collection(); //illegal List justAsBad = new List(); //illegal List list = new ArrayList(); //legal ArrayList aList = new ArrayList(); //legal Collection col = new ArrayList(); //legal ** ** Legal only if using methods from Collection interface In the example above, ray is an ArrayList that stores Object references. In order to call non-Object methods on a spot in ray, casting would be required. ray.add(0,"hello"); out.println(((String)ray.get(0)).charAt(0)); list, aList, and col store Object references. © A+ Computer Science -

© A+ Computer Science - www.apluscompsci.com ArrayList © A+ Computer Science - www.apluscompsci.com Lab 16.

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