The ArrayList class CSC 123 Fall 2018 Howard Rosenthal

Transcription

1 The ArrayList class CSC 123 Fall 2018 Howard Rosenthal

2 Lesson Goals Describe the ArrayList class Discuss important methods of this class Describe how it can be used in modeling Much of the information in this section is adapted from: 2

3 Why Use The ArrayList class Programs frequently keep data in lists Often, arrays are used for this and arrays are a fundamental feature of most programming languages But because lists are so useful, Java includes the ArrayList class, which works much like an array with extra methods and features Like an array, an ArrayList contains elements that are accessed using an integer index However, unlike an array, the size of an ArrayList is not fixed Remember that once you declared String [] list = new String[3]; the length of list could not be changed An ArrayList expands as needed as items are added to it An ArrayList has easy to use methods for inserting and removing elements from it You can implement anything in ArrayList using an array, but ArrayList save us a lot of work 3

4 Drawbacks Of The ArrayList class The elements of an ArrayList must be object references, not primitive data like int or double Remember how we used arrays of String objects, or even an array of arrays ArrayList operations are slightly slower than array operations Question: How can we use the ArrayList with primitive types? 4

5 A Simple Example Look at ArrayListExample1.java The example program creates an ArrayList that holds String references and then adds references to three Strings. The statement ArrayList<String> names = new ArrayList<String>(); creates an ArrayList of String references. The phrase <String> can be read as "of String references Note that there are "angle (sometimes called diamond> brackets" on each side of <String>. The phrase ArrayList<String> describes both the type of the object that is constructed (an ArrayList) and the type of data it will hold (references to String). The names.get( int index ) method returns the reference in the cell at index If the cell is empty, an IndexOutOfBoundsException will be thrown and the program will halt (unless there is code to handle the exception) Unlike in an array, cells don t automatically default to the null value 5

6 Picturing Instantiation Of An ArrayList of String Objects Amy names names[0] names[1] names[2] ArrayList methods method1 method2 o o o String methods method1 method2 o o o Cindy String methods method1 method2 o o o Bob String methods method1 method2 o o o names[0] through names[2] are reference variables, each of which refers to a String The methods of each of the String variables are String methods, while the methods of names are ArrayList methods 6

7 Some Basic Rules And Definitions A program must import the java.util package to use ArrayList import java.util.arraylist; By default, an ArrayList starts out with 10 empty cells, although (as we will see, you can specify a different number) As we said, unlike in an array, cells don t automatically default to the null value ArrayList is a generic type, This means that its constructor specifies both the type of object to construct and the type that the new object will hold The type the object will hold is placed inside angle brackets like this: <DataType> Generics are in effect parameterized types, meaning that the data type will be defined at the time that the client class declares and instantiates an object of that class Now, when the ArrayList object is constructed, it will hold data of type "reference to DataType This is a collection of data of that particular type When we build our own classes starting with lesson 9 this will become more helpful Question: What type of data will values hold in each cell? ArrayList<Integer> values = new ArrayList<Integer>(); 7

8 Constructing An ArrayList Object (1) To declare a reference variable for an ArrayList do this: ArrayList<E> myarray; // E is any class that is available to the program myarray is a reference to a future ArrayList object. The future ArrayList object will contain an array of references to objects of type E or to a descendant class of E. Construct the ArrayList later on in the code: myarray = new ArrayList<E>(); The array has an initial capacity of 10 cells, although the capacity will increase as needed as references are added to the list Cells will contain references to objects of type E (or a descendant class) Since constant expansion may not be very efficient, f you have an idea of what the capacity you need, start the ArrayList with that capacity myarray = new ArrayList<E>(int initialcapacity); 8

9 Constructing An ArrayList Object (2) Just as with any other variables you can combine the two steps to both declare a reference variable and construct an ArrayList: ArrayList<E> myarray = new ArrayList<E>(); To declare a variable and to construct a ArrayList with a specific initial capacity do this: ArrayList<E> myarray = new ArrayList<E>( int initialcapacity ); The initial capacity is the number of cells that the ArrayList starts with. It can expand beyond this capacity if you add more elements. Expanding the capacity of an ArrayList is slow so avoid this, estimate how many elements are needed and construct an ArrayList of that many plus some extra 9

10 Null and ArrayList An ArrayList element can be an object reference or the value null When a cell contains null, the cell is not considered to be empty. The picture shows empty cells with an "X" and cells that contain a null with null Remember in an ArrayList a cell is not automatically initialized to null You d need to say: names.add(null); no quotes 10

11 Spot Check - Formulating An ArrayList Object Say that you are writing a program to keep track of the students in a course. There are usually about 25 students, but a few students may add the class, and a few students may drop the class. Data for a student is kept in a Student object. Declare and construct an ArrayList suitable for this situation. 11

12 Answer To Spot Check Question Say that you are writing a program to keep track of the students in a course. There are usually about 25 students, but a few students may add the class, and a few students may drop the class. Data for a student is kept in a Student object. Declare and construct an ArrayList suitable for this situation. ArrayList<Student> students = new ArrayList<Student>( 30 ); 12

13 Quick Check Here is a declaration and construction of a ArrayList: ArrayList<String> data = new ArrayList<String>(10); Which of the following statements will work? data.add( "Irene Adler" ); data.add( new String("Laura Lyons") ); data.add( 221 ); data.add( new Integer( 221 ) ); 13

14 Quick Check - Answers Here is a declaration and construction of a ArrayList: ArrayList<String> data = new ArrayList<String>(10); Which of the following statements will work? data.add( "Irene Adler" ); // OK data.add( new String("Laura Lyons") ); //OK data.add( 221 ); // wrong type of argument data.add( new Integer( 221 ) ); // wrong type of argument 14

15 Capacity And Size The capacity is the total number of cells in an ArrayList Capacity cannot be retrieved The size is the number of cells that have data in them. Cells 0 up through size-1 have data in them Data are added in order. Before cell N gets data, cells 0, 1, 2,... N- 1 must hold data The size increases by one each time an element is added However, the capacity remains unchanged until the ArrayList is full When an element is added to a full list, the Java runtime system will greatly increase the capacity of the list so that many more elements can be added. To find out the current size of an ArrayList use its size() method: names.size() returns the size of names 15

16 Common Methods Of The ArrayList class (1) Method Name Description Example void add(int index, Object element) boolean add(object o) void clear() void ensurecapacity(int mincapacity) Object get(int index) Inserts the specified element at the specified position index in the ArrayList. Throws IndexOutOfBoundsException if the specified index is out of range (index < 0 index > size()) Appends the specified element to the end of this list Removes all of the elements from this ArrayList Manually increases the capacity of this ArrayList instance, if necessary, to ensure that it can hold at least the number of elements specified by the minimum capacity argument. Returns the element at the specified position in this list. Throws IndexOutOfBoundsException if the specified index is out of range (index < 0 index >= size()). See ArrayListExampleAddAtIndex.java See ArrayListExample1.java names.clear() empties the cell, it does not place the null address in the cell. Size() returns 0 names.ensurecapacity(15); increases capacity to at least 15 For our example of names names.get(1) returns Bob 16

17 Common Methods Of The ArrayList class (2) Method Name Description Example int indexof(object o) int lastindexof(object o) Object remove(int index) Object set(int index, Object element) int size() void trimtosize() Returns the index in the ArrayList of the first occurrence of the specified element, or -1 if the ArrayList does not contain this element. Returns the index in the ArrayList of the last occurrence of the specified element, or -1 if the ArrayList does not contain this element. Removes the element at the specified position in the ArrayList. Throws IndexOutOfBoundsException if the index out is of range (index < 0 index >= size()). Replaces the element at the specified position in the ArrayList with the specified element. Throws IndexOutOfBoundsException if the specified index is out of range (index < 0 index >= size()). Returns the number of elements in the ArrayList. Trims the capacity of this ArrayList instance to be the ArrayList s current size. See ArrayListExampleRemove.java See ArrayListExampleRemove.java See ArrayListExampleRemove.java See ArrayListExampleSet.java See ArrayListExample1.java 17

18 Understanding Remove When you remove an element from the middle of an ArrayList the other element move up 18

19 The ArrayList tostring method The ArrayList class has a tostring method that returns a String representing all of the items stored in an ArrayList object. For example, suppose we have set up the namelist object with the Strings James, Catherine, and Bill. We could use the following statement to display all of the names: System.out.printf( %s\n, namelist); The contents of the ArrayList will be displayed in the following manner: [James, Catherine, Bill] See ArrayListToString.java 19

20 Using The Iterator And iterator() Method An Introduction Often you wish to access the elements of an ArrayList one by one, in order You could write a counting loop You can use an Iterator object. To get an Iterator object, use the iterator method of ArrayList: Iterator<E> iterator() // Returns an iterator i.e. Iterator<String> iter = names.iterator(); //creates an Iterator object ArrayList implements the Iterable interface (to be discussed later). iterator() is the only method in this interface An iterator object is used to visit the elements of a list one by one It visits only the cells that have data (so you don't need to worry about going past the end of data) This is more convenient than writing a loop. 20

21 Iterator Methods An iterator implements the Iterator<E> interface, which has the following methods: boolean hasnext() // Returns true if not all elements have been visited E next() // Returns the next element of the list, a reference to type E void remove() // Remove from the list the element just returned by next() You need to import the Iterator class: import java.util.iterator; See ArrayListExampleIterator.java 21

22 Using The Enhanced For Loop With The ArrayList Class The enhanced for loop (sometimes called a "for each" loop) can be used with any class that implement the Iterable interface, such as ArrayList for (ClassName currentobject: ArrayListName) { Process currentobject } The program does the same thing as we showed with the Iterator The enhanced for loop accesses the String references in names and assigns them one-by-one to currentobject With an enhanced for loop there is no danger an index will go out of bounds The enhanced for loop only visits those cells that are not empty, beginning with cell zero Since an ArrayList never has gaps between cells, all occupied cells are visited See ArrayListExampleIterator.java 22

23 IntegersInArrayList Programming Exercise 1 Request and read in 10 integers from the terminal and put into an ArrayList. Remember, an ArrayList can only store object references, so you will need to take advantage of autoboxing. Then add the numbers together that have been placed in the array, first with the use of an iterator, and then with an enhanced for loop, printing out the results in each case. 23

24 Programming Exercise 2 CharacterArrayListExample In main read in a String using nextline (allowing blanks). Convert it to a StringBuilder object. The input String can consist of any type of characters. Pass the StringBuilder object reference to a method called intoarraylist. In intoarraylist extract the characters one at a time and place into an ArrayList called charlist (remember you will need to use the Character wrapper class). Pass a reference to charlist back to main. Then have main call a method called charsum with a reference to the previously created ArrayList. charsum returns an integer which is the sum of the Unicode values of the characters in the ArrayList. The main method then prints out the ArrayList with 4 spaces between each character, taking advantage of tostring. Underneath each character you should then print the Unicode value of the character. On the final line print out the returned integer which is the sum of those Unicode values. 24

25 Some Samples: Please enter a String of any length: ABC A B C Programming Exercise 2 My Number 123!# M y N u m b e r 1 2 3! # The sum of the Unicode values is