Chapter 13: Arrays of Objects

Transcription

1 Chapter 13: Arrays of Objects Think Java: How to Think Like a Computer Scientist by Allen B. Downey As in Chapter 11, we will be following a more standard approach than the one taken by Downey in this chapter. Instead, please refer to Chapter 5 Sections and Chapter 7 of Eck, "Java Notes", also linked from the class website.

2 Review: Arrays An array is a set of values where each value is identified by an index. array elements int size = 6; array index 1-2

3 Access Array Elements using the [ ] operator Note: as with String, elements go from 0 to 3 count[4] = 2; ArrayOutOfBounds Execption 1-3

4 Arrays of Primitives (char, int, double, boolean) When we create an array of primitive data, the data is stored in each element of the array: char[] grades = {'A','B','D','F','B'}; grades 'A' 'B' 'D' 'F' 'B' boolean[] pass = new boolean[grades.length]; pass false false false false false

5 Arrays of Primitives We usually process arrays using for loops for (int k = 0; k < grades.length; k++) if (grades[k] >= 'A' && grades[k] <= 'C') pass[k] == true; k grades 'A' 'B' 'D' 'F' 'B' pass false false false false false

6 We can also create our own Objects Remember, object variables store a reference to where the object is stored Time t = new Time(11,30,40); t hour 11 minute 30 second

7 Arrays of Objects: Each element is a reference to an object Time [] appts = new Time[5]; appts[0] = new Time(9,20,0); appts[1] = new Time(11,15,0); appts null null null null null hour 9 minute 20 hour 11 minute 15 second 0 second 0 1-7

8 To modify an object in an array, specify array[index] first appts[0].hour = 12; appts[1].minute = 30; appts[2].second = 15; // Null Pointer Exception! appts null null null null null hour 9 minute 20 hour 11 minute 15 second 0 second 0 1-8

9 If we make the instance variables private, we have to use the set methods appts[0].sethour(12); appts[1].setminute(30); appts[2].setsecond(15); // Null Pointer Exception! appts null null null null null hour 9 minute 20 hour 11 minute 15 second 0 second 0 1-9

10 You can use a for loop to create all the objects for (int k = 0; k < appts.length; k++) appts[k] = new Time(); appts

11 And another for loop to print them all out for (int k = 0; k < appts.length; k++) appts[k].print(); Console 12:00:00 12:00:00 12:00:00 12:00:00 12:00:00 appts

12 For Lab 13 we will practice creating arrays of different objects 1-12

13 Writing Methods that Process Arrays This method would be defined in the ArrayOfObjects class public static void printalltimes(time s [] times) { } for (int k = 0; k < times.length; k++) times[k].print(); System.out.println(""); // go to new line Calling the method from main: printalltimes(appts); 1-13

14 Other Classes we will work with in Lab13 String: String name = "Jasper"; Employee: Employee cpa = new Employee("Bill",1111,37000); 1-14

15 Arrays of Objects you can create Array of String: String [] dictionary = new String[100000]; String [] names = {"Joe","Allie","Sam","Fay"}; Array of Scrabble Tile objects: Tile [] letterbag = new Tile[99]; Tile [] player1rack = new Tile[7]; Array of Employee: Employee [] marketingteam = new Employee[15]; Array of Card: Card [] pokerhand = new Card[4]; 1-15

16 Do Lab 13 Problems 0 through 3 First define the Employee class (review) 1-16

17 Part B Lecture Additional OOP (object) methods tostring equals compareto A Card class Reading data from a file into an array Working with partially filled arrays 1-17

18 Our OOP Model for objects Each class that can make objects has private instance variables accessor methods (get) modifier methods (set) print method 1-18

19 Example: Time class instance variables: int hour, minute; double second; accessor methods gethour, getminute, getsecond modifier methods sethour, setminute, setsecond print method print We are going to add 3 more items to this list 1-19

20 Additional methods for OOP The following slides illustrate three other very useful methods you will want to incorporate into your own defined classes to make it easy to work with their objects (instances) tostring equals compareto We will demonstrate implementing these for the Time and Employ class in lab

21 The tostring method Every object type has a method tostring returns a string representation of the object When you System.out.print an object Java calls the object s tostring method automatically Default version just returns the object's Hex address System.out.print("The time is " + now); prints: The time is Time@80cc7c0 1-21

22 Define a tostring method for Time Can override the default behavior with own def: public String tostring() { } return hour + ":" + minute + ":" + second; Allows for better output: System.out.print("The time is " + now); prints: The time is 11:35:

23 You could also invoke tostring explicitly Just like any other object method Time now = new Time(12,23,47); String s = now.tostring(); 1-23

24 Try this out now in your lab13 project write the tostring method for Time and Employee class demo by changing the print statements in ArraysOfObjects 1-24

25 Notions of same-ness with objects What is the meaning of "same"? "Chris and I have the same car" Same make and model But two different cars "Chris and I have the same mother" his mother and mine are one person 1-25

26 Card Class class Card{ private char suit; private int rank; This was part of assignment 11 Cards are represented by char for suit: 'C' = Clubs, 'D' = Diamonds, 'H' = Hearts, 'S' = Spades And an integer for rank: 1 = Ace, 2-10, 11=Jack, 12=Queen, 13=King Card mystery = new Card('H',13); 1-26

27 Both possibilities of "sameness" with objects in Java Card1 and Card2 contain the same data Card1 and Card2 refer to the same object 1-27

28 Two references to the same object To see if two references refer to the same object, we use the == operator References to the same object are identical 1-28

29 Two Objects with the Same Data To see if two object contain the same data, write a method, equals, in the Card class References to objects with same data are equivalent. 1-29

30 The equals Method To check equivalence, it is standard practice to write a method with the name equals. public boolean equals(card that) { return (this.suit == that.suit && this.rank == that.rank;) } 1-30

31 Application to String Objects Using == to check for String equivalence is an error: String s1 = "hello"; String s2 = "hello"; if (s1 == s2) false, not identical System.out.println("same"); else Two different objects, with same data System.out.println("Not the same"); 1-31

32 Correct way to check for String Equivalence The.equals method for String objects String s1 = "hello"; String s2 = "hello"; if (s1.equals(s2)) true, equivalent objects System.out.println("same"); else Two different objects, with same data System.out.println("Not the same"); 1-32

33 Try this out now in your lab13 project write an equals method for Time and Employee class demo by comparing Time and Employee objects in ArraysOfObjects 1-33

34 Comparing Primitives and Objects Comparing primitives: use <,>,==,<=,>=,!= int x = 4, y = 6; if (x < y) System.out.println("x < y"); Comparing String objects: String s1 = "apple", s2 = "apply"; if (s1.compareto(s2) < 0) System.out.println("s1 before s2"); 1-34

35 The compareto Method How to order cards? By suit, then by rank? or, by rank, then by suit? Cards come ordered by suit, then rank First compare suit: if (this.suit > that.suit) return 1; if (this.suit < that.suit) return -1; Then compare rank: if (this.rank > that.rank) return 1; if (this.rank < that.rank) return -1; If neither of the above is true, must be same just return

36 The compareto method for Card class class Card{ private char suit; private int rank;... public int compareto(card that) { } } if (this.suit > that.suit) return 1; if (this.suit < that.suit) return -1; if (this.rank > that.rank) return 1; if (this.rank < that.rank) return -1; return 0; 1-36

37 Try this out now in your lab13 project write the compareto method for Time class compare hour, then min, then sec for Employee class compare name, then if they are the same, by id demo by comparing Time and Employee objects in ArraysOfObjects 1-37

38 Arrays of Cards Create an array to hold 52 cards The array contains references to objects does not hold objects themselves cards[0].print(); // Null Pointer Exception! 1-38

39 Creating a Deck of Cards 1 way is to have the user enter all the values for a deck of Cards not practical Another way is to read a list of Card values from a file, say cards.txt 1-39

40 Read a Deck of Cards from a file Declare a file Scanner: Scanner inputfile = new Scanner(new File("cards.txt")); Read file data into temp local variables char tempsuit = inputfile.next().charat(0); int temprank = inputfile.nextint(); Create a new Card object based on values read Card tempcard = new Card(tempSuit, temprank); Store tempcard in array deck[index] = tempcard; Put in a loop that repeats 52 times 1-40

41 Read a Deck of Cards from a file You will also need to import: import java.util.scanner; import java.io.file; import java.io.filenotfoundexception; And tell Java your main method (potentially) throws FileNotFoundException 1-41

42 The resulting array of cards after loop 1-42

43 The printdeck method Encode a traversal of cards array into a method public static void printdeck(card [] cards) { } for (int k = 0; k < cards.length; k++) cards[k].print(); System.out.println(""); // go to newline 1-43

44 Partially Filled Arrays Typically, if the amount of data that an array must hold is unknown: size the array to the largest expected number of elements. use a counting variable to keep track of how much valid data is in the array. int[] array = new int[100]; int count = 0; System.out.print("Enter a number or -1 to quit: "); number = keyboard.nextint(); while (number!= -1 && count <= 99) { array[count] = number; count++; System.out.print("Enter a number or -1 to quit: "); number = keyboard.nextint(); } input, number and keyboard were previously declared and keyboard references a Scanner object 8-44

45 Processing a Partially Filled Array After the code in the previous slide, we can process the array with a loop that goes up to count: for ( int k = 0; k < count; k++) System.out.println( array[k] ); Only processes the cells that have been filled with data the ArrayList class (next Lecture) shows a better way to handle arrays with fluctuating size 8-45

46 Start Assignment