Object-Oriented Design and Programming (Java)

2 Topics Covered Today 2.1 Implementing Classes –2.1.3 Method equals and Method toString –1.2.6 Implementing Classes the Library System

3 The Object Class Java defines the class java.lang.Object that is defined as a superclass for all classes. If a class doesn’t specify explicitly which class it is derived from, then it will be implicitly derived from class Object. So, in the previous example, Employee was derived from Person which in turn was derived from Object.

4 Person Employee Object Class Hierarchy Diagram

5 Method toString in Object Class The Object class defines a set of methods that are inherited by all classes. One of these is the toString() method that is used whenever we want to get a String representation of an object. The version of toString method defined in class Object returns a String with the following format: When you define a new class, you can override the toString() method in order to have a suitable representation of the new type of objects as Strings.

6 Point public class Point { private int x,y; public Point(int initialX, int initialY) { x = initialX; y = initialY; } public int getX() { return x; } public int getY() { return y; } public String toString() { return "(" + getX() + "," + getY() + ")"; }

7 Method toString Point pointOne = new Point(10, 100); Point pointTwo = new Point(-20, 200); Point pointThree = new Point(50, -500); System.out.println(pointOne); System.out.println(pointTwo); System.out.println(pointThree); System.out.println(); Output ： (10,100) (-20,200) (50,-500)

8 Method equals In JAVA, all classes inherit the equals() method of Object class. This default implementation is exactly the same as obj1 == obj2 (compare pointers) boolean equals(Object obj)

9 Compare Point objects Point pointOne = new Point(10, 100); Point pointTwo = new Point(10, 100); Point pointThree = pointOne; if (pointOne.equals(pointTwo)) { System.out.println("pointOne and pointTwo are equal"); } else { System.out.println("pointOne and pointTwo are different"); } if (pointOne.equals(pointThree)) { System.out.println("pointOne and pointThree are equal"); } else { System.out.println("pointOne and pointThree are different");}

10 Compare Point objects pointOnepointTwo pointThree (null) Point pointOne = new Point(10, 100); Point pointTwo = new Point(10, 100); Point pointThree = pointOne;

11 Compare Point objects pointOnepointThree (null) Point pointOne = new Point(10, 100); Point pointTwo = new Point(10, 100); Point pointThree = pointOne; pointTwo

12 Compare Point objects pointOnepointThree Point pointOne = new Point(10, 100); Point pointTwo = new Point(10, 100); Point pointThree = pointOne; pointTwo

13 Compare Point objects Point pointOne = new Point(10, 100); Point pointTwo = new Point(10, 100); Point pointThree = pointOne; pointOne pointThree X=10 Y=100 pointTwo X=10 Y=100

14 Override method equals public boolean equals(Object object) { if (object instanceof Point) { Point point = (Point) object; return point.getX() == getX() && point.getY() == getY(); } else { return false; } } 在大部分类中继承自 Object 类的 equals() 方法都不太适用， 因此每个类都需要重写该方法； 该方法比较两个对象，只有当两个对象的状态都一样时， 返回 true ；

15 Override method equals Point pointOne = new Point(10, 100); Point pointTwo = new Point(10, 100); Point pointThree = new Point(50, 500); if (pointOne.equals(pointTwo)) { System.out.println("pointOne and pointTwo are equal"); } else { System.out.println("pointOne and pointTwo are different"); } if (pointOne.equals(pointThree)) { System.out.println("pointOne and pointThree are equal"); } else { System.out.println("pointOne and pointThree are different"); }

overload 和 override 的区别 16 override （重写） 1 、方法名、参数、返回值相 同。 2 、子类方法不能缩小父类方 法的访问权限。 3 、子类方法不能抛出比父类 方法更多的异常 ( 但子类方法 可以不抛出异常 ) 。 4 、存在于父类和子类之间。 5 、方法被定义为 final 不能被 重写。 overload （重载） 1 、参数类型、个数、顺 序至少有一个不相同。 2 、不能重载只有返回值 不同的方法名。 3 、存在于父类和子类、 同类中。

Implementing Classes the Library System

18 Part of Class Diagram

19 Class CatalogItem public class CatalogItem { /* Code of the item. */ private String code; /* Title of the item. */ private String title; /* Year the item was published. */ private int year; /* Indicates if the item is available */ private boolean available;

20 Class CatalogItem /** * Constructs a CatalogItem object. * * Sets the instance variable available * to true. * * initialCode the code of the item. initialTitle the title of the item. initialyear the year the item was published. */ public CatalogItem(String initialCode, String initialTitle, int initialyear) { code = initialCode; title = initialTitle; year = initialyear; available = true; }

21 Class CatalogItem /** * Returns the code of this item. * the code of this item. */ public String getCode() { return code; } /** * Returns the title of this item. * the title of this item. */ public String getTitle() { return title; } /** * Returns the year this item was published. * the year this item was published. */ public int getYear() { return year; }

22 Class CatalogItem /** * Sets the value of instance variable available. * value the new value. */ public void setAvailable(boolean value) { available = value; } /** * Returns true if the item is available. * true if the item is available; * false otherwise. */ public boolean isAvailable() { return available; }

23 Class CatalogItem /** * Returns true if the code of this catalog item * is equal to the code of the argument * * object object with which this catalog item is compared. true if the code of this catalog item is * equal to the code of the argument; false * otherwise. */ public boolean equals(Object object) { return object instanceof CatalogItem && getCode().equals(((CatalogItem) object).getCode()); }

24 Class CatalogItem /** * Returns the string representation of this catalog item. * the string representation of this catalog item. */ public String toString() { return getCode() + "_" + getTitle() + "_" + getYear() + "_" + isAvailable(); } }

25 Class Book public class Book extends CatalogItem { /* Author of the book.*/ private String author; /* Number of pages in the book.*/ private int numberOfPages;

26 Class Book /** * Constructs a Book object. * initialCode the code of the book. initialTitle the title of the book. initialYear the year the book was published. initialAuthor the author of the book. initialNumberOfPages the number of pages in the book. */ public Book(String initialCode, String initialTitle, int initialYear, String initialAuthor, int initialNumberOfPages) { super(initialCode, initialTitle, initialYear); author = initialAuthor; numberOfPages = initialNumberOfPages; }

27 Class Book /** * Returns the author of this book. * the author of this book. */ public String getAuthor() { return author; } /** * Returns the number of pages in this book. * the number of pages in this book. */ public int getNumberOfPages() { return numberOfPages; }

28 Class Book /** * Returns the string representation of this book. * the string representation of this book. */ public String toString() { return super.toString() + "_" + getAuthor() + "_" + getNumberOfPages(); } }

29 Class Recording /** * This class models a recording. It extends CatalogItem} and * adds the following information: * * the performer of the recording, a String * the format of the recording, a String * * author name CatalogItem */ public class Recording extends CatalogItem { /* Performer of the recording. */ private String performer; /* Format of the recording. */ private String format;

30 Class Recording /** * Constructs an Recording object. * initialCode the code of the catalog item. initialTitle the title of the catalog item. initialYear the year of the catalog item. initialPerformer the performer of the recording. initialFormat the format of the recording. */ public Recording(String initialCode, String initialTitle, int initialYear, String initialPerformer, String initialFormat) { super(initialCode, initialTitle, initialYear); performer = initialPerformer; format = initialFormat; }

31 Class Recording /** * Returns the performer of this recording. * the performer of this recording. */ public String getPerformer() { return performer; } /** * Returns the format of this recording. * the format of this recording. */ public String getFormat() { return format; }

32 Class Recording /** * Returns the string representation of this recording. * the string representation of this recording. */ public String toString() { return super.toString() + "_" + getPerformer() + "_ “ + getFormat(); } }

33 Class Borrower /** * This class models a library user. It contains the following * information: * * the id of the borrower, a String * the name of the borrower, a String * * author name */ public class Borrower { /* Identification number of the borrower.*/ private String id; /* Name of the borrower.*/ private String name;

34 Class Borrower /** * Constructs a Borrower object. * initialId the identification number of the borrower. initialName the name of the borrower. */ public Borrower(String initialId, String initialName) { id = initialId; name = initialName; }

35 Class Borrower /** * Returns the identification number of this borrower. * the identification number of this borrower. */ public String getId() { return id; } /** * Returns the name of this borrower. * the name of this borrower. */ public String getName () { return name; }

36 Class Borrower /** * Returns true if the id of this borrower is * equal to the id of the argument. * * object object with which this borrower is compared. true if the id of this borrower is * equal to the id of the argument; false * otherwise. */ public boolean equals(Object object) { return object instanceof Borrower && getId().equals(((Borrower) object).getId()); }

37 Class Borrower /** * Returns the string representation of this borrower. * the string representation of this borrower. */ public String toString() { return getId() + "_" + getName(); } }