Object-Oriented Design and Programming (Java)

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Topics Covered Today

• 2.1 Implementing Classes – 2.1.3 Method equals and Method toString – 1.2.6  Implementing Classes the Library System

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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.

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Person

Employee

Object

Class Hierarchy Diagram

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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: ClassName@number

• 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.

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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() + ")";

} }

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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)

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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)

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Compare Point objectsPoint 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");}

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Compare Point objects

pointOne pointTwo pointThree(null) (null)

Point pointOne = new Point(10, 100);Point pointTwo = new Point(10, 100);

Point pointThree = pointOne;

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Compare Point objects

pointOnepointThree(null)

Point pointOne = new Point(10, 100);Point pointTwo = new Point(10, 100);

Point pointThree = pointOne;

pointTwo

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Compare Point objects

pointOnepointThree

Point pointOne = new Point(10, 100);Point pointTwo = new Point(10, 100);

Point pointThree = pointOne;

pointTwo

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Compare Point objects

Point pointOne = new Point(10, 100);Point pointTwo = new Point(10, 100);

Point pointThree = pointOne;

pointOne

pointThree X=10Y=100

pointTwo

X=10Y=100

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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 ；

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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

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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。

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1.2.6  Implementing Classes the Library System

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Part of Class Diagram

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Class CatalogItempublic 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;

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Class CatalogItem /**

* Constructs a <code>CatalogItem</code> object.

* <p>

* Sets the instance variable <code>available</code>

* to <code>true</code>.

* <p>

*

* @param initialCode the code of the item.

* @param initialTitle the title of the item.

* @param initialyear the year the item was published.

*/

public CatalogItem(String initialCode, String initialTitle, int initialyear) {

code = initialCode;

title = initialTitle;

year = initialyear;

available = true;

}

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Class CatalogItem/*** Returns the code of this item.

* * @return the code of this item. */

public String getCode() { return code;}

/** * Returns the title of this item. * * @return the title of this item. */

public String getTitle() { return title; }

/** * Returns the year this item was published. * * @return the year this item was published. */

public int getYear() { return year; }

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Class CatalogItem/**

* Sets the value of instance variable <code>available</code>. * * @param value the new value. */ public void setAvailable(boolean value) {

available = value;}

/** * Returns <code>true</code> if the item is available. * * @return <code>true</code> if the item is available; * <code>false</code> otherwise. */ public boolean isAvailable() { return available; }

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Class CatalogItem /**

* Returns <code>true</code> if the code of this catalog item

* is equal to the code of the argument

* </p>

*

* @param object object with which this catalog item is compared.

* @return <code>true</code> if the code of this catalog item is

* equal to the code of the argument; <code>false</code>

* otherwise.

*/

public boolean equals(Object object) {

return object instanceof CatalogItem &&

getCode().equals(((CatalogItem) object).getCode());

}

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Class CatalogItem/**

* Returns the string representation of this catalog item.

*

* @return the string representation of this catalog item.

*/

public String toString() {

return getCode() + "_" + getTitle() + "_" + getYear() + "_" + isAvailable();

}

}

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Class Bookpublic class Book extends CatalogItem { /* Author of the book.*/ private String author;

/* Number of pages in the book.*/ private int numberOfPages;

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Class Book /** * Constructs a <code>Book</code> object.

* * @param initialCode the code of the book. * @param initialTitle the title of the book. * @param initialYear the year the book was published. * @param initialAuthor the author of the book. * @param 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; }

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Class Book/*** Returns the author of this book.*

* @return the author of this book. */ public String getAuthor() {

return author;}

/** * Returns the number of pages in this book. * * @return the number of pages in this book. */

public int getNumberOfPages() { return numberOfPages; }

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Class Book/**

* Returns the string representation of this book. * * @return the string representation of this book. */ public String toString() {

return super.toString() + "_" + getAuthor() + "_" + getNumberOfPages();

}}

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Class Recording/** * This class models a recording. It extends {@link CatalogItem} and * adds the following information: * <ol> * <li>the performer of the recording, a <code>String</code></li> * <li>the format of the recording, a <code>String</code></li> * </ol> * * @author author name * @version 1.0.0 * @see CatalogItem */public class Recording extends CatalogItem { /* Performer of the recording. */ private String performer;

/* Format of the recording. */ private String format;

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Class Recording/**

* Constructs an <code>Recording</code> object. * * @param initialCode the code of the catalog item. * @param initialTitle the title of the catalog item. * @param initialYear the year of the catalog item. * @param initialPerformer the performer of the recording. * @param 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; }

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Class Recording/*** Returns the performer of this recording.

* * @return the performer of this recording. */ public String getPerformer() { return performer; }

/** * Returns the format of this recording. * * @return the format of this recording. */ public String getFormat() { return format; }

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Class Recording/**

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

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Class Borrower/** * This class models a library user. It contains the following * information: * <ol> * <li>the id of the borrower, a <code>String</code></li> * <li>the name of the borrower, a <code>String</code></li> * </ol> * * @author author name * @version 1.0.0 */public class Borrower { /* Identification number of the borrower.*/ private String id;

/* Name of the borrower.*/ private String name;

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Class Borrower /**

* Constructs a <code>Borrower</code> object. * * @param initialId the identification number of the borrower. * @param initialName the name of the borrower. */ public Borrower(String initialId, String initialName) {

id = initialId; name = initialName;

}

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Class Borrower /** * Returns the identification number of this borrower.

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

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Class Borrower /**

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

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Class Borrower /**

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