Building Java Programs.ppt

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1、Building Java Programs,Chapter 8 Lecture 8-2: Constructors and Encapsulationreading: 8.4 - 8.5 self-checks: #10-17 exercises: #9, 11, 14, 16,2,Object initialization: constructors,reading: 8.4self-check: #10-12 exercises: #9, 11, 14, 16,Initializing objects,Currently it takes 3 lines to create a Poin

2、t and initialize it:Point p = new Point(); p.x = 3; p.y = 8; / tediousWed rather pass the fields initial values as parameters:Point p = new Point(3, 8); / better!We are able to this with most types of objects in Java.,Constructors,constructor: Initializes the state of new objects.public type(paramet

3、ers) statements;runs when the client uses the new keyword does not specify a return type; it implicitly returns the new object being createdIf a class has no constructor, Java gives it a default constructor with no parameters that sets all fields to 0.,Constructor example,public class Point int x;in

4、t y;/ Constructs a Point at the given x/y location.public Point(int initialX, int initialY) x = initialX;y = initialY;public void translate(int dx, int dy) x += dx;y += dy; ,Tracing a constructor call,What happens when the following call is made?Point p1 = new Point(7, 2);,public Point(int initialX,

5、 int initialY) x = initialX;y = initialY; public void translate(int dx, int dy) x += dx;y += dy; ,Client code, version 3,public class PointMain3 public static void main(String args) / create two Point objectsPoint p1 = new Point(5, 2);Point p2 = new Point(4, 3);/ print each pointSystem.out.println(“

6、p1: (“ + p1.x + “, “ + p1.y + “)“);System.out.println(“p2: (“ + p2.x + “, “ + p2.y + “)“); / move p2 and then print it againp2.translate(2, 4);System.out.println(“p2: (“ + p2.x + “, “ + p2.y + “)“); OUTPUT: p1: (5, 2) p2: (4, 3) p2: (6, 7),Common constructor bugs,Accidentally writing a return type s

7、uch as void:public void Point(int initialX, int initialY) x = initialX;y = initialY;This is not a constructor at all, but a method!Storing into local variables instead of fields (“shadowing“):public Point(int initialX, int initialY) int x = initialX;int y = initialY;This declares local variables wit

8、h the same name as the fields, rather than storing values into the fields. The fields remain 0.,Multiple constructors,A class can have multiple constructors. Each one must accept a unique set of parameters.Write a constructor for Point objects that accepts no parameters and initializes the point to

9、the origin, (0, 0)./ Constructs a new point at (0, 0). public Point() x = 0;y = 0; ,10,Encapsulation,reading: 8.5 - 8.6 self-check: #13-17 exercises: #5,Encapsulation,encapsulation: Hiding implementation details of an object from its clients.Encapsulation provides abstraction. separates external vie

10、w (behavior) from internal view (state) Encapsulation protects the integrity of an objects data.,Private fields,A field can be declared private. No code outside the class can access or change it.private type name;Examples:private int id;private String name;Client code sees an error when accessing pr

11、ivate fields:PointMain.java:11: x has private access in Point System.out.println(“p1 is (“ + p1.x + “, “ + p1.y + “)“);,Accessing private state,We can provide methods to get and/or set a fields value:/ A “read-only“ access to the x field (“accessor“)public int getX() return x;/ Allows clients to cha

12、nge the x field (“mutator“)public void setX(int newX) x = newX;Client code will look more like this:System.out.println(“p1: (“ + p1.getX() + “, “ + p1.getY() + “)“);p1.setX(14);,Point class, version 4,/ A Point object represents an (x, y) location. public class Point private int x;private int y;publ

13、ic Point(int initialX, int initialY) x = initialX;y = initialY;public double distanceFromOrigin() return Math.sqrt(x * x + y * y);public int getX() return x;public int getY() return y;public void setLocation(int newX, int newY) x = newX;y = newY;public void translate(int dx, int dy) x = x + dx;y = y

14、 + dy; ,Client code, version 4,public class PointMain4 public static void main(String args) / create two Point objectsPoint p1 = new Point(5, 2);Point p2 = new Point(4, 3);/ print each pointSystem.out.println(“p1: (“ + p1.getX() + “, “ + p1.getY() + “)“);System.out.println(“p2: (“ + p2.getX() + “, “

15、 + p2.getY() + “)“); / move p2 and then print it againp2.translate(2, 4);System.out.println(“p2: (“ + p2.getX() + “, “ + p2.getY() + “)“); OUTPUT: p1 is (5, 2) p2 is (4, 3) p2 is (6, 7),Benefits of encapsulation,Provides abstraction between an object and its clients.Protects an object from unwanted

16、access by clients. A bank app forbids a client to change an Accounts balance.Allows you to change the class implementation. Point could be rewritten to use polar coordinates (radius r, angle ), but with the same methods.Allows you to constrain objects state (invariants). Example: Only allow Points with non-negative coordinates.,