dispatch Dynamic again Static members
Extra office hours this week
Extra
office hours this week
Brent:
Wednesday, Moore 100A it is, 5-7PM Alexey: Thursday 5-7, location TBA
No
office hours for Brent on Friday
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Sample Exams
Posted
on class web site
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HW2
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Inheritance of fields
public class Item { protected Container container; protected String description; ... } public class Character extends Item { ... }
Since Character extends Item, every Character will automatically inherit the fields container and description no need for Character to declare its own container or description!
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CONSTRUCTORS AGAIN
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Constructors and Overloading
Constructors can be overloaded: A class can have many constructors, differing in the number and/or types of parameters Well come back to the topic of overloading later, but heres a little example...
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public class Rectangle { private Point lowerLeft; private double width, height; public Rectangle(Point lowerLeft, double width, double height) { this.lowerLeft = lowerLeft; this.width = width; this.height = height; } public Rectangle(double width, double height) { this.lowerLeft = new Point(0,0); this.width = width; this.height = height; } public double getX() { ... } public double getY() { ... } public void move(double dx, double dy) { ... } }
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... Point p = new Point(0,0); Rectangle r1 = new Rectangle(p,1,1); Rectangle r2 = new Rectangle(2,2);
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Constructors and Inheritance
Constructors are not inherited But one constructor can invoke another...
A constructor can use either this or super in its first line (only), to invoke another constructor in the same class definition (this) or its superclass (super) Otherwise, an implicit call to super() is inserted at the beginning
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public class Rectangle { private Point lowerLeft; private double width, height; public Rectangle(Point lowerLeft, double width, double height) { this.lowerLeft = lowerLeft; this.width = width; this.height = height; } public Rectangle(double width, double height) { this(new Point(0,0), width, height); } public double getX() { ... } public double getY() { ... } public void move(double dx, double dy) { ... } }
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public class Square extends Rectangle { public Square(Point lowerLeft, double side) { // Call superclass constructor with sides equal super(lowerLeft, side, side); } public Square(Point lowerLeft) { // Call constructor above with default side length this(lowerLeft, 1); } }
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... Point p = new Point(0,0); Rectangle r1 = new Square(p,2); Rectangle r2 = new Square(p); ... r1.move(3,0); ... System.out.println(r2.getX()); ... What gets printed? Why?
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stack
heap
Rectangle r1 Rectangle r2
Square Point lowerLeft double width double height Square Point lowerLeft double width double height Point double x double y 0 0
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2 2
1 1
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public class ColoredSquare extends Square { String color; public ColoredSquare(Point lowerLeft, double side, String color) { // Call superclass constructor with sides equal super(lowerLeft, side, side); this.color = color; } }
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INHERITANCE AND DYNAMIC DISPATCH
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Dynamic Dispatch
Standard method call in Java Determine which method to invoke based on dynamic type of object To trace b.m() b: A must model heap: Overriding replaces methods in subclasses, so knowing the dynamic type is crucial
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public class A { int a; public A() { a = 5; } public void m () { a = a + 1; } } public class B extends A { int b; public B() { b = 3; } public void m () { super.m(); b = b + 1; } } ... A x = new A(); B y = new B();
What does the memory configuration look like at the end? What happens if we do y.m()?
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public class C { int c; public C() { c = 2; } public void p() { this.q(); } public void q() { c = 3; } } public class D extends C { int d; public D() { c = c+1; d = 3; } public void q() { d = 5; super.q(); } } D y = new D(); ... y.p(); ...
What does the memory configuration look like at the end? What are the values of the instance variables?
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STATIC MEMBERS
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Different roles for classes
Models
for object creation and behavior for new class creation
instance
variables: object data methods: object functionality
Templates
Extend
abstract and concrete classes
Containers
for functionality and data that does not need multiple instances (static methods and data)
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Classes as containers
Container
for a set of related static methods and variables To access static members of class C
variable:
C.var method: C.meth(...)
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Example: the Math class
Defines
common mathematical constants and functions
Math.PI
3.141592653589793
Math.sin(Math.PI/2) Math.log(Math.E) Math.pow(2,3)
1.0
1.0
8.0
Math.pow(2, 0.5)
1.4142135623730951
Math.sqrt(3*3 4*4)
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5.0
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What + goes into a class
Dynamic:
different for different
instances
Variables:
separate values Methods: computation that depends on instance variables, this
Static:
independent of class instance
Variables:
shared value for all instances Methods: same computation
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Quiz
interface A { public int m(); } class B0 implements A { public int m() { return 0; } } class B1 implements A { public int m() { return 1; } } A a0 = new B0(); A a1 = new B1();
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1.
2.
3. 4.
What are the static (compile time) types of a0 and a1? What are the dynamic (run time) types of a0 and a1? What is the result of a0.m()? What is the result of a1.m()?
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Static vs. Dynamic
The keyword static has many meanings in Java.
static type, static data, static method, static nested class... Static information can be determined just by looking at the text of the program Dynamic information requires reasoning about the runtime memory configuration
Roughly:
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Static fields and methods
Information and behavior that is common to all instances of a class
public class Ticket { private static int issued; public static int getIssued(){ return Ticket.issued; } private int number; public Ticket() { this.number = Ticket.issued; Ticket.issued++; } public int getNumber() { return this.number; } }
static
dynamic
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Issuing tickets
The static variable issued tracks the total number of Ticket objects The instance (dynamic) variable number stores the number of a particular Ticket object
Ticket t1 = new Ticket(); t1.getNumber() 0 Ticket t2 = new Ticket(); t2.getNumber() 1 Ticket.getIssued() 2
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Memory model
t1: t2:
Class Ticket Ticket int number : 1 int issued : 2;
Ticket int number : 0
Location of issued known statically Activation record for getIssued has no reference to this.
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How to refer to things
Class
C, o instance of C static dynamic variable dv o.dv
dv this.dv
what from anywhere static methods of C dynamic methods of C
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variable sv C.sv o.sv sv C.sv
sv C.sv this.sv
method sm C.sm() o.sm() sm() C.sm()
sm() C.sm() this.sm()
method dm o.dm()
dm() this.dm()
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Access Control
public private package (package private) protected
Why control access?
public class Point implements Displaceable { public double x, y; public Point(double anX, double aY) { x = anX; y = aY; check(); } public double getX() { return x; } public double getY() { return y; } public void move(double dx, double dy) { x += dx; y += dy; check(); } public void check() { if (x < 0.0) { x = 0.0; } if (y < 0.0) { y = 0.0; } } }
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Why control access?
public class Point implements Displaceable { private double x, y; public Point(double anX, double aY) { x = anX; y = aY; check(); } public double getX() { return x; } public double getY() { return y; } public void move(double dx, double dy) { x += dx; y += dy; check(); } public void check() { if (x < 0.0) { x = 0.0; } if (y < 0.0) { y = 0.0; } } }
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Package access
package cis120.lecture10; package declaration public class Counter { int count; public void setCount(int val) { make visible in if (val >= 0) other packages count = val; } public int getCount() { return count; } }
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Top-level access
Classes
may have either public or package access.
package cis120.lecture10; public class A { . } class B { }
Package access
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Package
Groups of classes that work together Named similarly to Unix directories (but with . instead of /)
java.nio java.nio.channels java.nio.channels.spi
The Java API is organized into a large number of packages Packages are also useful for your own Java programs
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Referring to package members
Explicitly
java.io.Reader r = new java.io.FileReader("myfile");
Importing classes from the package
import java.io.Reader; import java.io.FileReader; Reader r = new FileReader("myfile");
Importing the whole package
import java.io.*; Reader r = new FileReader("myfile");
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Package hierarchy
java.io.Reader Reader class in java.io java.util.regex.* All classes in java.util.regex java.util.*
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All classes in java.util
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Creating our own packages
1. 2. 3. 4.
Build a corresponding directory hierarchy Point Java to the root of the directory hierarchy Add package declarations Make public the names we want to see from anywhere
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The directory hierarchy
/lectures root of the hierarchy /lectures/cis120 /lectures/cis120/lecture10 /lectures/cis120/lecture10/Counter.java /lectures/cis120/lecture10/Counter.class /lectures/cis120/lecture10/Ticket.java /lectures/cis120/lecture10/Ticket.class
The Java classpath tells Java where the roots of your class hierarchies are DrJava knows how to set the classpath to find the classes you create and compiled in it
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