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Chapter 6
Defining New Classes
CMPU-101: Problem-Solving and Abstraction Marc Smith Vassar College Spring 2007 2
Recap
Chapter 3 Using predefined classes & objects System.out.println("hello"); Chapters 4 & 5 Define instances of predefined classes String s = "hello".toUpperCase(); Scanner sc = new Scanner(System.in); Now… Define new classes 3
Prog 0601 — Driver
Driver refers to code, commonly found in the main method, that creates objects and sends them messages to get the program work started. What do these statements tell us about class Teacher? Teacher t = new Teacher(); t.lecture(); Teacher has a “zero-parameter” constructor and a method called lecture with prototype void lecture() 4
Prog 0601 — class Teacher
public Teacher() {} //------------------------------------------------------------- public void lecture() { System.out.println("do your homework!"); } //-------------------------------------------------------------
Style Used in the Text
Classes are separated by /////////////////////////////////////////// Methods are surrounded by //----------------------------------------- Classes end with } // end class className
Prog 0602 — Overload the lecture Method
public void lecture ( String name ) { System.out.println ( name + ", do your homework!"); } This version of lecture takes a single String parameter The prototype of this method is void lecture(String)
7
Tip Revisited
When testing features of a new language, make a minimal change to a working program. Starting with a blank template file ensures that you start with a working program. 8
class Animal in UML Notation
Animal
void speak()
String name
String sound
instance variables methods 9
Prog 0603 — Driver
Animal a1 = new Animal("fluffy","meow"); a1.speak(); Animal a2 = new Animal("fido","woof"); a2.speak(); Animal a3 = a2; a3.speak(); a1 = new Animal("bessie","moo"); a1.speak(); The Driver creates three variables of type Animal: a1, a2, and a3. 10 Animal a1 = new Animal("fluffy","meow"); fluffy meow a Animal a2 = new Animal("fido","woof"); fluffy meow a fido woof a
fluffy
meow
a
fido
woof
a
a
Animal a3 = a2;
19 Animal a1 = new Animal("fluffy","meow"); a1.speak(); Animal a2 = new Animal("fido","woof"); a2.speak(); Animal a3 = a2; a3.speak(); a1 = new Animal("bessie","moo"); a1.speak(); What is the output from this snippet of code? 20
In-Class Exercise
Write public void sleeps() so that a1.sleeps(); prints out fluffy is sleeping 21
In-Class Exercise
Write public void twice() so that a1.twice(); prints out meow, meow 22 Naming Constructor Parameters Stylistic Considerations (1) public Animal ( String theName, String theSound ) { name = theName; sound = theSound; } public Animal ( String name_, String sound_ ) { name = name_; sound = sound_; } Naming Constructor Parameters Stylistic Considerations (2) public Animal ( String a, String b ) { name = a; sound = b; } Naming Constructor Parameters Stylistic Considerations (3) a and b are Bad Variable Names (BVN) because they are non-descriptive
25 public Animal ( String name, String sound ) { this.name = name; this.sound = sound; } The winner! (this is the style we will adopt in this course) Naming Constructor Parameters Stylistic Considerations (4) 26
The this Pointer
For a constructor, this refers to the instance being constructed. For any other method, this refers to the receiver of the message. ! instance = instance of a class = object 27
Getters — Accessors
public String getSound() { return this.sound; } Methods intended to return the value of an instance variable In general, these methods have no parameters and a non-void return type 28
Setters — Mutators
public void setSound ( String newSound ) { this.sound = newSound; } Methods intended to set the value of an instance variable In general, these methods have a void return type and a single parameter. User-Defined Classes as Parameters public void chases ( Animal target ) { System.out.println ( this.name + " is chasing " + target.name ); } Objects of a particular type can receive messages containing parameters that are other objects of the same type.
User-Defined Class as a Return Type
receiver values result values name sound name sound "fido" "woof" "big fido" "WOOF" "fluffy" "meow" "big fluffy" "MEOW" Method makeBig
37 Let's make System.out.println(new Animal("fido","woof"); print out fido (woof) We need to write a toString method for class Animal. 38
Prog 0610 — toString
public String toString() { return name + " (" + sound + ")"; } 39
toString is used by
println System.out.println(a1); System.out.println(new Animal("leo","roar")); concatenation (+) System.out.println("I think that " + a1 + " is more faithful than " + a2 + "."); 40 Reading an Object from the Keyboard We want the dialog: Reading an Animal ... Enter name: fido Enter sound: woof See Prog0611.
static read Method
public static Animal read ( PrintStream ps, Scanner sc ) { ps.println("Reading an Animal ..."); ps.print("Enter name: "); String name = sc.nextLine(); ps.print("Enter sound: "); String sound = sc.nextLine(); return new Animal(name,sound); }
Calling a static read Method
Scanner kb = new Scanner(System.in); Animal a = Animal.read(System.out,kb); System.out.println(); System.out.println(a);
43
Instance Method vs Class Method
instance method class method (static method) called by tacking to the right of an instance of the class called by tacking to the right of the class name requires an instance of the class does not require an instance of the class 44
Prog 0612 — Reading from a File
System.out.print("Enter the name of a file of Animals: "); String filename = kb.nextLine(); Scanner sc = new Scanner(new File(filename)); Animal a1 = Animal.read(System.out,sc); Animal a2 = Animal.read(System.out,sc); Prompts are unnecessary for this case! 45
static Methods in Class Math
Math.abs(…) Math.sin(…) Math.sqrt(…) 46
public static void main
We need to be able to call the main method even though we haven't created an instance of the driver class. This requires that main be static. (Why must main be public?) ! How do we call the main method? (hint: In DrJava, from the interactions pane, how do you run a program that has a main method?)
static Methods Can't Refer to Instance Variables
class A { private int var; //----------------------------------------------------------------------- public static void f() { System.out.println(this.var);! Note instance variable! } //----------------------------------------------------------------------- } This won't compile! ! If you follow our rule of referring to instance variables using this, you should never see a static method that contains a this!
Sharing Data Structure
Animal a1 = new Animal("fluffy","meow"); Animal a2 = a1; // Beware! // Memory structure is // being shared. a1.speak(); a2.speak(); a2.setName("pretzel"); System.out.println(); a1.speak(); a2.speak(); In other words, a1 and a Both refer to the same Animal object in memory! Draw the picture…
