Review questions. Review questions, cont d. Class Definition. Methods. Class definition: methods. April 1,

Transcription

1 April 1, Previous Lecture: Intro to OOP Class definition: instance variables & methods Today s Lecture: Instance methods Constructors Methods with input parameters Review questions Where do you put declarations for instance and static variables? Immediately inside the class definition (immediately inside )? or Inside a method definition (e.g., inside method main)? Reading: Sec 4.2, 5.0 April 1, 2003 Lecture 19 2 Review questions, cont d If a variable does not store a value of primitive type, what does it store? What is the keyword for protecting instance variables? (I.e., not allow access from another class.) What is the keyword for instantiating an object? Class Definition public class class-name { declaration (and initialization) constructor methods April 1, 2003 Lecture 19 3 April 1, 2003 Lecture 19 4 Class definition: methods public double { Methods Modifier Return type Method name public double { Parameter list (if any) April 1, 2003 Lecture 19 5 April 1, 2003 Lecture 19 6

2 April 1, Methods A method is a named, parameterized group of statements modifier return-type method-name ( parameter-list ) { statement-list return-type void means nothing is returned from the method There must be a return statement, unless return-type is void Class definition public double { April 1, 2003 Lecture 19 7 April 1, 2003 Lecture 19 8 Class definition: public double { // Setter method public void { range = 4; April 1, 2003 Lecture 19 9 Calling an instance method Interval ; = new ; double x; x =.; x 0.0 April 1, 2003 Lecture Interval = new ; Interval = new ; Interval in2 = new ; if (. > in2. ) System.out.println( blah... ); in2

3 April 1, Constructor A constructor is used to create objects Each class has a default constructor Can define your own constructor: modifier class-name ( parameter-list ) { statements-list Use public as the modifier for now Note: an instance method that has no return type Constructor invocation new class-name ( expression-list ) The value of above expression is a reference to a new object of the given class-name The defined (or default) constructor is invoked on the new object created by new April 1, 2003 Lecture April 1, 2003 Lecture private double range; // interval width // Constructor public Interval(double b, double r) { base = b; range = r; public double { Creating an object Interval ; = new Interval( 0.5,1); base 0.5 range 1.0 April 1, 2003 Lecture April 1, 2003 Lecture public Interval(double b, double r) { this.base = b; this.range = r; Keyword this returns a reference to the object itself Use keyword this only when it is necessary. (It is not necessary in the example above.) April 1, 2003 Lecture private double range; // interval width // Constructor public Interval(double base, double r){ this.base = base; range = r; public double { April 1, 2003 Lecture 19 18

4 April 1, Method with input parameter Write an instance method expand(double f) that expands the Interval by a factor of f. Where do you put this method? What should be the method header? // Expand interval by factor of f Parameter of primitive type: pass by value I.e., value is copied April 1, 2003 Lecture April 1, 2003 Lecture Interval i1 = new Interval(0.2,0.7); double x = 2; i1.expand(x); System.out.println(i1.); i1 base 0.2 range 0.7 x 2.0 April 1, 2003 Lecture April 1, 2003 Lecture Method with input parameter Write an instance method isin(interval i) that returns the boolean value true if the instance is in Interval i. Return false otherwise. Parameter of non-primitive type: pass by reference I.e., Reference is copied; object itself is not copied Be sure to read Sec 5.0 April 1, 2003 Lecture April 1, 2003 Lecture 19 24

5 April 1, Interval i1 = new Interval(0.2,0.7); Interval i2 = new Interval( Math.random(),0.2); System.out.println(i2.isIn(i1)); i1 base 0.2 range 0.7 i2 base 0.1 range 0.2 isin() isin() April 1, 2003 Lecture April 1, 2003 Lecture return base>=i.base && <=i.; boolean in = base>=i.base && <=i.; return in; April 1, 2003 Lecture return base>=i.base && <=i.; boolean in = (base>=i.base && <=i.); if ( in==true ) return true; else return false; April 1, 2003 Lecture 19 28