1 Programming with methods and classes Chapter 7 Fall 2005 CS 101 Aaron Bloomfield 2 Static vs. non-static 3 Methods Instance (or member) method Operates on a object (i.e., and instance of the class) String s = new String("Help every cow reach its " + "potential!"); int n = s.length(); Class (i.e. static) method Service provided by a class and it is not associated with a particular object String t = String.valueOf(n); Instance method Class method 4 Variables Instance variable and instance constants Attribute of a particular object Usually a variable Point p = new Point(5, 5); int px = p.x; Class variables and constants Collective information that is not specific to individual objects of the class Usually a constant Color favoriteColor = Color.MAGENTA; double favoriteNumber = Math.PI - Math.E; Instance variable Class constants 5 static and non- static rules Member/instance (i.e. non-static) fields and methods can ONLY be accessed by the object name Class (i.e. static) fields and methods can be accessed by Either the class name or the object name Non-static methods can refer to BOTH class (i.e. static) variables and member/instance (i.e. non-static) variables Class (i.e. static) methods can ONLY access class (i.e. static) variables 6 Static vs. non-static Consider the following code: public class Staticness { private int a = 0; private static int b = 0; public void increment() { a++; b++; } public String toString() { return "(a=" + a + ",b=" + b + ")"; } 7 Static vs. non-static And the code to run it: public class StaticTest { public static void main (String[] args) { Staticness s = new Staticness(); Staticness t = new Staticness(); s.increment(); t.increment(); System.out.println (s); System.out.println (t); } 8 Static vs. non-static Execution of the code Output is: (a=1,b=3) (a=2,b=3) 9 Staticness s = new Staticness(); Staticness t = new Staticness(); s.increment(); t.increment(); System.out.println (s); System.out.println (t); Staticness s = new Staticness(); Staticness t = new Staticness(); s.increment(); t.increment(); System.out.println (s); System.out.println (t); Static vs. non-static: memory diagram Staticness - a = 0 t Staticness - a = 0 s 0 b Staticness - a = 1 Staticness - a = 1 Staticness - a = 10 Program demo StaticTest.java StaticTest.java 11 Yale vs. Harvard Web references: Web references: 12 Conversion.java 13 Task Conversion.java Support conversion between English and metric values d degrees Fahrenheit = (d 32)/1.8 degrees Celsius 1 mile = kilometers 1 gallon = liters 1 ounce (avdp) = grams 1 acre = square miles = hectares 14 Conversion Implementation public class Conversion { // conversion equivalencies private static final double KILOMETERS_PER_MILE = ; private static final double LITERS_PER_GALLON = ; private static final double GRAMS_PER_OUNCE = ; private static final double HECTARES_PER_ACRE = ; 15 Conversion implementation public static double fahrenheitToCelsius (double f) { return (f - 32) / 1.8; } } Modifier public indicates other classes can use the method Modifier static indicates the method is a class method No use of member/instance variables!!! 16 Conversion Implementation // temperature conversions methods public static double fahrenheitToCelsius(double f) { return (f - 32) / 1.8; } public static double celsiusToFahrenheit(double c) { return 1.8 * c + 32; } // length conversions methods public static double kilometersToMiles(double km) { return km / KILOMETERS_PER_MILE; } 17 Conversion Implementation // mass conversions methods public static double litersToGallons(double liters) { return liters / LITERS_PER_GALLON; } public static double gallonsToLiters(double gallons) { return gallons * LITERS_PER_GALLON; } public static double gramsToOunces(double grams) { return grams / GRAMS_PER_OUNCE; } public static double ouncesToGrams(double ounces) { return ounces * GRAMS_PER_OUNCE; } 18 Conversion Implementation // area conversions methods public static double hectaresToAcres(double hectares) { return hectares / HECTARES_PER_ACRE; } public static double acresToHectares(double acres) { return acres * HECTARES_PER_ACRE; } 19 Conversion use Scanner stdin = new Scanner (System.in); System.out.print("Enter a length in kilometers: "); double kilometers = stdin.nextDouble(); double miles = Conversion.kilometersToMiles(kilometers); System.out.print("Enter a mass in liters: "); double liters = stdin.nextDouble(); double gallons = Conversion.litersToGallons(liters); System.out.print("Enter a mass in grams: "); double grams = stdin.nextDouble(); double ounces = Conversion.gramsToOunces(grams); System.out.print("Enter an area in hectares: "); double hectares = stdin.nextDouble(); double acres = Conversion.hectaresToAcres(hectares); 20 A Conversion use System.out.println(kilometers + " kilometers = " + miles + " miles "); System.out.println(liters + " liters = " + gallons + " gallons"); System.out.println(grams + " grams = " + ounces + " ounces"); System.out.println(hectares + " hectares = " + acres + " acres"); 2.0 kilometers = miles 3.0 liters = gallons 4.0 grams = ounces 5.0 hectares = acres 21 A preferred Conversion use NumberFormat style = NumberFormat.getNumberInstance(); style.setMaximumFractionDigits(2); style.setMinimumFractionDigits(2); System.out.println(kilometers + " kilometers = " + style.format(miles) + " miles "); System.out.println(liters + " liters = " + style.format(gallons) + " gallons"); System.out.println(grams + " grams = " + style.format(ounces) + " ounces"); System.out.println(hectares + " hectares = " + style.format(acres) + " acres"); 2.0 kilometers = 1.24 miles 3.0 liters = 0.79 gallons 4.0 grams = 0.14 ounces 5.0 hectares = acres Part of java.text 22 Program Demo Conversion.java Conversion.java 23 Fractals 24 Parameter passing 25 Java parameter passing The value is copied to the method Any changes to the parameter are forgotten when the method returns 26 Java parameter passing Consider the following code: static void foobar (int y) { y = 7; } public static void main (String[] args) { int x = 5; foobar (x); System.out.println(x); } What gets printed? formal parameter actual parameter y 5 x 5 y 7 27 Java parameter passing Consider the following code: static void foobar (String y) { y = 7; } public static void main (String[] args) { String x = 5; foobar (x); System.out.println(x); } What gets printed? formal parameter actual parameter yx 5"7" 28 Java parameter passing Consider the following code: static void foobar (Rectangle y) { y.setWidth (10); } public static void main (String[] args) { Rectangle x = new Rectangle(); foobar (x); System.out.println(x.getWidth()); } What gets printed? formal parameter actual parameter yx width = 0width = 10 29 Java parameter passing Consider the following code: static void foobar (Rectangle y) { y = new Rectangle(); y.setWidth (10); } public static void main (String[] args) { Rectangle x = new Rectangle(); foobar (x); System.out.println(x.getWidth()); } What gets printed? formal parameter actual parameter yx width = 0 width = 10 30 Java parameter passing The value of the actual parameter gets copied to the formal parameter This is called pass-by-value C/C++ is also pass-by-value Other languages have other parameter passing types Any changes to the formal parameter are forgotten when the method returns However, if the parameter is a reference to an object, that object can be modified Similar to how the object a final reference points to can be modified 31 Method invocations Actual parameters provide information that is otherwise unavailable to a method When a method is invoked Java sets aside memory for that particular invocation Called the activation record Activation record stores, among other things, the values of the formal parameters Formal parameters initialized with values of the actual parameters After initialization, the actual parameters and formal parameters are independent of each other Flow of control is transferred temporarily to that method 32 Value parameter passing demonstration public class ParameterDemo { public static double add(double x, double y) { double result = x + y; return result; } public static double multiply(double x, double y) { x = x * y; return x; } public static void main(String[] args) { double a = 8, b = 11; double sum = add(a, b); System.out.println(a + " + " + b + " = " + sum); double product = multiply(a, b); System.out.println(a + " * " + b + " = " + product); } } 33 Value parameter passing demonstration The file/class is actually called ParameterDemo.java 34 Program demo ParameterDemo.java ParameterDemo.java 35 ParameterDemo.java walkthrough double sum = add(a, b); public static double add (double x, double y) { double result = x + y; return result; } Initial values of formal parameters come from the actual parameters 36 ParameterDemo.java walkthrough double multiply = multiply(a, b); public static double multiply (double x, double y) { x = x * y; return x; } Initial values of formal parameters come from the actual parameters 37 End of lecture on 28 Nov 2005 Also talked about HW J8 38 PassingReferences.java 39 PassingReferences.java import java.awt.*; public class PassingReferences { public static void f(Point v) { v = new Point(0, 0); } public static void g(Point v) { v.setLocation(0, 0); } public static void main(String[] args) { Point p = new Point(10, 10); System.out.println(p); f(p); System.out.println(p); g(p); System.out.println(p); } } 40 PassingReferences.java run g() can change the attributes of the object to which p refers 41 Program demo PassingReferences.java PassingReferences.java 42 PassingReferences.java public static void main(String[] args) { Point p = new Point(10, 10); System.out.println(p); f(p); java.awt.Point[x=10,y=10 ] 43 PassingReferences.java public static void f(Point v) { v = new Point(0, 0); } 44 PassingReferences.java public static void main(String[] args) { Point p = new Point(10, 10); System.out.println(p); f(p); System.out.println(p); g(p); java.awt.Point[x=10,y=10 ] 45 PassingReferences.java public static void g(Point v) { v.setLocation(0, 0); } 46 PassingReferences.java public static void main(String[] args) { Point p = new Point(10, 10); System.out.println(p); f(p); System.out.println(p); g(p); System.out.println(p); java.awt.Point[x=10,y=10 ] java.awt.Point[x=0,y=0] 47 Star Wars Episode 3 Trailer 48 Star Wars Episode 3 Trailer That was a edited version That was a edited version I changed the PG-rated trailer to a G-rated trailer The original one can be found atThe original one can be found atOr Google for star wars parody 49 Casting 50 Casting Weve seen casting before: double d = (double) 3; int x = (int) d; Aside: duplicating an object String s = foo; String t = s.clone(); Causes an error: inconvertible types (Causes another error, but we will ignore that one) What caused this? 51 Casting, take 2 .clone() returns an object of class Object (sic) More confusion: You can also have an object of class Class Thus, you can have an Object class and a Class object Got it? We know its a String (as it cloned a String) Thus, we need to tell Java its a String via casting Revised code: String s = foo; String t = (String) s.clone(); Still causes that other error, but we are still willfully ignoring it 52 Casting, take 3 That other error is because String does not have a.clone() method Not all classes do! We just havent seen any classes that do have.clone() yet Check in the documentation if the object you want to copy has a.clone() method A class that does: java.util.Vector Vector s = new Vector(); Vector t = s.clone(); Vector u = (Vector) s.clone(); Causes the inconvertible types error 53 Casting, take 4 What happens with the following code? Vector v = new Vector(); String s = (String) v; Java will encounter a compile-time error inconvertible types What happens with the following code? Vector v = new Vector(); String s = (String) v.clone(); Java will encounter a RUN-time error ClassCastException 54 New 2005 demotivatiors! 55 Wrapper classes 56 What about adding variables to a Vector? The add method takes an Object as a parameter public void add (Object o) { Although we havent seen it yet, this means you can add any object you want to the vector Primitive types (i.e. variables) are not objects How can they be added? The solution: wrapper classes! 57 The Integer wrapper class This is how you add an int variable to a Vector: int x = 5; Integer i = new Integer(x); vector.add (i); // Integer j = (Integer) v.get(0); int y = j.intValue(); Pretty annoying syntax well see how to get around it in a bit 58 More on wrapper classes All the primitive types have wrapper classes Usually, the names are just the capitalized version of the type I.e. Double for double, Byte for byte, etc. Two exceptions: int and char int has Integer char has Character 59 More on wrapper classes Consider this code: int x = 5; vector.add (x); // int y = vector.get(0); Does this code work? It shouldnt As we are adding a variable (not an object) to a vector But it does work! Why? 60 Auto-boxing Java 1.5 will automatically wrap a primitive value into its wrapper class when needed And automatically unwrap a wrapper object into the primitive value So Java translates the previous code into the following: int x = 5; vector.add (new Integer(x)); // int y = ((Integer)vector.get(0)).intValue(); This is called autoboxing And auto-unboxing (unauto-boxing?) This does not work in Java 1.4 or before 61 More on auto-boxing Consider the following code: Double d = 7.5; Double e = 6.5; Double f = d + e; System.println (f); This is doing a lot of auto-boxing (and auto-unboxing): Double d = new Double(7.5); Double e = new Double(6.5); Double f = new Double(d.doubleValue() + e.doubleValue()); System.println (f); 62 Todays demotivators 63 Triple.java 64 Task Triple.java Represent objects with three integer attributes public Triple() Constructs a default Triple value representing three zeros public Triple(int a, int b, int c) Constructs a representation of the values a, b, and c public int getValue(int i) Returns the i-th element of the associated Triple public void setValue(int i, int value) Sets the i-th element of the associated Triple to value 65 Task Triple.java Represent objects with three integer attributes public String toString() Returns a textual representation of the associated Triple public Object clone() Returns a new Triple whose representation is the same as the associated Triple public boolean equals(Object v) Returns whether v is equivalent to the associated Triple 66 Triple.java implementation // Triple(): default constructor publicTriple() { this(0, 0, 0); } The new Triple object (the this object) is constructed by invoking the Triple constructor expecting three int values as actual parameters publicTriple() { inta=0; intb =0; intc = 0; this(a, b, c); } Illegal this() invocation. A this() invocation must begin its statement body 67 Triple.java implementation // Triple(): default constructor public Triple() { this (0,0,0); } // Triple(): specific constructor public Triple(int a, int b, int c) { setValue(1, a); setValue(2, b); setValue(3, c); } // Triple(): specific constructor - alternative definition public Triple(int a, int b, int c) { this.setValue(1, a); this.setValue(2, b); this.setValue(3, c); } 68 Triple.java implementation Class Triple like every other Java class Automatically an extension of the standard class Object Class Object specifies some basic behaviors common to all objects These behaviors are said to be inherited Three of the inherited Object methods toString() clone() equals() 69 Recommendation Classes should override (i.e., provide a class-specific implementation) toString() clone() equals() By doing so, the programmer-expected behavior can be provided System.out.println(p); // displays string version of // object referenced by p System.out.println(q); // displays string version of // object referenced by q 70 Triple.java toString() implementation public String toString() { int a = getValue(1); int b = getValue(2); int c = getValue(3); return "Triple[" + a + ", " + b + ", " + c+ "]"; } Consider Triple t1 = new Triple(10, 20, 30); System.out.println(t1); Triple t2 = new Triple(8, 88, 888); System.out.println(t2); Produces Triple[10, 20, 30] Triple[8, 88, 888] 71 A new take on an old song AlienSong.mpeg AlienSong.mpeg 72 Triple.java clone() implementation public Object clone() { int a = getValue(1); int b = getValue(2); int c = getValue(3); return new Triple(a, b, c); } Consider Triple t1 = new Triple(9, 28, 29); Triple t2 = (Triple) t1.clone(); System.out.println("t1 = " + t1); System.out.println("t2 = " + t2); Produces Triple[9, 28, 29] Must cast! 73 Triple.java equals() implementation public boolean equals(Object v) { if (v instanceof Triple) { int a1 = getValue(1); int b1 = getValue(2); int c1 = getValue(3); Triple t = (Triple) v; int a2 = t.getValue(1); int b2 = t.getValue(2); int c2 = t.getValue(3); return (a1 == a2) && (b1 == b2) && (c1 == c2); } else { return false; } } Cant be equal unless its a Triple Compare corresponding attributes 74 Triple.java equals() Triple e = new Triple(4, 6, 10); Triple f = new Triple(4, 6, 11);, Triple g = new Triple(4, 6, 10); Triple h = new Triple(4, 5, 11); boolean flag1 = e.equals(f); 75 Triple.java equals() Triple e = new Triple(4, 6, 10); Triple f = new Triple(4, 6, 11);, Triple g = new Triple(4, 6, 10); Triple h = new Triple(4, 5, 11); boolean flag2 = e.equals(g); 76 Triple.java equals() Triple e = new Triple(4, 6, 10); Triple f = new Triple(4, 6, 11);, Triple g = new Triple(4, 6, 10); Triple h = new Triple(4, 5, 11); boolean flag3 = g.equals(h); 77 Using our Triple class 78 Program demo TripleDemo.java TripleDemo.java 79 Yet more new demotivators!!! 80 End of lecture on 30 Nov 2005 Wont be going over the rest of the slides in this set 81 Scope 82 Whats wrong with this code? class Scope { public static void f(int a) { int b = 1; // local definition System.out.println(a); // print 10 a = b; // update a System.out.println(a); // print 1 } public static void main(String[] args) { int i = 10; // local definition f(i); // invoking f() with i as parameter System.out.println(a); System.out.println(b); } } Variables a and b do not exist in the scope of method main() 83 Program demo Scope.java (just the compilation) Scope.java (just the compilation) 84 Blocks and scope rules A block is a list of statements nested within braces A method body is a block A block can be placed anywhere a statement would be legal A block contained within another block is a nested block A formal parameter is considered to be defined at the beginning of the method body A local variable can be used only in a statement or nested blocks that occurs after its definition An identifier name can be reused as long as the blocks containing the duplicate declarations are not nested one within the other Name reuse within a method is permitted as long as the reuse occurs in distinct blocks 85 Legal class Scope2 { public static void main(String[] args) { int a = 10; f(a); System.out.println(a); } public static void f(int a) { System.out.println(a); a = 1; System.out.println(a); } } 86 Legal but not recommended public void g() { { int j = 1; // define j System.out.println(j); // print 1 } { int j = 10; // define a different j System.out.println(j); // print 10 } { char j = // define a different j System.out.println(j); // print } } 87 Program demo Scope2.java (just the compilation) Scope2.java (just the compilation) 88 Whats the output? for (int i = 0; i < 3; ++i) { int j = 0; ++j; System.out.println(j); } The scope of variable j is the body of the for loop j is not in scope when ++i j is not in scope when i < 3 are evaluated j is redefined and re-initialized with each loop iteration 89 Overloading 90 Overloading Have seen it often before with operators int i = ; double x = ; String s = "April " + "June"; Java also supports method overloading Several methods can have the same name Useful when we need to write methods that perform similar tasks but different parameter lists Method name can be overloaded as long as its signature is different from the other methods of its class Difference in the names, types, number, or order of the parameters 91 Legal public static int min(int a, int b, int c) { return Math.min(a, Math.min(b, c)); } public static int min(int a, int b, int c, int d) { return Math.min(a, min(b, c, d)); } 92 Legal public static int power(int x, int n) { int result = 1; for (int i = 1; i