1
Programming withmethods and classes
Chapter 7Fall 2006CS 101Aaron Bloomfield
22
Static vs. non-staticStatic 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();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();t.increment();System.out.println (s);System.out.println (t);
Staticness s = new Staticness();Staticness t = new Staticness();s.increment();t.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
0b
Staticness- a = 1
Staticness- a = 1
Staticness- a = 2
123
1010
Program demoProgram demo StaticTest.javaStaticTest.java
11
How well do you feel How well do you feel you understand static-ness?you understand static-ness?a)a) Very well! This stuff is so easy.Very well! This stuff is so easy.b)b) With a little review, I’ll be good.With a little review, I’ll be good.c)c) Not very well at all.Not very well at all.d)d) I’m so lost. What’s static again?I’m so lost. What’s static again?e)e) I’d rather not answer this question, I’d rather not answer this question,
thanks.thanks.
1212
Hand PaintingsHand Paintings
1313
Conversion.javaConversion.java
14
Task – Conversion.java Support conversion between English and metric values
d degrees Fahrenheit = (d – 32)/1.8 degrees Celsius 1 mile = 1.609344 kilometers 1 gallon = 3.785411784 liters 1 ounce (avdp) = 28.349523125 grams 1 acre = 0.0015625 square miles = 0.40468564 hectares
15
Conversion Implementationpublic class Conversion {
// conversion equivalenciesprivate static final double
KILOMETERS_PER_MILE = 1.609344;private static final double
LITERS_PER_GALLON = 3.785411784;private static final double
GRAMS_PER_OUNCE = 28.349523125;private static final double
HECTARES_PER_ACRE = 0.40468564;
16
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!!!
17
Conversion Implementation// temperature conversions methodspublic static double fahrenheitToCelsius(double f) {
return (f - 32) / 1.8;}
public static double celsiusToFahrenheit(double c) {return 1.8 * c + 32;
}
// length conversions methodspublic static double kilometersToMiles(double km) {
return km / KILOMETERS_PER_MILE;}
18
Conversion Implementation// mass conversions methodspublic 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;
}
19
Conversion Implementation// area conversions methodspublic static double hectaresToAcres(double hectares) {
return hectares / HECTARES_PER_ACRE;}
public static double acresToHectares(double acres) {return acres * HECTARES_PER_ACRE;
}
20
Conversion useScanner 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);
21
A Conversion useSystem.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 = 1.242742384474668 miles3.0 liters = 0.7925161570744452 gallons4.0 grams = 0.14109584779832166 ounces5.0 hectares = 12.355269141746666 acres
22
A preferred Conversion useNumberFormat 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 miles3.0 liters = 0.79 gallons4.0 grams = 0.14 ounces5.0 hectares = 12.36 acres
Part of java.text
2323
Program DemoProgram Demo Conversion.javaConversion.java
24
How well do you feel How well do you feel you understand you understand
Conversion.java?Conversion.java?a)a) Very well! This stuff is so easy.Very well! This stuff is so easy.b)b) With a little review, I’ll be good.With a little review, I’ll be good.c)c) Not very well at all.Not very well at all.d)d) I’m so lost. What’s a class again?I’m so lost. What’s a class again?e)e) I’d rather not answer this question, I’d rather not answer this question,
thanks.thanks.
2525
FractalsFractals
2727
Parameter passingParameter passing
28
Java parameter passing The value is copied to the method
Any changes to the parameter are forgotten when the method returns
29
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
30
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
y
x “5"
“7"
31
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
y
x width = 0width = 10
32
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
y
x width = 0
width = 0width = 10
33
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
34
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
35
Value parameter passing demonstrationpublic 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);
}}
36
Value parameter passing demonstration
The file/class is actually called ParameterDemo.java
3737
Program demoProgram demo ParameterDemo.javaParameterDemo.java
38
8.0x
add()
11.0y
-result
8.0x
add()
11.0y
19.0result
ParameterDemo.java walkthrough
8.0a
main()
11.0b
-sum
-product
8.0a
main()
11.0b
19.0sum
-product
double sum = add(a, b);
public static double add (double x, double y) { double result = x + y;return result;
}
Initial values of formal parameterscome from the actual parameters
39
8.0x
multiply()
11.0y
88.0x
multiply()
11.0y
8.0a
main()
11.0b
19.0sum
-product
8.0a
main()
11.0b
19.0sum
88.0product
ParameterDemo.java walkthroughdouble multiply = multiply(a, b);
public static double multiply (double x, double y) { x = x * y;
return x;}
Initial values of formal parameterscome from the actual parameters
40
How well do you feel How well do you feel you understand parameter you understand parameter
passing?passing?a)a) Very well! This stuff is so easy.Very well! This stuff is so easy.b)b) With a little review, I’ll be good.With a little review, I’ll be good.c)c) Not very well at all.Not very well at all.d)d) I’m so lost. What’s a parameter I’m so lost. What’s a parameter
again?again?e)e) I’d rather not answer this question, I’d rather not answer this question,
thanks.thanks.
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Honda’s best commercialHonda’s best commercial cog.movcog.mov
4242
CastingCasting
43
Casting We’ve 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?
44
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 it’s a String (as it cloned a String) Thus, we need to tell Java it’s 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…
45
Casting, take 3 That “other” error is because String does not have a .clone()
method Not all classes do! We just haven’t 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
46
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
47
How well do you feel How well do you feel you understand casting?you understand casting?
a)a) Very well! This stuff is so easy.Very well! This stuff is so easy.b)b) With a little review, I’ll be good.With a little review, I’ll be good.c)c) Not very well at all.Not very well at all.d)d) I’m so lost. What’s a parameter I’m so lost. What’s a parameter
again?again?e)e) I’d rather not answer this question, I’d rather not answer this question,
thanks.thanks.
4848
OverloadingOverloading
49
Overloading Have seen it often before with operators
int i = 11 + 28;double x = 6.9 + 11.29;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
50
Legalpublic 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));
}
51
Legalpublic static int power(int x, int n) {
int result = 1;for (int i = 1; i <= n; ++i) {
result *= x;}return result;
}
public static double power(double x, int n) {double result = 1;for (int i = 1; i <= n; ++i) {
result *= x;}return result;
}
52
What’s the output?public static void f(int a, int b) {
System.out.println(a + b);}
public static void f(double a, double b) {System.out.println(a - b);
}
public static void main(String[] args) {int i = 19;double x = 54.0;
f(i, x); }
53
How well do you feel How well do you feel you understand you understand
overloading?overloading?a)a) Very well! This stuff is so easy.Very well! This stuff is so easy.b)b) With a little review, I’ll be good.With a little review, I’ll be good.c)c) Not very well at all.Not very well at all.d)d) I’m so lost. What’s a parameter I’m so lost. What’s a parameter
again?again?e)e) I’d rather not answer this question, I’d rather not answer this question,
thanks.thanks.
5454
2004 IOCCC winners2004 IOCCC winners 2004 winners:2004 winners:
– 2004 2004 anonymousanonymous Rendering of a stroked fontRendering of a stroked font– 2004 2004 arachnidarachnid Curses maze displayer/navigator with only line-of-sightCurses maze displayer/navigator with only line-of-sight
visibilityvisibility– 2004 burley 2004 burley A Poker gameA Poker game– 2004 2004 gavaregavare A ray tracerA ray tracer– 2004 gavin 2004 gavin Mini-OSMini-OS– 2004 hibachi 2004 hibachi A CGI capable HTTP serverA CGI capable HTTP server– 2004 2004 hoylehoyle Curses based polynomial graphing with auto-scaleCurses based polynomial graphing with auto-scale– 2004 jdalbec 2004 jdalbec Conway's look'n'say sequence split into elementsConway's look'n'say sequence split into elements– 2004 kopczynski 2004 kopczynski OCR of 8, 9, 10 and 11OCR of 8, 9, 10 and 11– 2004 2004 newbernnewbern Renders arbitary bitmapped fontsRenders arbitary bitmapped fonts– 2004 omoikane 2004 omoikane A CRC inserterA CRC inserter– 2004 schnitzi 2004 schnitzi Editor animationEditor animation– 2004 sds 2004 sds Space/tab/linefeed steganographySpace/tab/linefeed steganography– 2004 vik1 2004 vik1 X Windows car racing gameX Windows car racing game– 2004 vik2 2004 vik2 Calculates prime numbers using only CPPCalculates prime numbers using only CPP
At At http://www1.us.ioccc.org/years.html#2004http://www1.us.ioccc.org/years.html#2004
5555
ScopeScope
56
What’s wrong with this code?class Scope {
public static void f(int a) {int b = 1; // local definitionSystem.out.println(a); // print 10a = b; // update aSystem.out.println(a); // print 1
}
public static void main(String[] args) {int i = 10; // local definitionf(i); // invoking f() with i as
parameterSystem.out.println(a); System.out.println(b);
}}
Variables a and b do not exist in the scope of method main()
5757
Program demoProgram demo Scope.java (just the compilation)Scope.java (just the compilation)
58
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
59
Legalclass 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);
}}
60
Legal but not recommendedpublic void g() {
{int j = 1; // define jSystem.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 '@'}
}
6161
Program demoProgram demo Scope2.java (just the compilation)Scope2.java (just the compilation)
62
What’s 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
63
How well do you feel How well do you feel you understand scoping?you understand scoping?
a)a) Very well! This stuff is so easy.Very well! This stuff is so easy.b)b) With a little review, I’ll be good.With a little review, I’ll be good.c)c) Not very well at all.Not very well at all.d)d) I’m so lost. What’s a parameter I’m so lost. What’s a parameter
again?again?e)e) I’d rather not answer this question, I’d rather not answer this question,
thanks.thanks.
6565
Triple.javaTriple.java
66
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
67
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
68
Triple.java implementation// Triple(): default constructor
public Triple() {this (0, 0, 0);
} The new Triple object (the this object) is constructedby invoking the Triple constructor expecting three int
values as actual parameters
public Triple() {int a = 0;int b = 0;int c = 0;this (a, b, c);
}Illegal this() invocation. A this() invocationmust begin its statement body
69
Triple.java implementation// Triple(): default constructorpublic Triple() {
this (0,0,0);}
// Triple(): specific constructorpublic 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);
}
70
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()
71
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
72
Triple.java toString() implementationpublic 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);
ProducesTriple[10, 20, 30]Triple[8, 88, 888]
73
Triple.java clone() implementationpublic 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);
ProducesTriple[9, 28, 29]Triple[9, 28, 29]
Must cast!
74
Triple.java equals() implementationpublic 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;}
}
Can’t be equal unless it’s a Triple
Compare corresponding attributes
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 flag1 = e.equals(f);
eTriple
x1: 4
f
g
h
x2: 6 x3: 10
Triple
x1: 4 x2: 6 x3: 11
Triple
x1: 4 x2: 6 x3: 10
Triple
x1: 4 x2: 5 x3: 11
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 flag2 = e.equals(g);
eTriple
x1: 4
f
g
h
x2: 6 x3: 10
Triple
x1: 4 x2: 6 x3: 11
Triple
x1: 4 x2: 6 x3: 10
Triple
x1: 4 x2: 5 x3: 11
77
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);
eTriple
x1: 4
f
g
h
x2: 6 x3: 10
Triple
x1: 4 x2: 6 x3: 11
Triple
x1: 4 x2: 6 x3: 10
Triple
x1: 4 x2: 5 x3: 11
78
Using our Triple class …
7979
Program demoProgram demo TripleDemo.javaTripleDemo.java
80
How well do you feel How well do you feel you understand Triple.java?you understand Triple.java?a)a) Very well! This stuff is so easy.Very well! This stuff is so easy.b)b) With a little review, I’ll be good.With a little review, I’ll be good.c)c) Not very well at all.Not very well at all.d)d) I’m so lost. What’s a parameter I’m so lost. What’s a parameter
again?again?e)e) I’d rather not answer this question, I’d rather not answer this question,
thanks.thanks.
8181
SummarySummary
82
Summary of key points The this keyword
Can be used to call another constructor Must be the FIRST thing called
Can be used as a reference to the current object Static vs. non-static
A static variable means there is only one such variable regardless of how many objects have been declared
A static method does not care about the “state” of the object
Various methods we may want to override: clone() toString() equals()
Using random numbers
8383
Cubic TragedyCubic Tragedy Cubic_tragedy_m640.movCubic_tragedy_m640.mov
8484
A two legged dog….A two legged dog….