Download Methods and classes

Programming with
methods and classes
Chapter 7
Fall 2005
CS 101
Aaron Bloomfield
1
Static vs. non-static
2
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();
Instance method
 Class (i.e. static) method
 Service provided by a class and it is not associated with a
particular object
String t = String.valueOf(n);
Class method
3
Variables
 Instance variable and instance constants
 Attribute of a particular object
 Usually a variable
Point p = new Point(5, 5);
int px = p.x;
Instance variable
 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;
Class constants
4
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
5
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 + ")";
}
}
6
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);
}
}
7
Static vs. non-static
 Execution of the code…
 Output is:
(a=1,b=3)
(a=2,b=3)
8
Static vs. non-static: memory diagram
Staticness s = new
Staticness t = new
s.increment();
t.increment();
t.increment();
System.out.println
System.out.println
Staticness();
Staticness();
(s);
(t);
s
t
Staticness
-a=1
0
Staticness
Staticness
-a=2
0
1
b
0
3
2
1
9
Program demo

StaticTest.java
10
Yale vs. Harvard

Web references: http://www.harvardsucks.org/,
http://www.yaledailynews.com/article.asp?AID=27506
11
Conversion.java
12
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
13
Conversion Implementation
public class Conversion {
// conversion equivalencies
private 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;
14
Conversion implementation
Modifier public indicates other classes can use the method
Modifier static indicates the method is a class method
public static double fahrenheitToCelsius (double f) {
return (f - 32) / 1.8;
}
No use of member/instance variables!!!
15
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;
}
16
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;
}
17
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;
}
18
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);
19
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
3.0
4.0
5.0
kilometers = 1.242742384474668 miles
liters = 0.7925161570744452 gallons
grams = 0.14109584779832166 ounces
hectares = 12.355269141746666 acres
20
A preferred Conversion use
Part of java.text
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
3.0
4.0
5.0
kilometers = 1.24 miles
liters = 0.79 gallons
grams = 0.14 ounces
hectares = 12.36 acres
21
Program Demo

Conversion.java
22
Fractals
23
Parameter passing
24
Java parameter passing
 The value is copied to the method
 Any changes to the parameter are forgotten when the
method returns
25
Java parameter passing
 Consider the following code:
static void foobar (int y) {
y = 7;
}
y
5
7
x
5
formal parameter
public static void main (String[] args) {
int x = 5;
actual parameter
foobar (x);
System.out.println(x);
}
 What gets printed?
26
Java parameter passing
 Consider the following code:
static void foobar (String y) {
y = “7”;
}
y
“7"
x
“5"
formal parameter
public static void main (String[] args) {
String x = “5”;
actual parameter
foobar (x);
System.out.println(x);
}
 What gets printed?
27
Java parameter passing
 Consider the following code:
static void foobar (Rectangle y) {
y.setWidth (10);
formal parameter
}
public static void main (String[] args) {
Rectangle x = new Rectangle();
foobar (x);
actual parameter
System.out.println(x.getWidth());
}
y
 What gets printed?
x
width
width==10
0
28
Java parameter passing
 Consider the following code:
static void foobar (Rectangle y) {
y = new Rectangle();
y.setWidth (10); formal parameter
}
public static void main (String[] args) {
Rectangle x = new Rectangle();
actual parameter
foobar (x);
System.out.println(x.getWidth());
}
 What gets printed?
y
width
width==10
0
x
width = 0
29
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
30
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
31
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);
}
}
32
Value parameter passing demonstration
 The file/class is actually called ParameterDemo.java
33
Program demo

ParameterDemo.java
34
ParameterDemo.java walkthrough
double sum = add(a, b);
Initial values of formal parameters
come from the actual parameters
public static double add (double x, double y) {
double result = x + y;
return result;
}
main()
add()
x
8.0
y
11.0
result
19.0
-
a
8.0
b
11.0
sum
19.0
-
product
-
35
ParameterDemo.java walkthrough
double multiply = multiply(a, b);
Initial values of formal parameters
come from the actual parameters
public static double multiply (double x, double y) {
x = x * y;
return x;
}
main()
multiply()
x
88.0
8.0
y
11.0
a
8.0
b
11.0
sum
19.0
product
88.0
-
36
PassingReferences.java
38
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);
}
39
PassingReferences.java run
g() can change the
attributes of the
object to which p
refers
40
Program demo

PassingReferences.java
41
PassingReferences.java
public static void main(String[] args) {
Point p = new Point(10, 10);
System.out.println(p);
f(p);
main()
Point
p
y: 10
x: 10
f()
v
java.awt.Point[x=10,y=10]
Met hod main()'s variable p
and met hod f()'s formal
paramet er v have t he same
value, which is a reference t o
an object represent ing
locat ion (10, 10) 42
PassingReferences.java
public static void f(Point v) {
v = new Point(0, 0);
}
main()
Point
p
y: 10
x: 10
f()
Point
v
y:
0
x:
0
43
PassingReferences.java
public static void main(String[] args) {
Point p = new Point(10, 10);
System.out.println(p);
f(p);
main()
Point
p
y: 10
x: 10
g()
System.out.println(p);
g(p);
v
java.awt.Point[x=10,y=10]
java.awt.Point[x=10,y=10]
Met hod main()'s variable p
and met hod g()'s formal
paramet er v have t he same
value, which is a reference t o
an object represent ing
locat ion (10, 10) 44
PassingReferences.java
public static void g(Point v) {
v.setLocation(0, 0);
}
main()
Point
p
y:
y: 10
0
x:
x: 10
0
g()
v
Met hod main()'s variable p
and met hod g()'s formal
paramet er v have t he same
value, which is a reference t o
an object represent ing
locat ion (10, 10) 45
PassingReferences.java
public static void main(String[] args) {
Point p = new Point(10, 10);
System.out.println(p);
f(p);
main()
Point
p
y: 0
x: 0
System.out.println(p);
g(p);
System.out.println(p);
java.awt.Point[x=10,y=10]
java.awt.Point[x=10,y=10]
java.awt.Point[x=0,y=0]
46
Star Wars Episode 3 Trailer
47
Star Wars Episode 3 Trailer

That was a edited version
– I changed the PG-rated trailer to a G-rated
trailer

The original one can be found at
http://www.sequentialpictures.com/
– Or Google for “star wars parody”
48
Casting
49
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?
50
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
51
ignoring it…
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
52
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
53
New 2005 demotivatiors!
54
Wrapper classes
55
What about adding variables to a Vector?
 The add method takes an Object as a parameter
 public void add (Object o) {
 Although we haven’t 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!
56
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 – we’ll see how to get around it in a
bit…
57
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
58
More on wrapper classes
 Consider this code:
int x = 5;
vector.add (x);
//…
int y = vector.get(0);
 Does this code work?
 It shouldn’t
 As we are adding a variable (not an object) to a vector
 But it does work!
 Why?
59
Auto-boxing
 Java 1.5 will automatically “wrap” a primitive value into it’s
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
60
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);
61
Today’s demotivators
62
Triple.java
63
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
64
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
65
Triple.java implementation
// Triple(): default constructor
public Triple() {
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
public
int
int
int
this
}
Triple() {
a = 0;
b = 0;
c = 0;
(a, b, c);
Illegal this() invocation. A this() invocation
must begin its statement body
66
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);
}
67
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()
68
Recommendation
 Classes should override
implementation)
 toString()
 clone()
 equals()
(i.e.,
provide
a
class-specific
 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
69
Triple.java toString() implementation
public
int
int
int
String toString() {
a = getValue(1);
b = getValue(2);
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]
70
A new take on an old song…

AlienSong.mpeg
71
Triple.java clone() implementation
public
int
int
int
Object clone() {
a = getValue(1);
b = getValue(2);
c = getValue(3);
return new Triple(a, b, c);
}
 Consider
Triple t1 = new Triple(9, 28, 29);
Triple t2 = (Triple) t1.clone();
Must cast!
System.out.println("t1 = " + t1);
System.out.println("t2 = " + t2);
 Produces
Triple[9, 28, 29]
Triple[9, 28, 29]
72
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
int a2
int b2
int c2
t
=
=
=
Can’t be equal
unless it’s a Triple
= (Triple) v;
t.getValue(1);
t.getValue(2);
t.getValue(3);
return (a1 == a2) && (b1 == b2) && (c1 == c2);
}
else {
return false;
}
Compare corresponding
attributes
}
73
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);
Triple
e
x1: 4
x2: 6
x3: 10
Triple
f
x1: 4
x2: 6
x3: 11
Triple
g
x1: 4
x2: 6
Triple
h
x1: 4
x2: 5
x3: 10
74
x3: 11
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);
Triple
e
x1: 4
x2: 6
x3: 10
Triple
f
x1: 4
x2: 6
x3: 11
Triple
g
x1: 4
x2: 6
Triple
h
x1: 4
x2: 5
x3: 10
75
x3: 11
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);
Triple
e
x1: 4
x2: 6
x3: 10
Triple
f
x1: 4
x2: 6
x3: 11
Triple
g
x1: 4
x2: 6
Triple
h
x1: 4
x2: 5
x3: 10
76
x3: 11
Using our Triple class
 …
77
Program demo

TripleDemo.java
78
Yet more new demotivators!!!
79
Scope
81
What’s 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);
}
}
82
Variables a and b do not exist in the scope of method main()
Program demo

Scope.java (just the compilation)
83
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
84
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);
}
}
85
Legal but not recommended
public void g() {
{
int j = 1;
System.out.println(j);
}
{
int j = 10;
System.out.println(j);
}
{
char j = '@';
System.out.println(j);
}
}
// define j
// print 1
// define a different j
// print 10
// define a different j
// print '@'
86
Program demo

Scope2.java (just the compilation)
87
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
88
Overloading
89
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
90
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));
}
91
Legal
public 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;
}
92
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);
}
93
Summary
94
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()
95
 Using random numbers