cmsc 150 classes cs 150: mon 6 feb 2012 images: library of congress
TRANSCRIPT
CMSC 150CLASSES
CS 150: Mon 6 Feb 2012 Images: Library of Congress
First: quick review of using methods
Remember types of methods:
Function: creates and “returns” a value
int score = game.getScore();
System.out.println( “You get “ + game.numPointsForWin() );
int newScore = game.getScore() – game.numPointsPerMove();
Room playerRoom = game.getRoom( playerRow, playerCol );
Remember to either: Save the returned value in a variable of the
appropriate type, orUse the returned value in an expression.
First: quick review of using methods
Remember types of methods:
Procedure: does work & returns no value (i.e., void return type)
game.updateScore(newScore);
game.updateMessage( “It’s the StayPuft Marshmallow Man!”);
roomForSlimer.addSlimer();
Remember: methods with void return type don’t create a value illogical to use them in an assignment or expression
Anatomy of a method call
game.updateScore( newScore );
An object reference is always needed to call a method
Object to call themethod with.
Anatomy of a method call
game.updateScore( newScore );
An object reference is always needed to call a method
Name of the methodto call.
Anatomy of a method call
game.updateScore( newScore );
An object reference is always needed to call a method
Parameters – input values forthe method.
One Exception
public class GhostBusters {
…
public boolean handleMove( ) {
…
boolean result = checkForLoss( );
}
public boolean checkForLoss( ) { … }
}
EXCEPT: if the method definition and the method call are in the same class, the object reference is “implicit”
Supplied automatically by Java
One other exception
Static methods are a little different int jenny = Integer.parseInt(“8675309”);
This is the name of a class, not a referenceto an object.
What is a Class?
A “blueprint” of your entity to be implemented
Defines and encapsulates your entity’s Characteristics (data) Behaviors (methods)
Example:
Example: Dog
What characteristics do dogs have in general? Your class’s data (instance variables)
What behaviors do dogs exhibit? Your class’s methods
But do these identify a specific dog?
Example: Dog
Do these identify a specific dog?
Example: Dog
Example: Dog
Class: Dog Characteristics: breed, eye color, weight Behaviors: fetch(), lick(), sit(), stay(), poop()
Science Lab Lilly: an instance (object) of class Dog chocolate lab, golden eyes, 65 lbs. can perform Dog behaviors
Darth Bailey: an instance (object) of class Dog black lab, brown eyes, 50 lbs. can perform Dog behaviors
In Summary
Write the class “blueprint” once
Create a specific object of that class Create another object of that class And another…
Consider a familiar example
Write Our Own String Class
public class SimpleString
{
// instance variables (data)
// methods
}
Notice no main() method Only when you want to directly execute that
class
Writing Your Own String Classpublic class SimpleString
{
// instance variables (data)
private char myFirstCharacter;
private char mySecondCharacter;
private int myLength;
// methods
}
Methods
Constructor must have same name as class syntax: public ClassName( parameters… ) creates the object of this class type SimpleString str = new SimpleString(‘Z’,’a’); no return type in method definition (implicit)
All other methods
Writing Your Own String Class
public class SimpleString
{
// instance variables (data)
private char myFirstCharacter;
private char mySecondCharacter;
private int myLength;
// constructor
public SimpleString( char charOne, char charTwo )
{
myFirstCharacter = charOne;
mySecondCharacter = charTwo;
myLength = 2;
}
}
In Another Class…
public class Tester
{
public static void main( String[] args )
{
SimpleString str1 = new SimpleString( ‘O’, ‘y’ );
SimpleString str2 = new SimpleString( ‘Z’, ‘a’ );
}
}
public class SimpleString
{
private char myFirstChar;
private char mySecondChar;
private int myLength;
public SimpleString(char char1, char char2)
{
myFirstChar = char1;
mySecondChar = char2;
myLength = 2;
}
}
public class Tester{
public static void main( String[] args ){
SimpleString str1 = new SimpleString(‘O’,’y’);
SimpleString str2 = new SimpleString(‘Z’,’a’);
}}
SimpleString.java
Tester.java
public class Tester{
public static void main( String[] args ){
SimpleString str1 = new SimpleString(‘O’,’y’);
SimpleString str2 = new SimpleString(‘Z’,’a’);
}}
SimpleString.java
Tester.java
Variables are of the class type SimpleString
public class SimpleString
{
private char myFirstChar;
private char mySecondChar;
private int myLength;
public SimpleString(char char1, char char2)
{
myFirstChar = char1;
mySecondChar = char2;
myLength = 2;
}
}
public class Tester{
public static void main( String[] args ){
SimpleString str1 = new SimpleString(‘O’,’y’);
SimpleString str2 = new SimpleString(‘Z’,’a’);
}}
SimpleString.java
Tester.java
Objects will be constructed using the constructor from that class
public class SimpleString
{
private char myFirstChar;
private char mySecondChar;
private int myLength;
public SimpleString(char char1, char char2)
{
myFirstChar = char1;
mySecondChar = char2;
myLength = 2;
}
}
The Two Programs Together
SimpleString.java: defines blueprint for a generic "SimpleString" object we do not directly run this
Tester.java: we run this (because it contains main()) uses SimpleString as a variable type creates objects of the class SimpleString
Let's see what happens in memory as we run Tester…
public class Tester{
public static void main( String[] args ){
SimpleString str1 = new SimpleString(‘O’,’y’);
SimpleString str2 = new SimpleString(‘Z’,’a’);
}}
SimpleString.java
Tester.java
str1
declare…
public class SimpleString
{
private char myFirstChar;
private char mySecondChar;
private int myLength;
public SimpleString(char char1, char char2)
{
myFirstChar = char1;
mySecondChar = char2;
myLength = 2;
}
}
127
128
129
130
131
132
133
134
135
136
137
138
public class Tester{
public static void main( String[] args ){
SimpleString str1 = new SimpleString(‘O’,’y’);
SimpleString str2 = new SimpleString(‘Z’,’a’);
}}
SimpleString.java
Tester.java‘O’
‘y’
character literals stored
elsewhere automatically
public class SimpleString
{
private char myFirstChar;
private char mySecondChar;
private int myLength;
public SimpleString(char char1, char char2)
{
myFirstChar = char1;
mySecondChar = char2;
myLength = 2;
}
}
str1 127
128
129
130
131
132
133
134
135
136
137
138
public class Tester{
public static void main( String[] args ){
SimpleString str1 = new SimpleString(‘O’,’y’);
SimpleString str2 = new SimpleString(‘Z’,’a’);
}}
SimpleString.java
Tester.java
reserve enough space for a
SimpleString object…
public class SimpleString
{
private char myFirstChar;
private char mySecondChar;
private int myLength;
public SimpleString(char char1, char char2)
{
myFirstChar = char1;
mySecondChar = char2;
myLength = 2;
}
}
‘O’
‘y’
str1 127
128
129
130
131
132
133
134
135
136
137
138
public class SimpleString
{
private char myFirstChar;
private char mySecondChar;
private int myLength;
public SimpleString(char char1, char char2)
{
myFirstChar = char1;
mySecondChar = char2;
myLength = 2;
}
}
public class Tester{
public static void main( String[] args ){
SimpleString str1 = new SimpleString(‘O’,’y’);
SimpleString str2 = new SimpleString(‘Z’,’a’);
}}
SimpleString.java
Tester.java
reserve enough space for a
SimpleString object…
mySecondChar
myLengthDATA
METHOD
STUFF
myFirstChar
‘O’
‘y’
str1 127
128
129
130
131
132
133
134
135
136
137
138
public class SimpleString
{
private char myFirstChar;
private char mySecondChar;
private int myLength;
public SimpleString(char char1, char char2)
{
myFirstChar = char1;
mySecondChar = char2;
myLength = 2;
}
}
public class Tester{
public static void main( String[] args ){
SimpleString str1 = new SimpleString(‘O’,’y’);
SimpleString str2 = new SimpleString(‘Z’,’a’);
}}
SimpleString.java
Tester.java
mySecondChar
myLength
METHOD
STUFF
myFirstChar
‘O’
‘y’
str1 127
128
129
130
131
132
133
134
135
136
137
138
invoke the constructor…
public class SimpleString
{
private char myFirstChar;
private char mySecondChar;
private int myLength;
public SimpleString(char char1, char char2)
{
myFirstChar = char1;
mySecondChar = char2;
myLength = 2;
}
}
public class Tester{
public static void main( String[] args ){
SimpleString str1 = new SimpleString(‘O’,’y’);
SimpleString str2 = new SimpleString(‘Z’,’a’);
}}
SimpleString.java
Tester.java
mySecondChar
myLength
METHOD
STUFF
myFirstChar
‘O’
‘y’
str1 127
128
129
130
131
132
133
134
135
136
137
138
which reserves spacefor the
parameters(like variables)…
char1
char2
public class SimpleString
{
private char myFirstChar;
private char mySecondChar;
private int myLength;
public SimpleString(char char1, char char2)
{
myFirstChar = char1;
mySecondChar = char2;
myLength = 2;
}
}
public class Tester{
public static void main( String[] args ){
SimpleString str1 = new SimpleString(‘O’,’y’);
SimpleString str2 = new SimpleString(‘Z’,’a’);
}}
SimpleString.java
Tester.java
mySecondChar
myLength
METHOD
STUFF
myFirstChar
‘O’
‘y’
str1 127
128
129
130
131
132
133
134
135
136
137
138
char1
char2
parameters
arguments
‘O’
public class SimpleString
{
private char myFirstChar;
private char mySecondChar;
private int myLength;
public SimpleString(char char1, char char2)
{
myFirstChar = char1;
mySecondChar = char2;
myLength = 2;
}
}
public class Tester{
public static void main( String[] args ){
SimpleString str1 = new SimpleString(‘O’,’y’);
SimpleString str2 = new SimpleString(‘Z’,’a’);
}}
SimpleString.java
Tester.java
mySecondChar
myLength
METHOD
STUFF
myFirstChar
‘O’
‘y’
str1 127
128
129
130
131
132
133
134
135
136
137
138
char1
char2
parameters
arguments
‘O’
‘y’
public class SimpleString
{
private char myFirstChar;
private char mySecondChar;
private int myLength;
public SimpleString(char char1, char char2)
{
myFirstChar = char1;
mySecondChar = char2;
myLength = 2;
}
}
public class Tester{
public static void main( String[] args ){
SimpleString str1 = new SimpleString(‘O’,’y’);
SimpleString str2 = new SimpleString(‘Z’,’a’);
}}
SimpleString.java
Tester.java
mySecondChar
myLength
METHOD
STUFF
myFirstChar
‘O’
‘y’
str1 127
128
129
130
131
132
133
134
135
136
137
138
char1
char2
‘O’
‘y’
execute the first statement in the
constructor…
‘O’
public class SimpleString
{
private char myFirstChar;
private char mySecondChar;
private int myLength;
public SimpleString(char char1, char char2)
{
myFirstChar = char1;
mySecondChar = char2;
myLength = 2;
}
}
public class Tester{
public static void main( String[] args ){
SimpleString str1 = new SimpleString(‘O’,’y’);
SimpleString str2 = new SimpleString(‘Z’,’a’);
}}
SimpleString.java
Tester.java
mySecondChar
myLength
METHOD
STUFF
myFirstChar
‘O’
‘y’
str1 127
128
129
130
131
132
133
134
135
136
137
138
char1
char2
‘O’
‘y’
execute the next statement in the
constructor…
‘O’
‘y’
public class SimpleString
{
private char myFirstChar;
private char mySecondChar;
private int myLength;
public SimpleString(char char1, char char2)
{
myFirstChar = char1;
mySecondChar = char2;
myLength = 2;
}
}
public class Tester{
public static void main( String[] args ){
SimpleString str1 = new SimpleString(‘O’,’y’);
SimpleString str2 = new SimpleString(‘Z’,’a’);
}}
SimpleString.java
Tester.java
mySecondChar
myLength
METHOD
STUFF
myFirstChar
‘O’
‘y’
str1 127
128
129
130
131
132
133
134
135
136
137
138
char1
char2
‘O’
‘y’
execute the next statement in the
constructor…
‘O’
‘y’
2
public class SimpleString
{
private char myFirstChar;
private char mySecondChar;
private int myLength;
public SimpleString(char char1, char char2)
{
myFirstChar = char1;
mySecondChar = char2;
myLength = 2;
}
}
public class Tester{
public static void main( String[] args ){
SimpleString str1 = new SimpleString(‘O’,’y’);
SimpleString str2 = new SimpleString(‘Z’,’a’);
}}
SimpleString.java
Tester.java
mySecondChar
myLength
METHOD
STUFF
myFirstChar
‘O’
‘y’
str1 127
128
129
130
131
132
133
134
135
136
137
138
as the constructor finishes, space for the parameters is
reclaimed…
‘O’
‘y’
2
public class SimpleString
{
private char myFirstChar;
private char mySecondChar;
private int myLength;
public SimpleString(char char1, char char2)
{
myFirstChar = char1;
mySecondChar = char2;
myLength = 2;
}
}
public class Tester{
public static void main( String[] args ){
SimpleString str1 = new SimpleString(‘O’,’y’);
SimpleString str2 = new SimpleString(‘Z’,’a’);
}}
SimpleString.java
Tester.java
mySecondChar
myLength
METHOD
STUFF
myFirstChar
‘O’
‘y’
str1 127
128
129
130
131
132
133
134
135
136
137
138
constructor call returns the memory address of the new
object, which is assigned to str1…
‘O’
‘y’
2
130
public class SimpleString
{
private char myFirstChar;
private char mySecondChar;
private int myLength;
public SimpleString(char char1, char char2)
{
myFirstChar = char1;
mySecondChar = char2;
myLength = 2;
}
}
public class Tester{
public static void main( String[] args ){
SimpleString str1 = new SimpleString(‘O’,’y’);
SimpleString str2 = new SimpleString(‘Z’,’a’);
}}
SimpleString.java
Tester.java
mySecondChar
myLength
METHOD
STUFF
myFirstChar
‘O’
‘y’
str1 127
128
129
130
131
132
133
134
135
136
137
138
str1 variable now “references” the
SimpleString object
‘O’
‘y’
2
130
public class SimpleString
{
private char myFirstChar;
private char mySecondChar;
private int myLength;
public SimpleString(char char1, char char2)
{
myFirstChar = char1;
mySecondChar = char2;
myLength = 2;
}
}
public class Tester{
public static void main( String[] args ){
SimpleString str1 = new SimpleString(‘O’,’y’);
SimpleString str2 = new SimpleString(‘Z’,’a’);
}}
SimpleString.java
Tester.java
mySecondChar
myLength
METHOD
STUFF
myFirstChar
‘O’
‘y’
str1 127
128
129
130
131
132
133
134
135
136
137
138
‘O’
‘y’
2
130
object (instance) of class SimpleString
variable of type SimpleString(containing reference)
public class SimpleString
{
private char myFirstChar;
private char mySecondChar;
private int myLength;
public SimpleString(char char1, char char2)
{
myFirstChar = char1;
mySecondChar = char2;
myLength = 2;
}
}
public class Tester{
public static void main( String[] args ){
SimpleString str1 = new SimpleString(‘O’,’y’);
SimpleString str2 = new SimpleString(‘Z’,’a’);
}}
SimpleString.java
Tester.java
mySecondChar
myLength
METHOD
STUFF
myFirstChar
str2 337
338
339
340
341
342
343
344
345
346
347
348
‘Z’
‘a’
2
340
next statement results in similar
picture elsewhere in memory
public class MyString
{
// instance variables (data)
private char myFirstCharacter;
private char mySecondCharacter;
private int myLength;
// default constructor
public SimpleString()
{
myFirstCharacter = ‘ ‘;
mySecondCharacter = ‘ ‘;
myLength = 0;
}
// constructor with two parameters
public SimpleString( char charOne, char charTwo )
{
myFirstCharacter = charOne;
mySecondCharacter = charTwo;
myLength = 2;
}
}
Methods
Constructor must have same name as class syntax: public ClassName( arguments… ) creates the object of this class type MyString str = new MyString(); no return type (implicit)
All other methods use any non-reserved name two types: perform action, return a value
Methods
All other methods use any non-reserved name two types: perform action, return a value
1. perform an action: don't return anything public void doSomething() { …
public void doSomethingElse() { …2. return a value: provide a specific return
type public int length() { … public String substring( int …
All other methods use any non-reserved name two types: perform action, return a value
1. perform an action: don't return anything public void doSomething( … )
{ … public void doSomethingElse( … ) { …2. return a value: provide a specific return type public int length() { … public String substring( int …
Methods
public class SimpleString
{
…
// these methods do something without
// needing to return any sort of value
public void printTheString()
{
System.out.println( “” + myFirstCharacter
+ mySecondCharacter );
}
public void clearTheString()
{
myFirstCharacter = ‘ ‘;
mySecondCharacter = ‘ ‘;
myLength = 0;
}
}
return type of method
public class SimpleString
{
…
// these methods do something without
// needing to return any sort of value
public void printTheString()
{
System.out.println( “” + myFirstCharacter
+ mySecondCharacter );
}
public void clearTheString()
{
myFirstCharacter = ‘ ‘;
mySecondCharacter = ‘ ‘;
myLength = 0;
}
}
public class SimpleString
{
…
// these methods do something without
// needing to return any sort of value
public void printTheString()
{
System.out.println( “” + myFirstCharacter
+ mySecondCharacter );
}
public void clearTheString()
{
myFirstCharacter = ‘ ‘;
mySecondCharacter = ‘ ‘;
myLength = 0;
}
}
void: no return
statement
public class SimpleString
{
…
// these methods return some sort of info
public int length()
{
return myLength; // return instance variable
}
public char charAt(int index)
{
char returnChar = ‘ ‘;
if (index == 0) {
returnChar = myFirstCharacter;
} else if (index == 1) {
returnChar = mySecondCharacter;
}
return returnChar;
}
}
public class SimpleString
{
…
// these methods return some sort of info
public int length()
{
return myLength; // return instance variable
}
public char charAt(int index)
{
char returnChar = ‘ ‘;
if (index == 0) {
returnChar = myFirstCharacter;
} else if (index == 1) {
returnChar = mySecondCharacter;
}
return returnChar;
}
}
return type of method
public class SimpleString
{
…
// these methods return some sort of info
public int length()
{
return myLength; // return instance variable
}
public char charAt(int index)
{
char returnChar = ‘ ‘;
if (index == 0) {
returnChar = myFirstCharacter;
} else if (index == 1) {
returnChar = mySecondCharacter;
}
return returnChar;
}
}
return statement using a
variable of type int
return statement using a
variable of type char