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interfaces: a quick review

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creates a new SkiJumper with no //training or jumps. private String myName; ... sOne = new String('one'); String sTwo = new String('two'); sTwo = sOne; sOne ' ... – PowerPoint PPT presentation

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Title: interfaces: a quick review

1
interfaces a quick review

public interface Athlete
void train(double dHours)
//increases the number of hours
//trained by dHours
public interface Flier
void fly() //increments the number
//of jumps
class SkiJumper
public SkiJumper(String sName)
//creates a new SkiJumper with no
//training or jumps
private String myName

2
interfaces a quick review

public interface Athlete
void train(double dHours)
public interface Flier
void fly()
class SkiJumper implements Athlete, Flier
public SkiJumper(String sName)
//creates a new SkiJumper with no
//training or jumps
public void train(double dHours)
public void fly()
private String myName
private double myHoursTrained

3
interfaces a quick review

class SkiJumper implements Athlete, Flier
public SkiJumper(String sName)
//creates a new SkiJumper with no
//training or jumps
public void train(double dHours)
myHoursTrained dHours
public void fly()
myJumps
private String myName
private double myHoursTrained
private int myJumps

4
interfaces a quick review

class SkiJumper implements Athlete, Flier
public SkiJumper(String sName)
//creates a new SkiJumper with no
//training or jumps
myName sName
myHoursTrained 0
myJumps 0
public void train(double dHours)
myHoursTrained dHours
public void fly()
myJumps
private String myName

5
interfaces a quick review

class SkiJumper implements Athlete, Flier
public SkiJumper(String sName)
myName sName
myHoursTrained 0
myJumps 0
public void train(double dHours)
myHoursTrained dHours
public void fly()myJumps
?? getHoursTrained() ??
?? getJumps() ??
private String myName
private double myHoursTrained
private int myJumps

6
interfaces a quick review

class SkiJumper implements Athlete, Flier
public SkiJumper(String sName)
myName sName
myHoursTrained 0
myJumps 0
public void train(double dHours)
myHoursTrained dHours
public void fly()myJumps
public double getHoursTrained()
return myHoursTrained
public int getJumps()
return myJumps

7
Applications vs. Applets

An applet is a program that runs within a browser
An application is a program that is installed on
a single computer
We can make applications in Java pretty much like
with did in C

8
Applications vs. Applets

public class AppDemo
public static void main(String args)
System.out.println("Hello World")

9
Applications vs. Applets

Q What is the String args for?
A You don't really care, but here's an example
public class AppDemo
public static void main(String args)
for(int nI 0 nI lt args.length nI)
System.out.println("Argument " nI
" " argsnI)

10
Applications vs. Applets

public class AppDemo
public static void main(String args)
for(int nI 0 nI lt args.length nI)
System.out.println("Argument " nI
" " argsnI)

11
Applications vs. Applets

To run our Java application, we need to Build it
first
Then we go to the command prompt and select the
correct folder
Typing java lets windows know what's coming up is
a java application
Then comes the name of the program AppDemo
And then the arguments
Testing out this silly program
C\folder namegtjava AppDemo Testing out
this silly program

12
Applications vs. Applets

You need to know that you can make both kinds of
programs in Java
But the AP exam doesn't test Applications or
Applets
On the AP you will only be asked to write classes
and methods
These classes and methods could be part of an
application or an applet

13
Handles

You know you don't get a new object in Java until
you ask for one
Thingy someThing
someThing new Thingy()
After the declaration Thingy someThing you have
a handle to a Thingy called someThing
Handles are also called "References" or "Pointers"

14
Only Objects have Handles

In Java, there are two types of data
Objects
Primitives
Objects are made with classes
Primitives are simple pieces of data like int,
double, char and boolean
int nNum 3
//creates an int

15
Handles

You can think of a handle as the name of an
object
String sString
Does NOT make a new String, it only makes a
handle to a String
sString can also be called a reference variable
sString new String("hello")
Now the handle is "hooked up" to a new "mailbox"

16
Handles

String sString

sString
17
Handles

String sString
sString new String("hello")

"hello"
sString
18
vs. .equals

String sString1 new String("hello")
String sString2 new String("hello")
System.out.println(sString1sString2)
System.out.println(sString1.equals(sString2))
/Sample Output
false
true /

19
vs. .equals

String sString1 new String("hello")
String sString2 new String("hello")
System.out.println(sString1sString2)
System.out.println(sString1.equals(sString2))

"hello"
sString1
"hello"
sString2
20
vs. .equals

String sString1 new String("hello")
String sString2 sString1
System.out.println(sString1sString2)
System.out.println(sString1.equals(sString2))
/Sample Output
true
true /

21
vs. .equals

String sString1 new String("hello")
String sString2 sString1
System.out.println(sString1sString2)
System.out.println(sString1.equals(sString2))

"hello"
sString1
sString2
22
vs. .equals

Rule always use .equals with objects unless you
are positive that you need

23
garbage

If no variables hold a reference to an object,
there is no way to find it. It becomes "garbage".
This can also be called an "orphan"
String sOne new String("one")
String sTwo new String("two")
sTwo sOne

"one"
sOne
"two"
sTwo
24
alias