Java - PowerPoint PPT Presentation

1 / 22

About This Presentation

Title:

Java

Description:

Calls each delegate when an event occurs. Java has to use a full object ... static void Main() { DelegateDemo t = new DelegateDemo ... – PowerPoint PPT presentation

Number of Views:20

Avg rating:3.0/5.0

Slides: 23

Provided by: julie57

Category:

more less

Transcript and Presenter's Notes

Title: Java

1
Java C

Summarise key similarities differences
Memory management type safety
Interface
Delegates, struct, properties, reference
parameter
Evaluate the similarities differences
Discuss different languages approach to OO

2
Overview

C and Java similarities
Garbage collection
Run-time checking
Types (casts), Array Bounds checking
Single inheritance
C and Java differences
Delegates
Exceptions
Value Reference variables
Properties

3
C Properties

C classes have methods properties
Good programming hides properties (why)
Only access properties through methods
In C can access properties by assignment
But actually uses Getter Setter functions
Just syntactic sugar simpler to read
Better linguistic match to property concept
Assignment indicates that something is being
changed

4
Properties Example

public class MyClass
private long m_long
//public property
public long Number
get
return (m_long)
set
m_long value
public MyClass()
m_long 0

MyClass m new MyClass()
m.Number 3
long m m.Number

The real data member is private
In Java, the getter setter would be explicit
methods Method call syntax is used.
5
Event Handling

Event-generating objects keep a list of handlers
When an event occurs,
run through the list calling a suitable method
Java Handlers are objects
C Handlers are pointers to methods
Delegates

6
Delegates

Like a function pointer in C
But can also be an instance method pointer
Keeps track of the owning object
Used for callbacks pass function to server
E.g. events use a delegate as an event handler
Component can keep a list of delegates
Calls each delegate when an event occurs
Java has to use a full object
But can create anonymous objects
Inline definition of method implementations for
interface

7
Java Event Handlers

public class MyButtonListener extends Applet
implements ActionListener
private Button m_acceptButton
private Button m_rejectButton
public MyButtonListener()
m_acceptButton new Button("OK")
m_acceptButton.AddActionListener(this)
m_rejectButton new Button("Cancel")
m_rejectButton.AddActionListener(this)
// code to add these Buttons to me and for
layout
//ActionListener events
public void actionPerformed(ActionEvent e)
if (e.getSource() m_rejectButton)
//write rejection code
else if (e.getSource()
m_acceptButton)
//write acceptance code

Add this object to the buttons list of handlers
The interface for the handler
The method that anything implementing
ActionListener must provide
8
C Delegate Event Handlers

public class MyButtonListener Form
public MyButtonListener()
Button accept new Button()
accept.Text "OK"
accept.Click new EventHandler(AcceptClick
)
Button reject new Button()
reject.Text "Cancel"
reject.Click new EventHandler(RejectClick
)
//Event handlers for buttons
private void AcceptClick(object sender,
EventArgs e)
private void RejectClick(object sender,
EventArgs e)

Added to the list of function pointers
function pointer No ()
Overloaded operator
9
Creating a Delegate

Defining a Delegate type
public delegate void EventHandler(object sndr,
EventArgs e)
Creating Using a Delegate object
public class Button
public String Text
public EventHandler Click new EventHandler()
public ClickTheButton(EventArgs e)
Click(this, e)

Essentially a class with a list of method pointers
All the method pointers will be called with these
parameters
10
Evaluation of Delegates

More type-safe than function pointers
No need to create objects to pass functions
around
class DelegateDemo
delegate int DelegateProcessInt(int i)
public int Square(int i) return ii
public int Cube(int i) return iii
void ShowResult(DelegateProcessInt d, String
s,int i)
Console.WriteLine("0 of 1 is 2", s, i,
d(i))
static void Main()
DelegateDemo t new DelegateDemo()
t.ShowResult(new DelegateProcessInt (t.Square),
"Square", 7)
t.ShowResult(new DelegateProcessInt (t.Cube),
"Cube", 7)

Equivalent to printf
11
Inheritance

C Java support only single inheritance
Can prevent class being a parent sealed/final
C doesnt automatically override methods
Methods only virtual if virtual keyword used
Must explicitly use override keyword
Prevents a child method being used if a new
method with same name added to ancestor
Can hide ancestor method use new keyword
Greater flexibility
But is it really valuable?

12
Interfaces

Like a class with just method signatures
Used in design-by-contract
A factory object can return any object that
implements the interface
i.e. implements all the methods
Avoids ambiguity in multiple inheritance
Java allows constants in interface
C allows different implementation of methods
with same name from different interfaces

13
Struct

C provides Struct
A lightweight class stored on the stack
Cant inherit from a Struct
Normal classes are stored on the heap
Struct variables are values
Class variables are references
MyStruct theStruct new MyStruct(1,2)
MyClass theClass new MyClass(1,2)

1
2
1
2
theStruct
theClass
14
Destructors

C Java have finalizers
Called by garbage collector when freeing object
Called at random time
May not be called if program aborted
Not good for general resource management
C defines the IDispose interface
Class implements IDispose
That is, provides a dispose() method
Explicitly call dispose() when finished with
object
Must ensure calling dispose twice isnt a problem

15
Exceptions Similarities

Handles system and application errors
Structured, uniform, and type-safe approach
All exceptions derived from one base clas
In C, an exception can be a value of any type
A finally block can provide termination code
Executes in normal and exceptional conditions
Well defined exceptions for system errors
overflow, divide-by-zero, null dereferences

16
Exceptions Differences

Java is fussier about exceptions
RunTimeException inherits from Exception
A method that could throw an Exception that isnt
a RunTimeException must declare this
void get(String url) throws MalFormedURLException
Allows compiler to check use
Allows IDE to prompt programmer

17
Access Protection