![setBackground(Color.cyan);
}
// Display msg in applet window.
public void paint(Graphics g)
{
g.drawString("This is in the applet window.", 10, 20);
showStatus("This is shown in the status window.");
}
}
Output:
The HTMLAPPLET Tag:
< APPLET
[CODEBASE = codebaseURL]
CODE = appletFile
[ALT = alternateText]
[NAME = appletInstanceName]
WIDTH = pixels
HEIGHT = pixels
[ALIGN = alignment]
[VSPACE = pixels]
[HSPACE = pixels]
>
[< PARAM NAME = AttributeName VALUE = AttributeValue>]
[< PARAM NAME = AttributeName2 VALUE = AttributeValue>]
. . .
[HTML Displayed in the absence of Java]
</APPLET>
Letâs take a look at each part now.
CODEBASE: CODEBASE is an optional attribute that specifies the base URL of the
applet code, which is the directory that will be searched for the appletâs executable
class file (specified by the CODE tag).
CODE :CODE is a required attribute that gives the name of the file containing your
appletâs compiled .class file. This file is relative to the code base URL of the applet,
which is the directory that the HTML file was in or the directory indicated by
CODEBASE if set.](https://image.slidesharecdn.com/oopswithjavamodulesiiiiv-231118063701-ef35b8c8/85/oops-with-java-modules-iii-iv-pptx-8-320.jpg)
![EXCEPTIONS METHODS
Following is the list of important medthods available in the Throwable class.
Methods with Description
1 public String getMessage()
Returns a detailed message about the exception that has occurred. This message is initialized in the
Throwable constructor.
2 public Throwable getCause()
Returns the cause of the exception as represented by a Throwable object
3 public String toString()
Returns the name of the class concatenated with the result of getMessage()
4 public voidprintStackTrace()
Prints the result of toString() along with the stack trace to System.err, the error output stream.
5 public StackTraceElement []getStackTrace()
Returns an array containing each element on the stack trace. The element at index 0 represents the top
of the call stack,and the last element in the array represents the method at the bottom of the call stack.
6 public Throwable fillInStackTrace()
Fills the stack trace of this Throwable object with the current stack trace, adding to any previous
information in the stacktrace.
CATCHING EXCEPTIONS
A method catches an exception using a combination of the try and catch keywords. A try/catch block is placed
around the code that might generate an exception. Code within a try/catch block is referred to as protected
code, and the syntax for using try/catchlooks like the following:
try
{
//Protected code
}catch(ExceptionName e1)
{
//Catch block
}
A catch statement involves declaring the type of exception you are trying to catch. If an exception occurs in
protected code, the catch block (or blocks) that follows the try is checked. If the type of exception that](https://image.slidesharecdn.com/oopswithjavamodulesiiiiv-231118063701-ef35b8c8/85/oops-with-java-modules-iii-iv-pptx-14-320.jpg)
![occurred is listed in a catch block, the exception is passed to the catch block much as an argument is passed
into a methodparameter.
EXAMPLE
The following is an array is declared with 2 elements. Then the code tries to access the 3rd element of the array
which throws anexception.
// File Name : ExcepTest.java
import java.io.*;
public class ExcepTest{
public static void main(String args[]){
try{
int a[] = new int[2];
System.out.println("Access element three :" + a[3]);
}catch(ArrayIndexOutOfBoundsException e){
System.out.println("Exception thrown :" + e);
}
System.out.println("Out of the block");
}
}
This would produce the followingresult:
Exception thrown :java.lang.ArrayIndexOutOfBoundsException: 3
Out of the block
MULTIPLE CATCH BLOCKS
A try block can be followed by multiple catch blocks. The syntax for multiple catch blocks looks like the
following:
try
{
//Protected code
}catch(ExceptionType1 e1)
{
//Catch block
}catch(ExceptionType2 e2)
{
//Catch block
}catch(ExceptionType3 e3)
{
//Catch block
}
The previous statements demonstrate three catch blocks, but you can have any number of them after a single
try. If an exception occurs in the protected code, the exception is thrown to the first catch block in the list. If
the data type of the exception thrown matches ExceptionType1, it gets caught there. If not, the exception
passes down to the second catch statement. This continues until the exception either is caught or falls through](https://image.slidesharecdn.com/oopswithjavamodulesiiiiv-231118063701-ef35b8c8/85/oops-with-java-modules-iii-iv-pptx-15-320.jpg)
![// Method implementation
}
//Remainder of class definition
}
THE FINALLY KEYWORD
The finally keyword is used to create a block of code that follows a try block. A finally block of code always
executes, whether or not an exception has occurred.
Using a finally block allows you to run any cleanupâtype statements that you want to execute, no matter what
happens in the protectedcode.
A finally block appears at the end of the catch blocks and has the following syntax:
try
{
//Protected code
}catch(ExceptionType1 e1)
{
//Catch block
}catch(ExceptionType2 e2)
{
//Catch block
}catch(ExceptionType3 e3)
{
//Catch block
}finally
{
//The finally block always executes.
}
EXAMPLE
public class ExcepTest{
public static void main(String args[]){
int a[] = new int[2];
try{
System.out.println("Access element three :" + a[3]);
}catch(ArrayIndexOutOfBoundsException e){
System.out.println("Exception thrown :" + e);
}
finally{
a[0] = 6;
System.out.println("First element value: " +a[0]);
System.out.println("The finally statement is executed");
}
}
}](https://image.slidesharecdn.com/oopswithjavamodulesiiiiv-231118063701-ef35b8c8/85/oops-with-java-modules-iii-iv-pptx-17-320.jpg)
![{
return amount;
}
}
To demonstrate using our userâdefined exception, the following CheckingAccount class contains a
withdraw() method that throws anInsufficientFundsException.
// File Name CheckingAccount.java
import java.io.*;
public class CheckingAccount
{
private double balance;
private int number;
public CheckingAccount(int number)
{
this.number = number;
}
public void deposit(double amount)
{
balance += amount;
}
public void withdraw(double amount) throws
InsufficientFundsException
{
if(amount <= balance)
{
balance -= amount;
}
else
{
double needs = amount - balance;
throw new InsufficientFundsException(needs);
}
}
public double getBalance()
{
return balance;
}
public int getNumber()
{
return number;
}
}
The following BankDemo program demonstrates invoking the deposit() and withdraw() methods of
CheckingAccount.
// File Name BankDemo.java
public class BankDemo
{
public static void main(String [] args)
{](https://image.slidesharecdn.com/oopswithjavamodulesiiiiv-231118063701-ef35b8c8/85/oops-with-java-modules-iii-iv-pptx-19-320.jpg)
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- 1. APPLETS A Java applet is a special kind of Java program that a browser enabled with Java technology can download from the internet and run. An applet is typically embedded inside a web page and runs in the context of a browser. An applet must be a subclass of the java.applet.Applet class. The Applet class provides the standard interface between the applet and the browser environment. The Applet class is contained in the java.applet package.Applet contains several methods that give you detailed control over the execution of your applet. In addition,java.applet package also defines three interfaces: AppletContext, AudioClip, and AppletStub. Applet Basics: All applets are subclasses of Applet. Thus, all applets must import java.applet. Applets must also import java.awt. AWT stands for the Abstract Window Toolkit. Since all applets run in a window, it is necessary to include support for that window by importing java.awt package. Applets are not executed by the console-based Java run-time interpreter. Rather, they are executed by either a Web browser or an applet viewer. Execution of an applet does not begin at main( ). Output to your appletâs window is not performed by System.out.println( ). Rather, it is handled with various AWT methods, such as drawString( ), which outputs a string to a specified X,Y location. Input is also handled differently than in an application. Once an applet has been compiled, it is included in an HTML file using theAPPLET tag. The applet will be executed by a Java-enabled web browser when it encounters the APPLET tag within the HTML file. To view and test an applet more conveniently, simply include a comment at the head of your Java source code file that contains the APPLET tag. Here is an example of such a comment: /* <applet code="MyApplet" width=200 height=60> </applet> */ This comment contains an APPLET tag that will run an applet called MyApplet in a window that is 200 pixels wide and 60 pixels high. Since the inclusion of an APPLET command makes testing applets easier, all of the applets shown in this tutorial will contain the appropriate APPLET tag embedded in a comment.
- 2. The Applet Class: Applet extends the AWT class Panel. In turn, Panel extends Container, which extends Component. These classes provide support for Javaâs window-based, graphical interface. Thus, Applet provides all of the necessary support for window-based activities Applet Architecture: An applet is a window-based program. As such, its architecture is different from the so-called normal, console-based programs . First, applets are event driven. it is important to understand in a general way how the event- driven architecture impacts the design of an applet. Here is how the process works. An applet waits until an event occurs. The AWT notifies the applet about an event by calling an event handler that has been provided by the applet. Once this happens, the applet must take appropriate action and then quickly return control to the AWT. An Applet Skeleton: Four methodsâinit( ), start( ), stop( ), and destroy( )âaredefined by Applet. Another, paint( ), is defined by the AWT Component class. All applets must import java.applet. Applets must also import java.awt. These five methods can be assembled into the skeleton shown here: // An Applet skeleton. import java.awt.*; import java.applet.*; /* <applet code="AppletSkel" width=300 height=100> </applet> */ public class AppletSkel extends Applet { // Called first. public void init() { // initialization } /* Called second, after init(). Also called whenever the applet is restarted. */ public void start() { // start or resume execution } // Called when the applet is stopped. public void stop() { // suspends execution
- 3. } /* Called when applet is terminated. This is the last method executed. */ public void destroy() { // perform shutdown activities } // Called when an applet's window must be restored. public void paint(Graphics g) { // redisplay contents of window } } Applet Initialization and Termination: It is important to understand the order in which the various methods shown in theskeleton are called. When an applet begins, the AWT calls the following methods, in this sequence: 1. init( ) 2. start( ) 3. paint( ) When an applet is terminated, the following sequence of method calls takes place: 1. stop( ) 2. destroy( ) init( ):init( ) method is called onceâthe first time an applet is loaded. The init( ) method is the first method to be called. This is where you should initialize variables. start():The start( ) method is called after init( ). It is also called to restart an applet after it has been stopped(i.e start() method is called every time, the applet resumes execution). Paint():The paint( ) method is called each time your appletâs output must be redrawn. This situation can occur for several reasons. For example, the window in which the applet is running may be overwritten by another window and then uncovered. Or the applet window may be minimized and then restored. paint( ) is also called when the applet begins execution. Whatever the cause, whenever the applet must redraw its output, paint( ) is called. The paint( ) method has one parameter of type Graphics. stop( ):The stop() method is called when the applet is stopped(i.e for example ,when the applet is minimized the stop method is called). destroy( ):The destroy( ) method is called when the environment determines that your applet needs to be removed completely from memory(i.e destroy() method is called when the applet is about to terminate).The stop( ) method is always called before destroy( ).
- 4. Simple Applet programe: SimpleApplet.java import java.awt.*; import java.applet.*; /* <applet code="SimpleApplet" width=300 height=100> </applet> */ public class SimpleApplet extends Applet { String msg=""; // Called first. public void init() { msg="Hello"; } /* Called second, after init(). Also called whenever the applet is restarted. */ public void start() { msg=msg+",Welcome to Applet"; } // whenever the applet must redraw its output, paint( ) is called. public void paint(Graphics g) { g.drawString(msg,20,20); } } Output:
- 5. How To Run an Applet Programe: There are two ways in which you can run an applet: â Executing the applet within a Java-compatible Web browser. â Using an applet viewer, such as the standard SDK tool, appletviewer. An applet viewer executes your applet in a window. This is generally the fastestand easiest way to test your applet. Using an applet viewer to run applet(demonstrates you to run SimpleApplet.java): Place the applet tag in comments in java source code. Note:Code attribute value must be equal to name of class which extends Applet class. Compiling: javac SimpleApplet.java Run: AppletViewer SimpleApplet.java Executing the applet within a Java-compatible Web browser(demonstrates you to run SimpleApplet.java): Compiling: javac SimpleApplet.java Create an Html file and embeded Applet tag in html file. Attributes in applet tag: Code(attribute):specify name of applet class to load into browser. Width(attribute):width of an applet. Height(attribute):height of an applet. SimpleApplet.html <html> <body> <applet code="SimpleApplet" width=300 height=100></applet> </body> </html> When you open SimpleApplet.html , SimpleApplet.class applet is loaded into browser. Note: The Browser must be java enabled to load applet programe. Simple Applet Display Methods: As weâve mentioned, applets are displayed in a window and they use the AWT to perform input and output.To output a string to an applet, use drawString( ), which is a member of the Graphics class.Graphics class is defined in java.awt package. void drawString(String message, int x, int y) Here, message is the string to be output and x and y are x-coordinate ,y-coordinate respectively. In a Java window, the upper-left corner is location 0,0.
- 6. To set the background color of an appletâs window, use setBackground( ). To set the foreground color (the color in which text is shown, for example), use setForeground( ). These methods are defined by Component, and they have the following general forms: void setBackground(Color newColor) void setForeground(ColornewColor) Here, newColor specifies the new color. The class Color defines the constants shown here that can be used to specify colors: Color.black Color.magenta Color.blue Color.orange Color.cyan Color.pink Color.darkGray Color.red Color.gray Color.white Color.green Color.yellow Color.lightGray For example, this sets the background color to green and the text color to red: setBackground(Color.green); setForeground(Color.red); Sample.java /* A simple applet that sets the foreground and background colors and outputs a string. */ import java.awt.*; import java.applet.*; /* <applet code="Sample" width=300 height=200> </applet> */ public class Sample extends Applet { String msg; public void init() { setBackground(Color.gray); setForeground(Color.white); msg = "Inside init( ) --"; } // Initialize the string to be displayed. public void start() { msg += " Inside start( ) --"; } // Display msg in applet window. public void paint(Graphics g) { msg += " Inside paint( )."; g.drawString(msg, 10, 30); } }
- 7. Output: Requesting Repainting: Whenever your applet needs to update the information displayed in its window, it simply calls repaint( ).The repaint( ) method is defined by the AWT. It causes the AWT run-time system to execute a call to your appletâs update( ) method, which, in its default implementation, calls paint( ). The simplest version of repaint( ) is shown here: void repaint( ) This version causes the entire window to be repainted. The following version specifies a region that will be repainted: void repaint(int left, int top, int width, int height) Here, the coordinates of the upper-left corner of the region are specified by left and top, and the width and height of the region are passed in width and height. These dimensions are specified in pixels. You save time by specifying a region to repaint. The other two versions of repaint(): void repaint(long maxDelay) void repaint(long maxDelay, int x, int y, int width, int height) Here, maxDelay specifies the maximum number of milliseconds that can elapse before update( ) is called. Using the Status Window: In addition to displaying information in its window, an applet can also output a message to the status window of the browser or applet viewer on which it is running. To do so, call showStatus( ) with the string that you want displayed. // Using the Status Window. import java.awt.*; import java.applet.*; /* <applet code="StatusWindow" width=300 height=300> </applet> */ public class StatusWindow extends Applet { public void init() {
- 8. setBackground(Color.cyan); } // Display msg in applet window. public void paint(Graphics g) { g.drawString("This is in the applet window.", 10, 20); showStatus("This is shown in the status window."); } } Output: The HTMLAPPLET Tag: < APPLET [CODEBASE = codebaseURL] CODE = appletFile [ALT = alternateText] [NAME = appletInstanceName] WIDTH = pixels HEIGHT = pixels [ALIGN = alignment] [VSPACE = pixels] [HSPACE = pixels] > [< PARAM NAME = AttributeName VALUE = AttributeValue>] [< PARAM NAME = AttributeName2 VALUE = AttributeValue>] . . . [HTML Displayed in the absence of Java] </APPLET> Letâs take a look at each part now. CODEBASE: CODEBASE is an optional attribute that specifies the base URL of the applet code, which is the directory that will be searched for the appletâs executable class file (specified by the CODE tag). CODE :CODE is a required attribute that gives the name of the file containing your appletâs compiled .class file. This file is relative to the code base URL of the applet, which is the directory that the HTML file was in or the directory indicated by CODEBASE if set.
- 9. ALT :The ALT tag is an optional attribute used to specify a short text message that should be displayed if the browser understands the APPLET tag but canât currently run Java applets. NAME: NAME is an optional attribute used to specify a name for the applet instance. Applets must be named in order for other applets on the same page to find them by name and communicate with them. WIDTH AND HEIGHT :WIDTH and HEIGHT are required attributes that give the size (in pixels) of the applet display area. ALIGN: ALIGN is an optional attribute that specifies the alignment of the applet.The possible values:LEFT, RIGHT, TOP, BOTTOM, MIDDLE, BASELINE, TEXTTOP, ABSMIDDLE,and ABSBOTTOM. VSPACE AND HSPACE :These attributes are optional. VSPACE specifies the space, in pixels, above and below the applet. HSPACE specifies the space, in pixels, on each side of the applet. PARAM NAME AND VALUE: The PARAM tag allows you to specify applet specific arguments in an HTML page. Applets access their attributes with the getParameter( ) method. Passing Parameters to Applets: The APPLET tag in HTML allows you to pass parameters to your applet. To retrieve a parameter, use the getParameter( ) method. It returns the value of the specified parameter in the form of a String object. Here is an example that demonstrates passing parameters: ParamDemo.java import java.awt.*; import java.applet.*; /* <applet code="ParamDemo" width=300 height=300> <param name=fontName value=Courier> <param name=fontSize value=14> <param name=leading value=2> <param name=accountEnabled value=true> </applet> */ public class ParamDemo extends Applet { String fn=null,fz=null,l=null,ae=null; public void init() { setBackground(Color.gray); setForeground(Color.white); }
- 10. public void start() { fn=getParameter("fontName"); fz=getParameter("fontSize"); l=getParameter("leading"); ae=getParameter("accountEnabled"); repaint(); } public void paint(Graphics g) { g.drawString("FontName:"+fn,10,10); g.drawString("FontSize:"+fz,10,30); g.drawString("Leading:"+l,10,50); g.drawString("AccountEnabled:"+ae,10,70); } } Output: getDocumentBase( ) and getCodeBase( ): Java will allow the applet to load data from the directory holding the HTML file that started the applet (the document base) and the directory from which the appletâs class file was loaded (the code base). These directories are returned as URL objects by getDocumentBase( ) and getCodeBase( ). Bases.java import java.awt.*; import java.applet.*; import java.net.*; /*<applet code="Bases" width=300 height=50></applet>*/ public class Bases extends Applet { // Display code and document bases. public void paint(Graphics g) { String msg; //URL class is defines in java.net package. URL url = getCodeBase(); // get code base
- 11. msg = "Code base: " + url.toString(); g.drawString(msg, 10, 20); url = getDocumentBase(); // get document base msg = "Document base: " + url.toString(); g.drawString(msg, 10, 40); } } Sample output from this program is shown here: AppletContext and showDocument( ): To allow your applet to transfer control to another URL, you must use the showDocument( ) method defined by the AppletContext interface. ACDemo.java import java.awt.*; import java.applet.*; import java.net.*; /*<applet code="ACDemo" width=300 height=50></applet>*/ public class ACDemo extends Applet { URL u; public void start() { AppletContext ac = getAppletContext(); URL url = getCodeBase(); // get url of this applet try { u=new URL(url+"Test.html"); ac.showDocument(u); } catch(MalformedURLException e) { showStatus("URL not found"); } } } Explanation:In this programe,the control is transfered from ACDemo.class (i.e applet) to Test.html file.
- 12. Two Types of Applets: There are two varieties of applets. The first are those based directly on the Applet class. These applets use the Abstract Window Toolkit (AWT) to provide the graphic user interface (or use no GUI at all). This style of applet has been available since Java was first created. The second type of applets are those based on the Swing class JApplet. Swing applets use the Swing classes to provide the GUI. Swing offers a richer and often easier-to-use user interface than does the AWT. Thus, Swing-based applets are now the most popular. JApplet inherits Applet, all the features of Applet are also available in Japplet.
- 13. JAVA EXCEPTIONS GENERAL An exception is a problem that arises during the execution of a program. An exception can occur for many different reasons, including thefollowing: ï· A user hasentered invalid data. ï· A file that needs to be opened cannot be found. ï· A network connection has been lost in the middle of communications or the JVM has run out of memory. Some of these exceptions are caused by user error, others by programmer error, and others by physical resources that have failed in somemanner. To understand how exception handling works in Java,you need to understand the three categories of exceptions: ï· Checked exceptions: A checked exception is an exception that is typically a user error or a problem that cannot be foreseen by the programmer. For example, if a file is to be opened, but the file cannot be found, an exception occurs. These exceptions cannot simply be ignored at the time of compilation. ï· Runtime exceptions: A runtime exception is an exception that occurs that probably could have been avoided by the programmer. As opposed to checked exceptions, runtime exceptions are ignored at the time of compilation. ï· Errors: These are not exceptions at all, but problems that arise beyond the control of the user or the programmer. Errors are typically ignored in your code because you can rarely do anything about an error. For example, if a stack overflow occurs, an error will arise. They are also ignored at the time of compilation. EXCEPTION HIERARCHY All exception classes are subtypes of the java.lang.Exception class.The exception class is a subclass of the Throwable class. Other than the exception class there is another subclass called Error which is derived from the Throwableclass. Errors are not normally trapped form the Javaprograms. These conditions normally happen in case of severe failures, which are not handled by the java programs. Errors are generated to indicate errors generated by the runtime environment. Example :JVM is out of Memory. Normally programs cannot recover from errors. The Exception class has two main subclasses: IOException class and RuntimeException Class.
- 14. EXCEPTIONS METHODS Following is the list of important medthods available in the Throwable class. Methods with Description 1 public String getMessage() Returns a detailed message about the exception that has occurred. This message is initialized in the Throwable constructor. 2 public Throwable getCause() Returns the cause of the exception as represented by a Throwable object 3 public String toString() Returns the name of the class concatenated with the result of getMessage() 4 public voidprintStackTrace() Prints the result of toString() along with the stack trace to System.err, the error output stream. 5 public StackTraceElement []getStackTrace() Returns an array containing each element on the stack trace. The element at index 0 represents the top of the call stack,and the last element in the array represents the method at the bottom of the call stack. 6 public Throwable fillInStackTrace() Fills the stack trace of this Throwable object with the current stack trace, adding to any previous information in the stacktrace. CATCHING EXCEPTIONS A method catches an exception using a combination of the try and catch keywords. A try/catch block is placed around the code that might generate an exception. Code within a try/catch block is referred to as protected code, and the syntax for using try/catchlooks like the following: try { //Protected code }catch(ExceptionName e1) { //Catch block } A catch statement involves declaring the type of exception you are trying to catch. If an exception occurs in protected code, the catch block (or blocks) that follows the try is checked. If the type of exception that
- 15. occurred is listed in a catch block, the exception is passed to the catch block much as an argument is passed into a methodparameter. EXAMPLE The following is an array is declared with 2 elements. Then the code tries to access the 3rd element of the array which throws anexception. // File Name : ExcepTest.java import java.io.*; public class ExcepTest{ public static void main(String args[]){ try{ int a[] = new int[2]; System.out.println("Access element three :" + a[3]); }catch(ArrayIndexOutOfBoundsException e){ System.out.println("Exception thrown :" + e); } System.out.println("Out of the block"); } } This would produce the followingresult: Exception thrown :java.lang.ArrayIndexOutOfBoundsException: 3 Out of the block MULTIPLE CATCH BLOCKS A try block can be followed by multiple catch blocks. The syntax for multiple catch blocks looks like the following: try { //Protected code }catch(ExceptionType1 e1) { //Catch block }catch(ExceptionType2 e2) { //Catch block }catch(ExceptionType3 e3) { //Catch block } The previous statements demonstrate three catch blocks, but you can have any number of them after a single try. If an exception occurs in the protected code, the exception is thrown to the first catch block in the list. If the data type of the exception thrown matches ExceptionType1, it gets caught there. If not, the exception passes down to the second catch statement. This continues until the exception either is caught or falls through
- 16. all catches, in which case the current method stops execution and the exception is thrown down to the previous method on thecall stack. EXAMPLE Here is code segment showing how to use multiple try/catch statements. try { file = new FileInputStream(fileName); x = (byte) file.read(); }catch(IOException i) { i.printStackTrace(); return -1; }catch(FileNotFoundException f) //Not valid! { f.printStackTrace(); return -1; } THE THROWS/THROW KEYWORDS If a method does not handle a checked exception, the method must declare it using the throws keyword. The throws keyword appears at the end of a method's signature. You can throw an exception, either a newly instantiated one or an exception that you just caught, by using the throw keyword. Try to understand the different in throws and throw keywords. The following method declares that it throws a RemoteException: import java.io.*; public class className { public void deposit(double amount) throws RemoteException { // Method implementation throw new RemoteException(); } //Remainder of class definition } A method can declare that it throws more than one exception, in which case the exceptions are declared in a list separated by commas. For example, the following method declares that it throws a RemoteException and an InsufficientFundsException: import java.io.*; public class className { public void withdraw(double amount) throws RemoteException, InsufficientFundsException {
- 17. // Method implementation } //Remainder of class definition } THE FINALLY KEYWORD The finally keyword is used to create a block of code that follows a try block. A finally block of code always executes, whether or not an exception has occurred. Using a finally block allows you to run any cleanupâtype statements that you want to execute, no matter what happens in the protectedcode. A finally block appears at the end of the catch blocks and has the following syntax: try { //Protected code }catch(ExceptionType1 e1) { //Catch block }catch(ExceptionType2 e2) { //Catch block }catch(ExceptionType3 e3) { //Catch block }finally { //The finally block always executes. } EXAMPLE public class ExcepTest{ public static void main(String args[]){ int a[] = new int[2]; try{ System.out.println("Access element three :" + a[3]); }catch(ArrayIndexOutOfBoundsException e){ System.out.println("Exception thrown :" + e); } finally{ a[0] = 6; System.out.println("First element value: " +a[0]); System.out.println("The finally statement is executed"); } } }
- 18. This would produce the followingresult: Exception thrown :java.lang.ArrayIndexOutOfBoundsException: 3 First element value: 6 The finally statement is executed NOTE ï· A catch clause cannot exist without a try statement. ï· It is not compulsory to have finally clauses when ever a try/catchblock is present. ï· The try block cannot be present without either catch clause or finally clause. ï· Any code cannot be present in between the try,catch, finally blocks. DECLARING YOU OWN EXCEPTION You can create your own exceptions in Java.Keep the following points in mind when writing your own exception classes: ï· All exceptions must be a child of Throwable. ï· If you want to write a checked exception that is automatically enforced by the Handle or Declare Rule, you need to extend the Exception class. ï· If you want to write a runtime exception, you need to extend the RuntimeException class. We can define our own Exception class as below: class MyException extends Exception{ } You just need to extend the Exception class to create your own Exception class. These are considered to be checked exceptions. The following InsufficientFundsException class is a userâdefined exception that extends the Exception class, making it a checked exception. An exception class is like any other class, containing useful fields and methods. EXAMPLE // File Name InsufficientFundsException.java import java.io.*; public class InsufficientFundsException extends Exception { private double amount; public InsufficientFundsException(double amount) { this.amount = amount; } public double getAmount()
- 19. { return amount; } } To demonstrate using our userâdefined exception, the following CheckingAccount class contains a withdraw() method that throws anInsufficientFundsException. // File Name CheckingAccount.java import java.io.*; public class CheckingAccount { private double balance; private int number; public CheckingAccount(int number) { this.number = number; } public void deposit(double amount) { balance += amount; } public void withdraw(double amount) throws InsufficientFundsException { if(amount <= balance) { balance -= amount; } else { double needs = amount - balance; throw new InsufficientFundsException(needs); } } public double getBalance() { return balance; } public int getNumber() { return number; } } The following BankDemo program demonstrates invoking the deposit() and withdraw() methods of CheckingAccount. // File Name BankDemo.java public class BankDemo { public static void main(String [] args) {
- 20. CheckingAccount c = new CheckingAccount(101); System.out.println("Depositing $500..."); c.deposit(500.00); try { System.out.println("nWithdrawing $100..."); c.withdraw(100.00); System.out.println("nWithdrawing $600..."); c.withdraw(600.00); }catch(InsufficientFundsException e) { System.out.println("Sorry, but you are short $" + e.getAmount()); e.printStackTrace(); } } } Compile all the above three files and run BankDemo, this would produce the following result: Depositing $500... Withdrawing $100... Withdrawing $600... Sorry, but you are short $200.0 InsufficientFundsException at CheckingAccount.withdraw(CheckingAccount.java:25) at BankDemo.main(BankDemo.java:13) COMMON EXCEPTIONS In Java,it is possible to define two catergories of Exceptions and Errors. ï· JVM Exceptions: â These are exceptions/errors that are exclusively or logically thrown by the JVM. Examples :NullPointerException, ArrayIndexOutOfBoundsException,ClassCastException, ï· Programmatic exceptions: â These exceptions are thrown explicitly by the application or the API programmers Examples: IllegalArgumentException, IllegalStateException.
- 21. byte a = 68; char a = 'A'; byte, int, long, and short can be expressed in decimal (base 10), hexadecimal (base 16) or octal (base 8) number systems as well. Prefix 0 is used to indicate octal and prefix 0x indicates hexadecimal when using these number systems for literals. FOR EXAMPLE: int decimal = 100; int octal = 0144; int hexa = 0x64; String literals in Javaare specified like they are in most other languages by enclosing a sequence of characters between a pair of doublequotes.
- 23. What is String? âą String is non-primitive data type and it is also class which is under java.lang.package. âą String is a collection of characters. âą String is immutable âą Without new keyword we can create object . âą It introduced in JDK1.1.
- 24. What is String literals? âą String s=âJavaâ; âą SCP is part of Heap memory. âJavaâ string literal which is stored in String Constant Pool (SCP) SCP
- 25. Why String is immutable? âą Strings are constant, values canât be changed after they are created. âą Because java uses the concept of string literal. âą Suppose, if one reference variable changes the value of the object, it will be affected to all the reference variables. That is why string objects are immutable in java. âą So, String is immutable.
- 26. Example 1: concat() method appends the string at the end so strings are immutable objects.
- 27. Java Programming Java String Constant Pool Heap Memory
- 28. Example 2: So, it assign it to the reference variable it prints Java Programming
- 29. Methods âą charAt(), contains(), equals(), equalsIgnoreCase(), toUpperCase, toLowerCase âą length(), compareTo, join(), isEmpty(), length(), replaceAll(), replaceFirst() âą trim(), index(), lastIndexOf(), toString(),concat(), replace(), equals() âą replace(), hashCode(), compareToIgnorCase() âą split() âą substring()
- 30. Example 3: subString() substring(int start/begin index) substring(int startindex, int endindex) Overloading Returns a new string that is a substring of this string.
- 31. Example 4: equals() Return type is boolean The String equals() method overrides the equals() method of Object class.
- 32. Example 5: equalsIgnoreCase() Return type is boolean
- 34. Example 7: replace() Return type is char, charSequence Replace methods introduced in JDK1.5
- 35. Example 8: split() It returns array of strings computed by splitting this string around matches of the given regular expression. It introduced in JDK 1.4.
- 36. Example 9: split() It returns array of strings computed by splitting this string around matches of the given regular expression. It introduced in JDK 1.4. Overloading
- 37. Example 10: concat() âą It returns a string that represents the concatenation of this object's characters followed by the string argument's characters.
- 38. Example 11: contains() Returns a boolean
- 39. Example 12: toString() Returns a hash code
- 40. Example 12: Using toString method to display a value
- 41. Example 13: toUpperCase(), toLowerCase(), chatAt()
- 42. Example 14: length(), join(), isEmpty()
- 43. Example 15: replaceAll(), replaceFirst()
- 48. Methods Name Introduced Version Description Return Type Parameter charAt() 1.0 The char value at the specified index. An index ranges from 0 to length() â 1(length in interface CharSequence) The char value at the specified index of this string. The first char value is at index 0. index - the index of the char value. IsEmpty() 1.6 If it is true, and only if, length() is 0. True if length() is 0, otherwise false No concat() 1.0 Concatenates the specified string to the end of this string.If the length of the argument string is 0, then this String object is returned A string that represents the concatenation of this object's characters followed by the string argument's characters. The String that is concatenated to the end of this String. compareTo() 1.0 The comparison is based on the Unicode value of each character in the strings. The value 0 if the argument string is equal to this string The String to be compared.
- 49. Methods Name Introduced Version Description Return Type Parameter contains() 1.5 True if and only if this string contains the specified sequence of char values. True if this string contains s, false otherwise The sequence to search for. trim() 1.0 a copy of the string, with leading and trailing whitespace omitted. Acopy of this string with leading and trailing white space removed, or this string if it has no leading or trailing white space. No equals IgnoreCas e() 1.0 Compares this String to another String, ignoring case considerations true if the argument is not null and it represents an equivalent String ignoring case; false otherwise. The String to compare this String against length() 1.0 The length of this string. The length is equal to the number of Unicode code units in the string. (charAt in interface CharSequence) the length of the sequence of characters represented by this object. No
- 50. Methods Name Introduced Version Description Return Type Parameter Exception replaceAll() 1.4 Replaces each substring of this string that matches the given regular expression with the given replacement. The resulting String. the sequence to search for PatternSyntaxE xception replaceFirst() 1.4 Replaces the first substring of this string that matches the given regular expression with the given replacement. The resulting String. the regular expression to which this string is to be matchedreplacement - the string to be substituted for the first match IndexOutOfBo undsException - compareTo 1.2 IgnoreCase() Compares two strings lexicographically, ignoring case differences. The specified String is greater than, equal to, or less than this String, ignoring case considerations. The String to be compared. No
- 51. Methods Name Introduced Version Description Return Type Parameter Exception toUpperCase() 1.0 Converts all of the characters in this String to upper case using the rules of the default locale. the String, converted to uppercase. No No toLoweCase() 1.0 Converts all of the characters in this String to lower case using the rules of the default locale. the String, converted to lowercase. No No join() 1.8 A string joined with given delimiter. In string join method, delimiter is copied for each elements. Joined string with delimiter. delimiter : char value to be added with each element elements : char value to be attached with delimiter NullPointerExce ption
- 52. Methods Name Introduced Version Description(Overrides a class Object) Return Type Parameter equals() 1.0 Compares this string to the specified object. The result is true if and only if the argument is not null and is a String object that represents the same sequence of characters as this object. True if the given object represents a String equivalent to this string, false otherwise The object to compare this String against hashCode() 1.0 A hash code for this string. A hash code value for No this object. toString() 1.0 The toString method for class Object returns a string consisting of the name of the class of which the object is an instance. This object (which is already a string!) is itself returned. Astring representation No of the object.
- 53. Methods Name Introduce d Version Description(Overloading) Return Type Parameter Exception substring() 1.0 Returns a new string that is a substring of this string. The substring begins with the character at the specified index and extends to the end of this string. the specified substring. beginIndex - the beginning index, inclusive. IndexOutOfBoun dsException - substring() 1.0 Returns a new string that is a substring of this string. The substring begins at the specified beginIndex and extends to the character at index endIndex - 1. Thus the length of the substring is endIndex-beginIndex. the specified substring. beginIndex - the beginning index, inclusive. endIndex - the ending index, exclusive. IndexOutOfBoun dsException -
- 54. Methods Name Introduce d Version Description(Overloading) Return Type Parameter Exception replace() 1.0 Returns a new string resulting from replacing all occurrences of oldChar in this string with newChar. A string derived from this string by replacing every occurrence of oldChar with ne wChar. oldChar - the old No character.newChar - the new character. replace() 1.5 Replaces each substring of this string that matches the literal target sequence with the specified literal replacement sequence. The resulting String. NullPointerExcept target - The sequence of char ion values to be replacedreplaceme nt - The replacement sequence of char values
- 55. Methods Name Introduced Version Description(Overloading) Return Type Parameter Exception split() 1.4 Splits this string around matches of the given regular expression The array of strings computed by splitting this string around matches of the given regular expression The delimiting regular expression. PatternSyntaxExce ption split() 1.4 Splits this string around matches of the given regular expression. the array of strings computed by splitting this string around matches of the given regular expression. regex - the delimiting regular expressionlimit - the result threshold, as described above PatternSyntaxExce ption -
- 56. Methods Name Introduced Version Description(Overloading) Return Type Parameter Exception index() 1.0 The index within this string of the first occurrence of the specified character. the index of the first occurrence of the character in the character sequence represented by this object a character (Unicode code point). No index() 1.0 â â fromIndex - the index to start the search from. No index() 1.0 â The index of the first occurrence of the specified substring the substring to search for. No index() 1.0 â â fromIndex - the index from which to start the search. No
- 57. Methods Name Introduced Version Description(Overloading) Return Type Parameter Exception lastIndex() 1.0 The index within this string of the last occurrence of the specified character. the index of the last occurrence of the character in the character sequence represented by this object a character (Unicode code point). No lastIndex() 1.0 â â fromIndex - the index to start the search from. No lastIndex() 1.0 â The index of the first occurrence of the specified substring the substring to search for. No lastIndex() 1.0 â â fromIndex - the index from which to start the search. No
- 58. What is String Buffer? âą StringBuffer is mutable String. âą Java StringBuffer class is (synchronized )thread-safe i.e. multiple threads cannot access it simultaneously. âą So it is safe and will result in an order.
- 59. Why String Buffer is mutable? append() it return many data types Overloading
- 60. Methods âą append(), replace(), setCharAt() âą insert() âą delete() âą reverse() âą length() âą charAt() âą deleteChatAt() âą setLength()
- 61. Methods Name Introduced Version Description Return Type Parameter Exception delete() 1.2 Removes the characters in a substring of this sequence. This object. start - The beginning index, inclusive.end - The ending index, exclusive. StringIndexOutOf BoundsException deleteCharAt() 1.2 Removes the char at the specified position in this sequence. This sequence is shortened by one char. â Index of char to remove â replace() 1.2 Replaces the characters in a substring of this sequence with characters in the specified String. â start - The beginning index, inclusive.end - The ending index, exclusive.str - String that will replace previous contents. â
- 62. Methods Name Introduced Version Description Return Type Parameter Exception reverse() 1.0.2 Causes this character sequence to be replaced by the reverse of the sequence A reference to this object. No No setLength() 1.0 Sets the length of the character sequence. The sequence is changed to a new character sequence whose length is specified by the argument No The new length IndexOutOfBoun dsException charAt() 1.0 Returns the char value in this sequence at the specified index. the char value at the specified index index - the index of the desired char value. IndexOutOfBoun dsException setCharAt() 1.0 The character at the specified No index is set. index - the index of the character to modify.ch - the new character. IndexOutOfBoun dsException
- 63. Methods Name Introduced Version Description(Overloading) Return Type Parameter Exception append() 1.0 Appends the specified string to this character sequence. A reference to this object. A string. No insert() 1.0 Inserts the string into this character sequence A reference to this object. offset - the offset.str - a string. StringIndexOutOf BoundsException Append() and insert() methods has many types in StringBuffer.
- 64. Example 1: delete(), deleteCharAt()
- 65. Example 2: reverse(), replace()
- 69. What is String Builder? âą StringBuilder is mutable String. âą The Java StringBuilder class is same as StringBuffer class except that it is non-synchronized (not thread-safe). âą It is available since JDK 1.5.
- 70. Methods âą append(), replace(), subsequence(), substring(), charAt() , trimToSize() âą insert() delete(), capacity(), ensureCapacity(), reverse(), length() âą StringBuffer methods is similar to StringBuilder
- 71. Methods Name Introduced Version Description(Overloading) Return Type Parameter Exception subsequence 1.5 () the new character sequence from given start index to exclusive end index value of this sequence. The subSequence(int start, int end) method returns the specified subsequence. The start and end index of a subsequence. IndexOutOfBou ndsException ) trimToSize( 1.5 To reduce the storage used for the character sequence. No No No reverse() 1.5 Causes this character sequence to be replaced by the reverse of the sequence A reference to this object. No No
- 72. Example 1:reverse() Overrides in a String Buffer
- 75. Difference String String Buffer It is immutable It is mutable. String is slow and consumes more memory String is fast and consumes less memory When you concat too many strings because every time it creates new instance. When you cancat strings. 1.0 1.0 Java.lang.package Java.lang.pakage
- 76. Difference StringBuffer String Builder It is immutable It is immutable. It is synchronized i.e. thread safe. It is non-synchronized i.e. not thread safe. It means two threads can't call the methods of StringBuffer simultaneously. It means two threads can call the methods of StringBuilder simultaneously. 1.0 1.5 It is less efficient. It is more efficient.