Odds and Ends. CS 21a 09/18/05 L14: Odds & Ends Slide 2 Copyright © 2005, by the authors of these slides, and Ateneo de Manila University. All rights

Odds and Ends

L14: Odds & EndsSlide 2

Copyright © 2005, by the authors of these slides, and Ateneo de Manila University. All rights reserved

CS 21a09/18/05

Odds and Ends

Some small topics on the side … Basic Exception Handling Class variables (static methods and fields) Command-line arguments Input through JOptionPane

Exceptions



CS 21a09/18/05

Basic Exception Handling Exception: something unexpected that

can occur in the execution of a program wrong number format NullPointerException ArrayIndexOutOfBoundsException divide by zero attempt to open a file that does not exist etc.

Java provides a way to handle exceptions that are thrown: the try-catch statement



CS 21a09/18/05

Try-catch Statement

Syntax:try { … } catch ( Exception e ) { ... }

Example:try { System.out.println( 5 / x );} catch ( Exception e ) { System.out.println( “div by zero” );}

Note the formatti

ng conventi

on.



CS 21a09/18/05

Breaking out of the try Block

try

{

statement1;

statement2; // if exception occurs here,

// statement3 will be skipped

statement3;

}

catch ( Exception e )

{

statement4; // executed after exception occurs

}



CS 21a09/18/05

Try-catch Chaintry { … file operation …}catch( FileNotFoundException se ) { … if file is not found …}catch( EOFException ee ){ … if no more data to read …}catch( IOException e ){ … for all other cases not yet

covered …}…

you can catch “Exception” to catch any kind of Exception works because of

polymorphism

OR … You can use a “try-catch chain” to catch specific exceptions



CS 21a09/18/05

Ignoring exceptions Some exceptions do not need to be caught but

will generate a runtime error if they occur Examples: NullPointerException,

ArrayIndexOutOfBoundsException If enclosed in a try-catch statement, users can

choose to handle them Other exceptions need to be caught

Examples: when dealing with Input/Output You may either: enclose the statements in a try-

catch statement and handle the situation or place the clause throws Exception at the end of the method header (recall console.readLine() example)



CS 21a09/18/05

More on Exceptions (CS 21b)

Stuff we’ll discuss in more detail in CS 21b Different types of Exceptions Generating your own exceptions Other features

Don’t worry about these for now!

Class (static) variables and methods



CS 21a09/18/05

Class (static) fields and methods

Things we’ve seen, but not fully explained Built-in Constants

Math.PI, FlowLayout.CENTER, Color.green, etc. Built-in functions

Math.sqrt(), Math.abs(), Integer.parseInt(), Double.isNaN()

Static methods public static void main( String[] args )

Static fields your own constants

public static final int MY_CONSTANT

These are all “static” fields or methods



CS 21a09/18/05

Static Fields

means that the field is shared by all instances of the same class

aka “class variable” as opposed to “instance variable”

e.g., in BankAccount, balance is an “instance

variable” – each instance has its own independent copy

However, if all BankAccounts share a minimum balance value, we can make a static field for that



CS 21a09/18/05

Example: Minimum Balance

Account

SV129

Account

SV506

Account

SV008

balance balance balance908.55 1304.98 354.00

Account

minBalance

100.00

There is one copy of minBalance for the whole class and shared by all instances.

There is one copy of minBalance for the whole class and shared by all instances.

The Accountclass

instances of the Account class



CS 21a09/18/05

Static Methods

Normally, a method applies to a particular instance e.g., you can’t just call deposit() from an applet.

(Which account are you deposting to?) you have to call aliceAccount.deposit(),

where aliceAccount is a pre-existing BankAccount a static method is a method that does not refer to a

particular instance That’s why we call them using ClassName.methodName()

there’s no instance that “owns” it. It “belongs” to the class in BlueJ, we right-click on the class, not on the instances

Useful for “functions” that don’t depend on an instance e.g., Math.sqrt( double d )

Note: they cannot refer to instance variables can only use static fields and methods that’s why convenience methods used by main have to be

static



CS 21a09/18/05

public class BankAccount{ private static int curNum = 0; private int balance;

public BankAccount( int initBal ) { balance = initBal; BankAccount.curnum++; } public void deposit( int amount ) { balance += amount; } public void withdraw( int amount ) { balance -= amount; } public int getBalance() { return balance; }}

Another example In this code, we use

the static field curNum to keep track of the number of BankAccounts created.

Whenever we create a bank account, the value of curNum increases by 1.

CurNum is accessible by all instances of BankAccount

Command-Line Arguments



CS 21a09/18/05

Using command-line arguments

Arguments entered from the command line e.g. java InputSampler abc 123

“abc” and “123” are command-line arguments

public static void main( String[] args ) String[] args stores these arguments args.length = 0 if no arguments are entered args.length = 2 if you run the example above

Usage String x = args[0]; //if args.length > 0



CS 21a09/18/05

Using command-line arguments

Very easy and straightforward to use. No need to include classes.

Only takes in Strings. You will need to convert to ints or doubles, if needed.

Works only if you put in command-line arguments. Otherwise, you get an ArrayIndexOutOfBounds exception if you try to access an index that does not exist. Hence, the need to check for the length of the array.

JOptionPane



CS 21a09/18/05

Using JOptionPane

import javax.swing.*; String showInputDialog( String prompt

) Usage:

String x = JOptionPane.showInputDialog( “Enter string: ” );



CS 21a09/18/05

Using JOptionPane

Provides a nice GUI window for user to provide input, when in a Java application

Need to import javax.swing to use it Only takes in Strings. You will need to

convert to ints or doubles, if needed.

Need to call System.exit( 0 ); at the last line of main() so that the program exits

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Odds and Ends. CS 21a 09/18/05 L14: Odds & Ends Slide 2 Copyright © 2005, by the authors of these slides, and Ateneo de Manila University. All rights