CS1101 Group1

Discussion 9

Lek Hsiang Hui

lekhsian @ comp.nus.edu.sg

http://www.comp.nus.edu.sg/~lekhsian/cs1101

Lab9 Taxi

• When implementing Type c, take note that a “greedy” algorithm will not work

i.e.Typical WRONG solution:counterA = passengers/4;passengerA = counterA * 4;passengerB = passengers – passengerA;lowest = checkPriceA( distance, passengerA);lowest = checkPriceB( distance, passengerB);

Lab9 Taxi

• Approach to solving type C:– Try all combinations– e.g. 12 people, try

0 standard, 2 Cab++1 standard, 2 Cab++2 standard, 1 Cab++3 standard, 0 Cab++etc

Exceptions

• What are exceptions?

• Something used for signaling erroneous situation

• E.g. your method should return a valuebut the parameters are wrong, you can throw an exception. (i.e. no need to return anything!)

Exceptions

• Different kinds of “Exceptions”

• Creating your own user-defined Exceptions

• throw new Exception

• throws Exception

• try-catch-finally

Different kinds of “Exceptions”

An Exception is really an Object (subclass of Object)

RuntimeException

• What are these?

• http://java.sun.com/javase/6/docs/api/java/lang/RuntimeException.html

• “A method is not required to declare in its throws clause any subclasses of RuntimeException that might be thrown during the execution of the method but not caught.”

Creating your own user-defined Exceptions

• Refer to http://www.comp.nus.edu.sg/~lekhsian/cs1101/d9

throw new Exception

• Syntax:throw new ExceptionClassName(…);

throw a new object

Handling Exceptions

• throws Exception

• try-catch-finally

Handling Exceptions : throws

• throws Exception

• If you do not want to explicitly handle the exception in the method, then put throws ExceptionName at the method

Exceptions

• Throwing an exceptionthrow new Exception(“some_exception_message”);

• Note that codes you will still have to handle codes in the catch block if there’s any code that throw an exceptioni.etry{ throw new Exception(“”);}catch(Exception e){ throw new Exception(“ “);}

Exceptions : try-catch-finally• Understand try – catch – finally

try {age = Integer.parseInt(inputStr);if (age < 0) { throw new Exception ("Negative age is invalid");}return age;

} catch (NumberFormatException e) {…

} catch (Exception e) {…

} finally{ …}

More SpecializedMore Specialized

More genera

l

More genera

l

Exceptions• Understand try – catch – finally

try {age = Integer.parseInt(inputStr);if (age < 0) { throw new Exception ("Negative age is invalid");}return age;

} catch (NumberFormatException e) { JOptionPane.showMessageDialog (null, "‘" + inputStr + "’ is invalid\nPlease enter digits only");} catch (Exception e) {

JOptionPane.showMessageDialog (null, "Error: " + e.getMessage());

} finally{ …}

More SpecializedMore Specialized

•finally:•Statements that would be executed no matter whether there’s an exception•So is there any difference between placing “ending codes” in the finallyblock and placing it after finally?

Exceptionspublic static void main(String args[]){

try{return;

}catch(Exception e){

System.out.println("in exception");}finally{

System.out.println("in finally");}

System.out.println("outside");}

• in finally is still being printed even though there’s a return statement in the try block

Exceptionspublic static void main(String args[]){

try{System.exit(1);return;

}catch(Exception e){

System.out.println("in exception");}finally{

System.out.println("in finally");}

System.out.println("outside");}

• However it is not always the case that for *all cases*, finally will be executed

Access rights

Member Restriction

this Subclass Package General

public

protected

default

private

Polymorphism

• What is polymorphism?

• Ability of a type/variable/object to take on different forms, so when you call a method, it will call the method specific to the real type

• Example: http://www.comp.nus.edu.sg/~lekhsian/cs1101/d9

Polymorphism

• abstract class• A class with at 1 abstract methodOr• A class which you don’t want anyone to

instantiatei.e.class abstract Animal{}Not allow to do:Animal a = new Animal();

Polymorphism

• abstract method• A method whose method body is not

implemented• Why is this useful?E.g.abstract class Shape{ public abstract int getArea();}//we don’t know what this shape will be//so can’t implement getArea

MyString

• By now you should have implemented the following methods:

– Constructors– append– charAt– ensureCapacity– insert– reverse– toString

MyString

• Try implementing :– delete– deleteCharAt– Equals– setCharAt