Exceptions and Assertions in Java: Handling Errors and Ensuring Code Correctness, Lecture notes of Java Programming

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LECTURE 08: EXCEPTIONS &

ASSERTIONS

o Exception

o Handle exception

o Checked vs Unchecked

o Assert

PLAN

Not Catching Exceptions

Scanner scanner = new Scanner(System.in);

System.out.println(“Enter integer:");

int number = scanner.nextInt();

Exception in thread “main” java.lang.InputMismatchException at java.util.Scanner.throwFor(Scanner.java:819) at java.util.Scanner.next(Scanner.java:1431) at java.util.Scanner.nextInt(Scanner.java:2040) at java.util.Scanner.nextInt(Scanner.java:2000) at Ch8Sample1.main(Ch8Sample1.java:35) Error message for invalid input

Catching an Exception

System.out.print(prompt);

try {

age = scanner.nextInt( );

} catch (InputMismatchException e){

System.out.println("Invalid Entry. "

+ "Please enter digits only");

try catch

o There are two methods we can call to get information about the

thrown exception:

• getMessage
• printStackTrace

Getting Information

try {

} catch (InputMismatchException e){

scanner.next(); //remove the leftover garbage

char

System.out.println(e.getMessage());

e.printStackTrace();

o A single try-catch

statement can

include multiple

catch blocks, one

for each type of

exception.

Multiple catch Blocks

try {

age = scanner.nextInt( );

val = cal.get(id); //cal is a GregorianCalendar

} catch (InputMismatchException e){

} catch (ArrayIndexOutOfBoundsException e){

o There are situations where we need to take certain actions

regardless of whether an exception is thrown or not.

o We place statements that must be executed regardless of

exceptions in the finally block.

The finally Block

try-catch-finally Control Flow

No Exception try { <t-stmt-1>

... <t-stmt-i> ... <t-stmt-n> } <catch-block-1> ... <catch-block-i> ... <catch-block-m> } finally { ... } Assume <t-stmt-i> throws an exception and <catch-block-i> is the matching block. Exception try { <t-stmt-1> ... <t-stmt-i> ... <t-stmt-n> } <catch-block-1> ... <catch-block-i> ... <catch-block-m> } finally { ... } finally block is executed. finally block is executed.

o We can write a method that throws an exception directly, i.e., this

method is the origin of the exception.

o Use the throw reserved to create a new instance of the Exception

or its subclasses.

o The method header includes the reserved word throws.

Throwing Exceptions

public void doWork(int num) throws Exception {

if (num != val) throw new Exception("Invalid val");

o When a method may throw an exception, either directly or

indirectly, we call the method an exception thrower.

o Every exception thrower must be one of two types:

• catcher.
• propagator.

Exception Thrower

Sample Call Sequence

try { B(); } catch (Exception e){

... }

Method A

try { C(); } catch (Exception e){

... }

Method B

try { D(); } catch (Exception e){

...

Method C

if (cond) { throw new Exception();

Method D

catcher propagator^ propagator

Stack Trace

A

B

C

D

o All types of thrown errors are instances of the Throwable class or

its subclasses.

o Serious errors are represented by instances of the Error class or

its subclasses.

o Exceptional cases that common applications should handle are

represented by instances of the Exception class or its subclasses.

Exception Types

o There are two types of exceptions:

• Checked.
• Unchecked.

o A checked exception is an exception that is checked at compile

time.

o All other exceptions are unchecked, or runtime, exceptions. As the

name suggests, they are detected only at runtime.

Checked vs. Runtime

o When calling a method that can throw checked exceptions

• use the try-catch statement and place the call in the try block,

or

• modify the method header to include the appropriate throws

clause.

o When calling a method that can throw runtime exceptions, it is

optional to use the try-catch statement or modify the method

header to include a throws clause.

Different Handling Rules