Application
Programming
Hend Alkittawi

Exception Handling
Introduction To Java Errors And
Exceptions

INTRODUCTION
-

In Java when things go wrong a java.lang.Exception object is
created.

-

For example,
-

if we add elements to an uninitialized arraylist
-

-

if we try to read from a file that doesn’t exist
-

-

FileNotFoundException

if we try to read past the end of the file
-

-

NullPointerException

IOException

if the file changes while we are reading it
-

IOException

THE CALL STACK
-

When a Java program runs, execution begins in the main()
method. The main() method creates objects and invokes methods
on them.

-

When execution moves to another method an entry is added to
the call stack.

-

When a method finishes executing, the entry

call stack

is removed from the call stack, and execution
returns to the next line in the main() method
-

obj.method()

this continues until the main method finishes
main()

THE CALL STACK
-

The call stack entry below, among other things, contains
-

the current method

-

where the call occurred in that method

NullPointerException:
at Student.getAverage(Student.java:79)
at Student.toString(Student.java:62)
at java.lang.String.valueOf(String.java:2615)
at java.io.PrintStream.print(PrintStream.java:616)
at java.io.PrintStream.println(PrintStream.java:753)
at Student.main(Student.java:120)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)

EXCEPTIONS IN JAVA
-

In Java, all exception classes inherit from the Exception class

-

Exceptions in Java are checked or unchecked!

-

Checked exceptions must be caught or thrown. Examples of
checked exceptions include:

-

-

IOException

-

FileNotFoundException

Unchecked exceptions should never be caught or thrown. Examples
of unchecked exceptions include:
-

NullPointerException

-

ArrayIndexOutOfBoundsException

EXCEPTIONS IN JAVA

EXCEPTION HANDLING
-

As developers, we must address any problems that might occur.

-

For unchecked exceptions, your code should follow best
practices in order to prevent exceptions occurrences. For
example

-

-

check for array bounds

-

check for null values

For checked exceptions, your code either throws the exception
or handles the exception with a try/catch block.

EXCEPTION HANDLING
-

Using try, catch, and finally blocks
-

Wrap all code that can cause a checked exception in try, catch
(and optionally finally) blocks

try {

try {

System.out.println("code here can cause
an exception");
} catch (Exception e) {

System.out.println("reading from a file …");
} catch (FileNotFoundException e) {

System.out.println("handle exception here");
System.out.println("handle exception here");
} finally { // optional
} finally { // optional
System.out.println("code that must be absolutely
executed after try block completes");
}

System.out.println("closing the file …");
}

EXCEPTION HANDLING
-

A try block can have multiple catch blocks.

-

The order of the catch blocks is important.

try {

try {
System.out.println("reading from a file …");

} catch (FileNotFoundException e) {

System.out.println("reading from a file …");
} catch (IOException e) {

System.out.println("code here will execute
when a FileNotFoundException is thrown!");
} catch (IOException e) {

System.out.println("code here will execute
when a FileNotFoundException is thrown!");
} catch (FileNotFoundException e) {

System.out.println("handle exception here");

System.out.println("code here will not execute
when a FileNotFoundException is thrown!");

} finally {
} finally {
System.out.println("closing the file …");
}

System.out.println("closing the file …");
}

THROWING EXCEPTIONS
-

An exception might be thrown, when there is nothing more you
can do about it!
public void methodA() {
try {
dangerZone();
} catch (Exception e) {
e.printStackTrace();
}
}
public void dangerZone() throws Exception {
throw new Exception();
}
}

EXCEPTION HANDLING EXAMPLE
import java.io.FileNotFoundException;
public class DemoExceptions {
public static void main(String[] args) {
try {
method(true);
System.out.println("returned from method()");
} catch (FileNotFoundException e) {
System.out.println("caught the exception, will handle it!");
e.printStackTrace();
} finally {
// code that must be absolutely executed after try block completes
System.out.println("finally will cleanup!");
}
}
public static void method(boolean exception) throws FileNotFoundException {
if(exception)
throw new FileNotFoundException();
System.out.println("method 1 executed successfully!");
}
}

HANDLING EXCEPTIONS
-

Handling exceptions improves the user experience!

-

Consider

-

-

Where can errors happen caused by our logic?

-

Where can exceptions happen?

-

Where can user error occur?

For each, how can we prevent or reduce these?
-

What would the user expect?

USER EXPERIENCE!
-

Suppose you are

-

Suppose you are

-

Suppose you are

exploring with Google

searching Google (web

shopping on Amazon

earth (desktop app),

app), you enter some

(mobile app), you tap

you click a button and

text, click the button

a button and

it

and

-

-

Closes and/or

refreshes, losing

program/window

your search text

Changes the size of

-

The result page

The app closes (and
maybe reopens)

The web page

reopens the

-

The entire style of
the app changes

-

GUI components move

the window

comes up, without

around the view

Moves GUI

results

(unexpectedly)

components around
the view

-

-

-

Does nothing!

-

Nothing happens!

-

Nothing happens!

CODE DEMO
- Demo exception handling concepts
in Eclipse!

DO YOU HAVE ANY
QUESTIONS?

THANK
YOU!

@

hend.alkittawi@utsa.edu

By Appointment
Online