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COS240 O-O Languages
AUBG, COS dept
Lecture 12
Title:
Exception Handling in Java
Reference: COS240 Syllabus
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
1
Lecture Contents:
Part
1
Brief introduction to EH
Part
2
Comprehensive introduction to EH
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
2
Part 1
Brief Intro to Exception Handling
Exceptions
An exception is an event that disrupts
normal program flow.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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Three approaches to error checking
1. Ignore all but the most important errors.
 The code is cleaner, but the program will misbehave
when it encounters an unusual error.
2. Do something appropriate for every error.
 The code is cluttered, but the program works better.
 You might still forget some error conditions.
3. Do the normal processing in one place, handle the
errors in another (this is the Java and C# way).
 The code is at least reasonably uncluttered.
4
Java and C# approach:
When an error occurs within a method (e.g. divide by zero;
array index out of bounds; etc.), the method creates an
object and hands it off to the runtime system.
The object, called an exception object, contains
information about the error, including its type and the
state of the program when the error occurred.
Creating an exception object and handing it to the runtime
system is called throwing an exception.
5
After a method throws an exception, the runtime system
attempts to find some code to handle it.
The set of possible code to handle the exception is the
ordered list of methods that had been called to get to the
method where the error occurred.
The list of methods is known as the call stack
The call stack.
6
The runtime system searches the call stack for a method
that contains a block of code that can handle the
exception.
This block of code is called an exception handler.
The search begins with the method in which the error
occurred and proceeds through the call stack in the
reverse order in which the methods were called.
When an appropriate handler is found, the runtime
system passes the exception to the handler.
An exception handler is considered appropriate if the type
of the exception object thrown matches the type that can
be handled by the handler.
7
The exception handler chosen is said to catch the
exception.
If the runtime system exhaustively searches all the
methods on the call stack without finding an appropriate
exception handler, the runtime system (and, consequently,
the program) terminates.
8
The try statement
• Java provides a control structure, the try statement (also
called the try-catch statement) to separate “normal”
code from error handling:
try {
do the “normal” code, ignoring possible
exceptions
}
catch (some exception) {
handle the exception
}
catch (some other exception) {
handle the exception
}
9
Example
try
{
result = numerator / denominator;
validResult = true;
}
catch (ArithmeticException e)
{
validResult = false;
}
10
try {
do the “normal” code, ignoring possible
exceptions
}
catch (some exception) {
handle the exception
}
finally {
...
}
11
Catching an Exception
try {
// normal code here!
}
catch(Exception e) {
System.out.println(e.getMessage());
e.printStackTrace(); // print call stack
}
• getMessage() is a method of the Throwable class that
returns the error message describing the exception.
• printStackTrace() is also a method of the Throwable
class
12
Multiple catch Statements
Order Matters!
Less general
first!
try {
// normal code here!
}
catch(ArithmeticException e) {
// Specific error handling here
}
catch(AnotherException e) {
// More general error handling here
}
13
Part 2
Exception Handling in Details
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
14
Part 2 Lecture Contents:












To get an overview of exceptions and exception handling.
To explore the advantages of using exception handling.
To distinguish exception types: Error (fatal) vs. Exception
(nonfatal), and checked vs. unchecked.
To declare exceptions in a method header.
To throw exceptions in a method.
To write a try-catch block to handle exceptions.
To explain how an exception is propagated.
To use the finally clause in a try-catch block.
To use exceptions only for unexpected errors.
To rethrow exceptions in a catch block.
To create chained exceptions.
To define custom exception classes.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
15
Introduction
When a program runs into a runtime error, the program
terminates abnormally.
In Java, runtime errors are caused by exceptions.
An exception is an object that represents an error or an
abnormal condition.
If the exception is not handled, the program will terminate
abnormally.
How can you handle the runtime error so that the program
can continue to run or terminate gracefully? This is the
Exception Handling subject.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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Exception-Handling Overview
 To
demonstrate EH, including how an exception
object is created and thrown:
 Open Quotient.java that reads two integers
and displays their quotient.
 Run program with regular data
 Run program with illegal data (integer zero div)
 FYI: FP real value divided by zero does not raise
an exception.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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Exception-Handling Overview
 Pure,

naked, risky assignment statement
quotient = dividend / divisor;
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
18
Exception-Handling Overview
 Simple
way to fix the problem is to add if stmt
to test the divisor:
 Open QuotientWithIf.java that reads two
integers and displays their quotient.
 Run program with regular data
 Run program with illegal data (integer zero div)
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
19
Exception-Handling Overview

User specified if statement
if (divisor != 0) {
quotient = dividend / divisor;
System.out.println(dividend+" / "+divisor+" is = "+quotient);
}
else
System.out.println("Divisor cannot be zero ");
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
20
Exception-Handling Overview
 To
demonstrate EH, including how to create,
throw, catch and handle an exception object:
 Open QuotientWithExcepionVerLiang.java
that reads two integers and displays their
quotient.
 Run program with regular data
 Run program with illegal data (integer zero div)
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
21
Exception-Handling Overview

Explicitly written code to throw an exception

try {
…
if (divisor == 0)
throw new ArithmeticException("Divisor cannot be zero ");

quotient = dividend / divisor;
System.out.println(dividend + " / " + divisor + " is = " + quotient);
} // end of try
catch (ArithmeticException aeRef) {
System.out.println(" Exception: integer cannot divide by zero ");
System.out.println(aeRef.getMessage());
aeRef.printStackTrace(); // print call stack
}
catch (InputMismatchException imeRef) {
System.out.println(" Exception " + imeRef.toString() );
}
finally {
System.out.println("\n\n Finally block executed");
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
22
Exception-Handling Overview
 To
demonstrate EH, including how to create,
throw, catch and handle an exception object:
 Open QuotientWithExcepionVerMalik.java
that reads two integers and displays their
quotient.
 Run program with regular data
 Run program with illegal data (integer zero div)
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
23
Exception-Handling Overview

No explicit code to throw an exception
Program implicitly creates exception and throws it

try {


…
quotient = dividend / divisor;
System.out.println(dividend + " / " + divisor + " is = " + quotient);
} // end of try
catch (ArithmeticException aeRef) {
System.out.println(" Exception: integer cannot divide by zero ");
System.out.println(aeRef.getMessage());
aeRef.printStackTrace(); // print call stack
}
catch (InputMismatchException imeRef) {
System.out.println(" Exception " + imeRef.toString() );
}
finally {
System.out.println("\n\n Finally block executed");
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
24
Exception-Handling Overview
Show runtime error
Quotient
Fix it using an if statement
QuotientWithIf
or using try … catch
QuotientWithException
What if the runtime error occurs in a called method?
QuotientWithMethod
See next slide
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
25
Exception-Handling Advantages
To
demonstrate EH advantages:
Open QuotientWithMethod.java that reads
two integers and displays their quotient.
Run program with regular data
Run program with illegal data (integer zero div)
 Now you see the advantages of using EH. It enables a
method to throw an exception (implicitly or explicitly) to
its caller. Without this capability, a method must handle
the exception or terminate the program.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
26
Exception-Handling Advantages
Exception
explicitly raised in one method
Exception caught in another method
public static int quotient(int num1, int num2) throws ArithmeticException {
if (num2 == 0)
throw new ArithmeticException("Divisor cannot be zero ");
return num1/num2;
}
public static void main(String[] args) {
try {
result = quotient(dividend, divisor);
System.out.println(dividend + " / " + divisor + " is = " + result);
} // end of try
catch (ArithmeticException aeRef) {
System.out.println(" Exception: an integer cannot be divided by zero ");
System.out.println(aeRef.getMessage());
aeRef.printStackTrace(); // print call stack
}
catch (InputMismatchException imeRef) {
System.out.println(" Exception " + imeRef.toString() );
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
27
Exception-Handling Advantages
Exception
implicitly raised in one method
Exception caught in another method
public static int quotient(int num1, int num2) throws ArithmeticException {
// if (num2 == 0)
//
throw new ArithmeticException("Divisor cannot be zero ");
return num1/num2;
}
public static void main(String[] args) {
try {
result = quotient(dividend, divisor);
System.out.println(dividend + " / " + divisor + " is = " + result);
} // end of try
catch (ArithmeticException aeRef) {
System.out.println(" Exception: an integer cannot be divided by zero ");
System.out.println(aeRef.getMessage());
aeRef.printStackTrace(); // print call stack
}
catch (InputMismatchException imeRef) {
System.out.println(" Exception " + imeRef.toString() );
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
28
Handling InputMismatchException
InputMismatchExceptionDemo
Open DemoMismatchException.java that reads
correctly input stream of integers.
By handling InputMismatchException, your
program will continuously read an input until it is
correct.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
29
Handling FileNotFoundException
FileNotFoundExceptionDemo
Open DemoFileNotFoundException.java that
raises an exception in case of file not found.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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The class Throwable has two direct subclasses,
Exception and Error.
Exceptions (members of the Exception family) are thrown
to signal abnormal conditions that can often be handled by
some code, though some may not be caught.
Errors (members of the Error family) are usually thrown
for more serious problems, such as OutOfMemoryError,
that may not be so easy to handle.
In general, code you write should throw only exceptions,
not errors.
Errors are usually thrown by the methods of the Java API,
or by the Java Virtual Machine itself.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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Exception Types
ClassNotFoundException
ArithmeticException
IOException
Exception
NullPointerException
RuntimeException
IndexOutOfBoundsException
Many more classes
Object
IllegalArgumentException
Throwable
Many more classes
LinkageError
Error
VirtualMachineError
Many more classes
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
33
System Errors
ClassNotFoundException
ArithmeticException
IOException
Exception
NullPointerException
RuntimeException
IndexOutOfBoundsException
Many more classes
Object
IllegalArgumentException
Throwable
System errors are thrown by JVM
and represented in the Error class.
The Error class describes internal
system errors. Such errors rarely
occur. If one does, there is little
you can do beyond notifying the
user and trying to terminate the
program gracefully.
Many more classes
LinkageError
Error
VirtualMachineError
Many more classes
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
34
Exceptions
Exception describes errors
caused by your program
and external
circumstances. These
errors can be caught and
handled by your program.
ClassNotFoundException
ArithmeticException
IOException
Exception
NullPointerException
RuntimeException
IndexOutOfBoundsException
Many more classes
Object
IllegalArgumentException
Throwable
Many more classes
LinkageError
Error
VirtualMachineError
Many more classes
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
35
Runtime Exceptions
ClassNotFoundException
ArithmeticException
IOException
Exception
NullPointerException
RuntimeException
IndexOutOfBoundsException
Many more classes
Object
IllegalArgumentException
Throwable
Many more classes
LinkageError
Error
VirtualMachineError
RuntimeException is caused by
programming errors, such as bad
casting, accessing an out-of-bounds
array, and numeric errors.
Many more classes
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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Checked Exceptions vs.
Unchecked Exceptions
RuntimeException, Error and their subclasses are
known as unchecked exceptions. All other
exceptions are known as checked exceptions,
meaning that the compiler forces the programmer
to check and deal with the exceptions.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
37
Unchecked Exceptions
ClassNotFoundException
ArithmeticException
IOException
Exception
NullPointerException
RuntimeException
IndexOutOfBoundsException
Many more classes
Object
IllegalArgumentException
Throwable
Many more classes
LinkageError
Error
VirtualMachineError
Unchecked
exception.
Many more classes
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
38
Declaring, Throwing, and
Catching Exceptions
Java’s EH model is based on three operations:
declaring an exception
throwing an exception
catching an exception
method1() {
try {
invoke method2;
}
catch (Exception ex) {
Process exception;
}
catch exception
declare exception
method2() throws Exception {
if (an error occurs) {
throw new Exception();
}
throw exception
}
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
39
Declaring Exceptions
Every method must state the types of checked
exceptions it might throw. This is known as
declaring exceptions.
public void myMethod() throws IOException
public void myMethod()
throws IOException, OtherException
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
40
Throwing Exceptions
When the program detects an error, the program can create
an instance of an appropriate exception type and throw it.
This is known as throwing an exception. Here is an example,
throw new TheException();
TheException ex = new TheException();
throw ex;
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
41
Throwing Exceptions Example
/** Set a new radius */
public void setRadius(double newRadius)
throws IllegalArgumentException {
if (newRadius >= 0)
radius = newRadius;
else
throw new IllegalArgumentException(
"Radius cannot be negative");
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
42
Catching Exceptions
When an exception is thrown, it can be caught and handled in a trycatch block as follows:
try {// Statements that may throw exceptions
statements;
}
catch (Exception1 exVar1) {
handler for exception1;
}
catch (Exception2 exVar2) {
handler for exception2;
}
...
catch (ExceptionN exVarN) {
handler for exceptionN;
}
43
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
Catching Exceptions pp465
try
try
try
catch
catch
catch
An exception
is thrown in
method3
Call Stack
method3
main method
method2
method2
method1
method1
method1
main method
main method
main method
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
44
Catch or Declare Checked Exceptions
Java forces you to deal with checked exceptions. If a method declares a
checked exception (i.e., an exception other than Error or
RuntimeException), you must invoke it in a try-catch block or declare to
throw the exception in the calling method. For example, suppose that
method p1 invokes method p2 and p2 may throw a checked exception (e.g.,
IOException), you have to write the code as shown in (a) or (b).
void p1() {
try {
p2();
}
catch (IOException ex) {
...
}
}
(a)
void p1() throws IOException {
p2();
}
(b)
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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Example: Declaring, Throwing, and
Catching Exceptions
 Objective:
This example demonstrates declaring,
throwing, and catching exceptions by modifying the
setRadius() method in the Circle class. The new
setRadius() method throws an exception if radius is
negative.
 Open CircleWithException.java file
TestCircleWithException
CircleWithException
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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E.g.: Declaring, Throwing, and Catching Exceptions
class Circle {
…
// methods mutators
public void setRadius(double par) throws IllegalArgumentException {
if (par >= 0.0) radius = par;
else throw new IllegalArgumentException("Radius cannot be negative");
}
…
} // end of class Circle
public class CircleWithException {
public static void main(String[] args) {
try {
Circle a = new Circle();
Circle b = new Circle(5.);
Circle c = new Circle(-5.);
}
catch (IllegalArgumentException ex){
System.out.println("\n-->>" + ex + "\n-->>");
System.out.println("\n-->>" + ex.getMessage() + "\n-->>");
ex.printStackTrace(); // print call stack
}
finally {
System.out.println("\n Finally block executed");
}
} // end of method main()
} // end of class
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
47
Defining Custom Exception Classes

Use the exception classes in the API whenever possible.

Define custom exception classes if the predefined
classes are not sufficient.

Define custom exception classes by extending
Exception or a subclass of Exception.


the setRadius() method throws an exception if the
radius is negative. Suppose you wish to pass the radius
to the handler, you have to create a custom exception
class.
Open CircleWithUserException.java file
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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Defining Custom Exception Classes
class IllegalRadiusException extends Exception {
private double rad;
public IllegalRadiusException(double radius) { // constructor
super("Invalid radius " + radius);
rad = radius;
}
public double getRad() { return rad; }
} // end of class IllegalRadiusException
class Circle {
// methods mutators
public void setRadius(double par) throws IllegalRadiusException {
if(par >= 0.0) radius = par;
else throw new IllegalRadiusException(par);
}
} // end of class Circle
public class CircleWithUserException {
public static void main(String[] args) {
try {
Circle a = new Circle(88.); //a.setRadius(-22.);
Circle b = new Circle(5.); // Circle c = new Circle(-5.);
}
catch (IllegalRadiusException ex){
System.out.println("\n-->>\nInvalid radius is " + ex.getRad() + "\n-->>");
}
} // end of method
main()
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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try {
// normal code here!
}
catch(Exception e) {
System.out.println(e.getMessage());
e.printStackTrace(); // print call stack
}

getMessage() is a method of the Throwable class that
returns the error message describing the exception.

printStackTrace() is also a method of the Throwable class
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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Rethrowing Exceptions
try {
statements;
}
catch(TheException ex) {
// perform operations before exits;
throw ex;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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51
The finally Clause
try {
statements;
}
catch(TheException ex) {
handling ex;
}
finally {
finalStatements;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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52
Cautions When Using Exceptions
 (+)
Exception handling separates error-handling
code from normal programming tasks, thus
making programs easier to read and to modify.
(
) Be aware, however, that exception handling
usually requires more time and resources
because it requires instantiating a new exception
object, rolling back the call stack, and
propagating the errors to the calling methods.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
53
When to Throw Exceptions
 An
exception occurs in a method.
 If
you want the exception to be processed by its
direct caller or by a method within the chain of
its callers, you should create an exception object
and throw it.
 If
you can handle the exception in the method
where it occurs, there is no need to throw it.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
54
When to Use Exceptions?
You should use it to deal with unexpected error
conditions. Do not use it to deal with simple,
expected situations. For example, the following code
try {
System.out.println(refVar.toString());
}
catch (NullPointerException ex) {
System.out.println("refVar is null");
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
55
When to Use Exceptions
is better to be replaced by
if (refVar != null)
System.out.println(refVar.toString());
else
System.out.println("refVar is null");
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
56
Assertions
An assertion is a Java statement that
enables you to assert an assumption about
your program.
 An assertion contains a Boolean expression
that should be true during program execution.
 Assertions can be used to assure program
correctness and avoid logic errors.

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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57
Declaring Assertions
An assertion is declared using the new Java keyword
assert in JDK 1.4 as follows:
assert assertion;
or
assert assertion : detailMessage;
where assertion is a Boolean expression and
detailMessage is a primitive-type or an Object value.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
58
Executing Assertions
When an assertion statement is executed, Java evaluates the
assertion. If it is false, an AssertionError will be thrown. The
AssertionError class has a no-arg constructor and seven
overloaded single-argument constructors of type int, long, float,
double, boolean, char, and Object.
For the first assert statement with no detail message, the no-arg
constructor of AssertionError is used. For the second assert
statement with a detail message, an appropriate AssertionError
constructor is used to match the data type of the message. Since
AssertionError is a subclass of Error, when an assertion becomes
false, the program displays a message on the console and exits.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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Executing Assertions Example
To
demonstrate effect of using assert keyword in
Java:
Open
AssertionDemo.java source file.
Compile
and run the demo program.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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Executing Assertions Example
public class AssertionDemo {
public static void main(String[] args) {
int i; int sum = 0;
for (i = 0; i <=10; i++) {
sum += i;
}
assert i == 12;
assert sum > 10 && sum < 5 * 10 : "sum is " + sum;
}
}
System.out.println("\n\nExecution continues ...");
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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Compiling Programs with
Assertions
Since assert is a new Java keyword introduced in
JDK 1.4, you have to compile the program using
a JDK 1.4 compiler. Furthermore, you need to
include the switch –source 1.4 in the compiler
command as follows:
javac –source 1.4 AssertionDemo.java
NOTE: If you use JDK 1.5, there is no need to
use the –source 1.4 option in the command.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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Running Programs with
Assertions
By default, the assertions are disabled at runtime. To
enable it, use the switch –enableassertions, or –ea for
short, as follows:
java –ea AssertionDemo
Assertions can be selectively enabled or disabled at
class level or package level. The disable switch is –
disableassertions or –da for short. For example, the
following command enables assertions in package
package1 and disables assertions in class Class1.
java –ea:package1 –da:Class1 AssertionDemo
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Using Exception Handling or
Assertions
Assertion should not be used to replace exception
handling. Exception handling deals with unusual
circumstances during program execution. Assertions are
to assure the correctness of the program. Exception
handling addresses robustness and assertion addresses
correctness. Like exception handling, assertions are not
used for normal tests, but for internal consistency and
validity checks. Assertions are checked at runtime and
can be turned on or off at startup time.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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Using Exception Handling or
Assertions, cont.
Do not use assertions for argument checking in public
methods. Valid arguments that may be passed to a
public method are considered to be part of the method’s
contract. The contract must always be obeyed whether
assertions are enabled or disabled. For example, the
following code should be rewritten using exception
handling as shown in Lines 28-35 in Circle.java in
Listing 13.8.
public void setRadius(double newRadius) {
assert newRadius >= 0;
radius = newRadius;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
65
Using Exception Handling or
Assertions, cont.
Use assertions to reaffirm assumptions. This gives you
more confidence to assure correctness of the program. A
common use of assertions is to replace assumptions with
assertions in the code.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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Using Exception Handling or
Assertions, cont.
Another good use of assertions is place assertions in a
switch statement without a default case. For example,
switch (month) {
case 1: ... ; break;
case 2: ... ; break;
...
case 12: ... ; break;
default: assert false : "Invalid month: " + month
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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67
Thank You
for
Your attention!
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68