Set Lecture #7:
Exceptions
& Mutability Issues
1.
2.
3.
4.
Break and Continue for Loops
Exceptions
Mutability/Immutability
Stringbuffer class
CMSC 131Fall 2009
Jan Plane (adapted from Bonnie Dorr)
break from loops
break can also be used to exit immediately from any loop
while
do-while
for
e.g. Read numbers from input until negative number encountered
Scanner sc = new Scanner (System.in);
int n;
while (true) {
n = sc.nextInt ();
if (n < 0)
break;
else
<process n>;
}
Loop only terminates when break executed
This only happens when n < 0
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
1
Warning about break
Undisciplined use of break can make loops
impossible to understand
Termination of loops without break can be understood
purely by looking while, for parts
W hen break included, arbitrary termination behavior can be
introduced
Rule of thumb: use break only when loop condition
is always true (i.e. break is only way to terminate
loop)
W hen you use it, make sure it has a good comment
explaining what is happening
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
2
1
continue Statement
continue can also be used to affect loops
break halts loops
continue jumps to bottom of loop body
Following prints even numbers between 0 and 10
for (int i = 0; i <= 10; i++){
if (i % 2 == 1)
continue;
System.out.println (i);
}
Effect of continue statement is to jump to bottom of loop immediately when i
is odd
This bypasses println!
continue should be avoided
Confusing
Easy equivalents exist (e.g. if-else)
Included in Java mainly for historical reasons
W hen you use it, make sure it has a good comment explaining what is
happening
CMSC 131 Fall 2009
Jan Plane (adapted from Bonnie Dorr)
3
Exceptions
Programs can generate errors
Arithmetic
Divide by zero, overflows,
Object / Array
Using a null reference, illegal array index,
File and I/O
Nonexistent file, attempt to read past the end of the file, (well see more
about file I/O later in course),
Application-specific
Errors particular to application (e.g., attempt to remove a nonexistent
customer from a database)
1.
2.
3.
4.
In Java: something that is outside the norm = exception
W hat to do when an error occurs?
Basically ignore it: Print an error message and terminate?
Have the method handle it internally: Handle error in the code where
the problem lies as best you can.
Have the method pass it off to someone else to handle: Return
error code so that whoever called this function can handle it.
Modern language approach: Cause exception to be thrown (and
caught (or processed) by any function up the stack trace)
CMSC 131Fall 2009
Jan Plane (adapted from Bonnie Dorr)
4
Exception Behavior
If program generates (throws) exception then
default behavior is:
Java clobbers (aborts) the program
Stack trace is printed showing where exception was
generated (red and blue in Eclipse window)
Example
public static int mpg(int miles, int
gallons){
return miles/gallons;
}
Throws an exception and terminates the program.
CMSC 131Fall 2009
Jan Plane (adapted from Bonnie Dorr)
5
2
Throwing Exceptions Yourself
To throw an exception, use throw command:
throw e;
e must evaluate to an exception object
You can create exceptions just like other objects, e.g.:
RuntimeException e = new RuntimeException(Uh oh);
RuntimeException is a class
Calling new this way invokes constructor for this class
RuntimeException generalizes other kinds of exceptions (e.g.
ArithmeticException)
CMSC 131Fall 2009
Jan Plane (adapted from Bonnie Dorr)
6
Exceptions, Classes and
Types
Exceptions are objects
Some examples from the Java class library (mostly java.lang):
ArithmeticException: Used e.g. for divide by zero
NullPointerException: attempt to access an object with a null
reference
IndexOutOfBoundsException: array or string index out of range
ArrayStoreException: attempting to store wrong type of object in array
EmptyStackException: attempt to pop an empty Stack (java.util)
IOException: attempt to perform an illegal input/output operation (java.io)
NumberFormatException: attempt to convert an invalid string into a
number (e.g., when calling Integer.parseInt( ) )
RuntimeException: general run-time error (subsumes above)
Exception: The most generic of type exception
CMSC 131Fall 2009
Jan Plane (adapted from Bonnie Dorr)
7
Java Exceptions in Detail
Exceptions are (special) objects in Java
They are created from classes
The classes are derived (inherit) from a special class,
Throwable
W e will learn more about inheritance, etc., later
Every exception object / class has:
Exception(String message)
Constructor taking an explanation as an argument
String getMessage()
Method returning the embedded message of the exception
void printStackTrace()
Method printing the call stack when the exception was thrown
CMSC 131Fall 2009
Jan Plane (adapted from Bonnie Dorr)
8
3
Handling Exceptions
Aborting program not always a good idea
E-mail: cant lose messages
E-commerce: must ensure correct handling of
private info in case of crash
Antilock braking, air-traffic control: must recover
and keep working
Java includes provides the programmer with
mechanisms for recovering from exceptions
CMSC 131Fall 2009
Jan Plane (adapted from Bonnie Dorr)
9
Java Exception Terminology
W hen an anomaly is detected during program
execution, the JVM throws a particular type of
exception
There are built-in exceptions
Users can also define their own (more later)
To avoid crashing, a program can catch a thrown
exception (if it isnt caught you see the red and
blue messages stack trace)
An exception generated by a piece of code can only
be caught if the program is alerted. This process is
called trying the piece of code.
CMSC 131Fall 2009
Jan Plane (adapted from Bonnie Dorr)
10
Exception Propagation
In previous example:
Exception thrown in one method
but caught in another
Java uses exception propagation to look for exception handlers
W hen an exception occurs, Java pops back up the call stack to
each of the calling methods to see whether the exception is being
handled (by a try-catch block). This is exception propagation
The first method it finds that catches the exception will have its
catch block executed. Execution resumes normally in the method
after this catch block
If we get all the way back to main and no method catches this
exception, Java catches it and aborts your program
CMSC 131Fall 2009
Jan Plane (adapted from Bonnie Dorr)
11
4
Exception Handling: Example
DateReader.java
Prompts user for a date in mm/dd/yyyy format
Prints year
Program uses:
substring method
May throw IndexOutOfBoundsException
Integer.parseInt method
May throw NumberFormatException
getYear method (if d is null)
May throw NullPointerException
How do we know about these exceptions? Javadoc!
http://java.sun.com/j2se/1.5.0/docs/api/java/lang/package-summary.html
CMSC 131Fall 2009
Jan Plane (adapted from Bonnie Dorr)
12
What about Strings and
Aliasing?
String objects are immutable; fields cannot be
changed once created
Mutable objects: fields (values of instance variables) can be
changed by a call to some function (e.g. Cat, Student, etc.)
Immutable objects: fields (values of instance variables)
cannot be changed by any call to any function
See String API:
http://java.sun.com/j2se/1.3/docs/api/java/lang/packagesummary.html
In the Cat and CatOwner example:
when one object is assigned to another, an alias is created
Cat a = new Cat(Fluffy);
Cat b = a;
CMSC 131 Fall 2008
Jan Plane (adapted from Bonnie Dorr)
13
Which picture represents the current
status of memory?
Stack
Heap
Stack
cat
a
b
Heap
cat
a
b
Fred
Fred
CMSC 131 Fall 2008
Jan Plane (adapted from Bonnie Dorr)
14
5
Mutable Strings
Strings are immutable
Once a String object is created, it cannot be altered
Sometimes mutable strings would be handy
Sometimes a small change needs to be made to a string
(e.g. misspelled name)
Dont want to create a whole new String object in this
case
StringBuffer: Javas class for mutable Strings
CMSC 131 Fall 2008
Jan Plane (adapted from Bonnie Dorr)
15
StringBuffer Basics
See documentation at:
http://java.sun.com/j2se/1.5.0/docs/api/java/lang/StringBuffer.html
Main methods
append: add characters to end
insert: add characters in middle
delete: remove characters
Note
append, insert return object of type StringBuffer
This is alias to object that the methods belong to!
See StringBufferExample.java
CMSC 131 Fall 2008
Jan Plane (adapted from Bonnie Dorr)
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