Download Static Methods, Variables, and Exceptions in CMSC 131 Spring 2007 - Prof. Bonnie J. Dorr and more Study notes Computer Science in PDF only on Docsity!
CMSC 131 Spring 2007 Jan Plane (adapted from Bonnie Dorr)
Lecture Set 6:
Static Methods & Variables
and Exceptions
1. Parameter Passing
2. Static variables and static
methods
3. Exceptions
CMSC 131 Spring 2007 Jan Plane (adapted from Bonnie Dorr)
1
Parameter Passing
Parameter List
Names of Parameters
Primitive type parameters
Reference type parameters
(See parameter passing example in CVS)
CMSC 131 Spring 2007 Jan Plane (adapted from Bonnie Dorr)
2
Why Have Static Variables /
Methods?
Sometimes info needs to be shared data among all
objects of a specific class type
e.g. How many objects in a class have been created?
A constant that needs to be the same for all objects of that
type
Sometimes it is useful to have methods that are in a
class that can be invoked without first creating
objects of that type
Static components help for these types of things
CMSC 131 Spring 2007 Jan Plane (adapted from Bonnie Dorr)
3
Declaring Static Methods (and
variables and constants)
Static methods
public static void main (…) { … }
public static void drawFlag(MyGrid grid, int
Ccode) { … }
How do we call static methods?
FlagMaker.drawFlag(grid, 1);
Can have static variables and constants too
public static int numStudents = 0;
public static final int MAX_ENROLLMENT = 10;
How do we use static variables and constants?
StudentRoster.numStudents
StudentRoster.MAX_ENROLLMENT
CMSC 131 Spring 2007 Jan Plane (adapted from Bonnie Dorr)
6
Calling one method from
another – static and non-static
static methods
when running they ARE NOT associated with a specific instance you do NOT have a “current object” but you do have a current class are usually called with: ClassName.sMethodName() if you are already in a static method, since you have a class name understood as the default, you can just use sMethodName()
non-static methods
when running they ARE associated with a specific instance you do have a “current object” are called with: objectName.nsMethodName() if you are already in a non-static method, since you have a current object assumed, you can just use nsMethodName() to call it on that current object since that non-static method must also be in the class, the class name is also understood as the default so you can use sMethodName() to call the static member of that class
CMSC 131 Spring 2007 Jan Plane (adapted from Bonnie Dorr)
7
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, (we’ll 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) In Java: error = exception What 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 131 Spring 2007 Jan Plane (adapted from Bonnie Dorr)
8
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 131 Spring 2007 Jan Plane (adapted from Bonnie Dorr)
9
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 131 Spring 2007 Jan Plane (adapted from Bonnie Dorr)
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Handling Exceptions
Aborting program not always a good idea
E-mail: can’t 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 131 Spring 2007 Jan Plane (adapted from Bonnie Dorr)
13
Java Exception Terminology
When 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 isn’t 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 131 Spring 2007 Jan Plane (adapted from Bonnie Dorr)
14
Exception Propagation
In previous example:
Exception thrown in one method …
… but caught in another
Java uses exception propagation to look for exception handlers
When 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 131 Spring 2007 Jan Plane (adapted from Bonnie Dorr)
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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
