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Java Exception Handling

admin — Sat, 19 Nov 2011 01:32:05 +0000

Java Exception Handling

I. Introduction

An exception is an error thrown by a class or method reporting an error in operation. For example, dividing by zero is undefined in mathematics, and a calculation can fail if this winds up being the case because of an error in user input. In this particular case an ArithmeticException is thrown, and unless the programmer looks for this exception and manually puts in code to handle it, the program will crash stating the exception thrown and a stack trace, which would be unhelpful to a casual user of a Java program. If the programmer handles the exception, he could deliver a useful error to the user and return the user to the beginning of the program so that they could continue to use it.

II. Common Exceptions

There are many different exceptions that can be thrown by a program, and the Java API contains quite a few. A lot are contained in the default package, java.lang; however, when you start using more functionality such as AWT, Swing, or java.io, the packages may also contain additional exceptions thrown by those libraries. As you start expanding the functionality, it might be a good idea to look at potential exceptions in the package and when they might be thrown in the course of your application. Here is a primer of some:

ArithmeticException

Thrown if a program attempts to perform division by zero

ArrayIndexOutOfBoundsException

–Thrown if a program attempts to access an index of an array that does not exist

StringIndexOutOfBoundsException

–Thrown if a program attempts to access a character at a non-existent index in a String

NullPointerException

–Thrown if the JVM attempts to perform an operation on an Object that points to no data, or null

NumberFormatException

–Thrown if a program is attempting to convert a string to a numerical datatype, and the string contains inappropriate characters (i.e. ‘z’ or ‘Q’)

ClassNotFoundException

–Thrown if a program can not find a class it depends at runtime (i.e., the class’s “.class” file cannot be found or was removed from the CLASSPATH)

IOException

–Actually contained in java.io, but it is thrown if the JVM failed to open an I/O stream

As I said before, many different exceptions exist, and you should probably use your API documentation to learn about additional exceptions that may apply to your particular application.

III. “Catching” Exceptions

The java language contains keywords used specifically for testing for and handling exceptions. The ones we will be using here are try and catch, and they must be used in conjunction with one another. They sort of work like if-else:

try{

Contains code to be tested

}catch(Exception e){

Contains code to be executed if instanced of Exception is caught

}

The catch statement can look for all exceptions, using the Exception superclass, or it can catch a specific exception that you think could be thrown in the code you are testing with the try block. You can even have multiple catch blocks to catch and execute custom code for a number of different errors. A good thing to note would be that any particular exception that is caught is compared with each catch statement sequentially; so it is a good idea to put more generic exceptions, like Exception, towards the bottom of the list.

IV. The Throwable Superclass

The catch statement also stores an instance of the exception that was caught in the variable that the programmer uses, in the previous example Exception e. While all exceptions are subclasses of Exception, Exception itself is a subclass of Throwable, which contains a nice suite of methods that you can use to get all kinds of information to report about your exceptions:

getMessage()–returns the error message reported by the exception in a String

printStackTrace()–prints the stack trace of the exception to standard output, useful for debugging purposes in locating where the exception occurred

printStackTrace(PrintStream s)–prints the stack trace to an alternative output stream

printStackTrace(PrintWriter s)–prints the stack trace to a file, this way you can log stack traces transparent to the user or log for later reference

toString()–if you just decide to print out the exception it will print out this: NAME_OF_EXCEPTION: getMessage().

Using the catch you now have control over what the error message is and where it goes.

V. Effectively using try-catch

In this section, I’m going to give you a few code samples on using try-catch blocks to give you an idea of the flexibility you as a programmer have over exceptions in your programs. The scenario is a user has input a file name to a program of a file that does not exist. In this scenario we are going to be using a text-based program; however, in a graphical environment you can easily use the catch block to draw dialog boxes. In the first example, we want to print a useful error message and exit the program gracefully, saving the user from the confusing stack trace with something useful:

try{

BufferedReader in = new BufferedReader(new FileReader(userInput));

System.out.println(in.readLine())

}catch(IOException e){

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

System.err.println(“Error: ” + userInput + ” is not a valid file. Please verify that the file exists and that you have access to it.”);

System.exit(1);

}

Here, System.err.println() prints the error message to standard error output which is a high priority buffer that is used to report error messages to the console. If you’re being a good programmer, you have separate methods that handle the different functionality of your programs; this way you can easily start the program from a previous place in the program before an exception occurs. In this next example, the user inputs the filename through the function getUserInput() elsewhere in the program; we want to report the “helpful error message” to the user and return the user to a place where he can enter a new filename.

public String getFileInput(String userInput){

try{

BufferedReader in = new BufferedReader(new FileReader(userInput));

return in.readLine();

}catch(IOException e){

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

System.err.println(“Error: ” + userInput + ” is not a valid file. Please verify that the file exists and that you have access to it.”);

return getFileInput(getUserInput());

}

Now you have an idea of how you can control your programs once an exception is thrown, and this should give you the basic idea of how to work with exceptions in the Java language.

VI. Final Thoughts on Exceptions

The best way to prevent your application from crashing from exceptions is to avoid exceptions in the first place. It is much more costly for a system to catch and handle exceptions than to account for potential errors directly in your code. A lot of times, an exception can point to a problem in the programmer’s logic rather than in the user’s use of the program. You should look for the cause of exceptions, and try to understand why it occurred and make sure that you really had considered everything. You will be making far more resilient and robust applications.

I. Introduction

II. Common Exceptions

ArithmeticException

Thrown if a program attempts to perform division by zero

ArrayIndexOutOfBoundsException

Thrown if a program attempts to access an index of an array that does not exist

StringIndexOutOfBoundsException

Thrown if a program attempts to access a character at a non-existent index in a String

NullPointerException

Thrown if the JVM attempts to perform an operation on an Object that points to no data, or null

NumberFormatException

Thrown if a program is attempting to convert a string to a numerical datatype, and the string contains inappropriate characters (i.e. ‘z’ or ‘Q’)

ClassNotFoundException

Thrown if a program can not find a class it depends at runtime (i.e., the class’s “.class” file cannot be found or was removed from the CLASSPATH)

IOException

Actually contained in java.io, but it is thrown if the JVM failed to open an I/O stream

As I said before, many different exceptions exist, and you should probably use your API documentation to learn about additional exceptions that may apply to your particular application.

III. “Catching” Exceptions

try{

Contains code to be tested

}catch(Exception e){

Contains code to be executed if instanced of Exception is caught

}

IV. The Throwable Superclass

getMessage()–returns the error message reported by the exception in a String

printStackTrace()–prints the stack trace of the exception to standard output, useful for debugging purposes in locating where the exception occurred

printStackTrace(PrintStream s)–prints the stack trace to an alternative output stream

printStackTrace(PrintWriter s)–prints the stack trace to a file, this way you can log stack traces transparent to the user or log for later reference

toString()–if you just decide to print out the exception it will print out this: NAME_OF_EXCEPTION: getMessage().

Using the catch you now have control over what the error message is and where it goes.

V. Effectively using try-catch

try{

BufferedReader in = new BufferedReader(new FileReader(userInput));

System.out.println(in.readLine())

}catch(IOException e){

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

System.err.println(“Error: ” + userInput + ” is not a valid file. Please verify that the file exists and that you have access to it.”);

System.exit(1);

}

public String getFileInput(String userInput){

try{

BufferedReader in = new BufferedReader(new FileReader(userInput));

return in.readLine();

}catch(IOException e){

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

System.err.println(“Error: ” + userInput + ” is not a valid file. Please verify that the file exists and that you have access to it.”);

return getFileInput(getUserInput());

}

Now you have an idea of how you can control your programs once an exception is thrown, and this should give you the basic idea of how to work with exceptions in the Java language.

VI. Final Thoughts on Exceptions

Some Examples of Exceptions

arithmetic.java

public class arithmetic {

public static void main(String[] args) {

int a,b,c;

a=5;

b=0;

c=a/b;

System.out.println(c);

}

Output

******

Exception in thread “main” java.lang.ArithmeticException: / by zero

at arithmetic.main(arithmetic.java:6

arrayindex.java

public class arrayindex {

public static void main(String[] args) {

int a[]= new int[5];

int b[]= {1,2,3,4,5,6};

for(int i=0;i<6;i++){

a[i]=b[i];

System.out.println(a[i]);

}

Output

******

Exception in thread “main” java.lang.ArrayIndexOutOfBoundsException: 5

at arrayindex.main(arrayindex.java:8)

StringIndexOutofBounds.java

public class StringIndexOutofBounds {

public static void main(String[] args) {

String str= “String Index out of Bounds Exception”;

String str2= str.substring(50);

System.out.println(str2);

}

Output

******

Exception in thread “main” java.lang.StringIndexOutOfBoundsException:

String index out of range: -14

at java.lang.String.substring(Unknown Source)

at StringIndexOutofBounds.main(StringIndexOutofBounds.java:7)

UnSupportedOperation.java

import java.util.ArrayList;

import java.util.Collections;

import java.util.List;

public class UnSupportedOperation {

public static void main(String[] args) {

al = new ArrayList();

al.add(2);

al.add(3);

al = Collections.unmodifiableList(al);

al.add(4);

}

Output

******

Exception in thread “main” java.lang.UnsupportedOperationException

at java.util.Collections$UnmodifiableCollection.add(Unknown Source)

at UnSupportedOperation.main(UnSupportedOperation.java:10)

ArrayStore.java

public class ArrayStore {

public static void main(String args[]) {

int a[]={};

double b[] = {2.4,5.3};

for(int i=0;i<2;i++)

a[i]=b[i];

}

Output

******

Exception in thread “main” java.lang.Error: Unresolved compilation problem:

Type mismatch: cannot convert from double to int

at ArrayStore.main(ArrayStore.java:6)

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PL/SQL Exception Handling Summary...

PL/SQL Exception Handling

admin — Wed, 13 Jul 2011 16:30:07 +0000

Summary of Predefined PL/SQL Exceptions

An internal exception is raised automatically if your PL/SQL program violates an Oracle rule or exceeds a system-dependent limit. PL/SQL predefines some common Oracle errors as exceptions. For example, PL/SQL raises the predefined exception NO_DATA_FOUND if a SELECT INTO statement returns no rows.

You can use the pragma EXCEPTION_INIT to associate exception names with other Oracle error codes that you can anticipate. To handle unexpected Oracle errors, you can use the OTHERS handler. Within this handler, you can call the functions SQLCODE and SQLERRM to return the Oracle error code and message text. Once you know the error code, you can use it with pragma EXCEPTION_INIT and write a handler specifically for that error.

PL/SQL declares predefined exceptions globally in package STANDARD. You need not declare them yourself. You can write handlers for predefined exceptions using the names in the following list:

Exception	Oracle Error
ACCESS_INTO_NULL	ORA-06530
CASE_NOT_FOUND	ORA-06592
COLLECTION_IS_NULL	ORA-06531
CURSOR_ALREADY_OPEN	ORA-06511
DUP_VAL_ON_INDEX	ORA-00001
INVALID_CURSOR	ORA-01001
INVALID_NUMBER	ORA-01722
LOGIN_DENIED	ORA-01017
NO_DATA_FOUND	ORA-01403
NOT_LOGGED_ON	ORA-01012
PROGRAM_ERROR	ORA-06501
ROWTYPE_MISMATCH	ORA-06504
SELF_IS_NULL	ORA-30625
STORAGE_ERROR	ORA-06500
SUBSCRIPT_BEYOND_COUNT	ORA-06533
SUBSCRIPT_OUTSIDE_LIMIT	ORA-06532
SYS_INVALID_ROWID	ORA-01410
TIMEOUT_ON_RESOURCE	ORA-00051
TOO_MANY_ROWS	ORA-01422
VALUE_ERROR	ORA-06502
ZERO_DIVIDE	ORA-01476

Brief descriptions of the predefined exceptions follow:

Exception	Raised when …
ACCESS_INTO_NULL	Your program attempts to assign values to the attributes of an uninitialized (atomically null) object.
CASE_NOT_FOUND	None of the choices in the WHEN clauses of a CASE statement is selected, and there is no ELSE clause.
COLLECTION_IS_NULL	Your program attempts to apply collection methods other than EXISTS to an uninitialized (atomically null) nested table or varray, or the program attempts to assign values to the elements of an uninitialized nested table or varray.
CURSOR_ALREADY_OPEN	Your program attempts to open an already open cursor. A cursor must be closed before it can be reopened. A cursor FOR loop automatically opens the cursor to which it refers. So, your program cannot open that cursor inside the loop.
DUP_VAL_ON_INDEX	Your program attempts to store duplicate values in a database column that is constrained by a unique index.
INVALID_CURSOR	Your program attempts an illegal cursor operation such as closing an unopened cursor.
INVALID_NUMBER	In a SQL statement, the conversion of a character string into a number fails because the string does not represent a valid number. (In procedural statements, VALUE_ERROR is raised.) This exception is also raised when the LIMIT-clause expression in a bulk FETCH statement does not evaluate to a positive number.
LOGIN_DENIED	Your program attempts to log on to Oracle with an invalid username and/or password.
NO_DATA_FOUND	A SELECT INTO statement returns no rows, or your program references a deleted element in a nested table or an uninitialized element in an index-by table. SQL aggregate functions such as AVG and SUM always return a value or a null. So, a SELECT INTO statement that calls an aggregate function never raises NO_DATA_FOUND. The FETCH statement is expected to return no rows eventually, so when that happens, no exception is raised.
NOT_LOGGED_ON	Your program issues a database call without being connected to Oracle.
PROGRAM_ERROR	PL/SQL has an internal problem.
ROWTYPE_MISMATCH	The host cursor variable and PL/SQL cursor variable involved in an assignment have incompatible return types. For example, when an open host cursor variable is passed to a stored subprogram, the return types of the actual and formal parameters must be compatible.
SELF_IS_NULL	Your program attempts to call a MEMBER method on a null instance. That is, the built-in parameter SELF (which is always the first parameter passed to a MEMBER method) is null.
STORAGE_ERROR	PL/SQL runs out of memory or memory has been corrupted.
SUBSCRIPT_BEYOND_COUNT	Your program references a nested table or varray element using an index number larger than the number of elements in the collection.
SUBSCRIPT_OUTSIDE_LIMIT	Your program references a nested table or varray element using an index number (-1 for example) that is outside the legal range.
SYS_INVALID_ROWID	The conversion of a character string into a universal rowid fails because the character string does not represent a valid rowid.
TIMEOUT_ON_RESOURCE	A time-out occurs while Oracle is waiting for a resource.
TOO_MANY_ROWS	A SELECT INTO statement returns more than one row.
VALUE_ERROR	An arithmetic, conversion, truncation, or size-constraint error occurs. For example, when your program selects a column value into a character variable, if the value is longer than the declared length of the variable, PL/SQL aborts the assignment and raises VALUE_ERROR. In procedural statements, VALUE_ERROR is raised if the conversion of a character string into a number fails. (In SQL statements, INVALID_NUMBER is raised.)
ZERO_DIVIDE	Your program attempts to divide a number by zero.