Download Because Murphy`s Law never fails

Java Errors and
Exceptions
Because Murphy’s Law never fails
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“Java is the most distressing thing to hit
computing since MS-DOS.”
– Alan Kay
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Corresponding Book
Sections
• Pearson Custom Computer Science:
Chapter 2, Sections 1-7
• Data Structures: Sections 2.3
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Programs Crash
• Programs can fail for any number of
reasons, including invalid input, failed
hardware, or even bad programming!
• Fortunately, Java has some mechanisms to
deal with failure
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Types of Exceptions/
Errors
Image from Pearson’s Textbook, page 39
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Java Errors
•
Errors - generally happen when the Java runtime
encounters a serious problem. A programmer is
generally not expected to handle these.
•
Examples:
•
OutOfMemoryError - occurs when the JVM
cannot allocate anymore memory for the
program to use
•
StackOverflowError - occurs if you call too
many methods consecutively and therefore run
out of stack space. Usually occurs when using
recursion when doing the recursive equivalent of
an infinite loop
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Java Exceptions
• Generally happen when a Java program
encounters a problem
• A programmer is generally expected to
handle these according to the Java API, and
the Java compiler (and therefore Eclipse)
will yell at you to do so
•
Exception to expected handled exceptions
is Runtime Exceptions
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Runtime Exceptions
• NullPointerException - Referencing an
object that is not there
• ArrayIndexOutOfBoundsException -
Accessing an array index that does not exist
• UndeclaredThrowableException - when a
method throws an exception (actually any
throwable object) of some type that it is not
declared to throw
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Other Exceptions
- input / output failed or
• IOException
stopped working
- you referenced
• ClassNotFoundException
a class that was not created/imported into
•
your program
UnsupportedFlavorException - happens
when you ask to try the 32nd flavor at
Baskin Robbins (actually for when a
DataFlavor type is not supported for a given
request, such as a copy/paste operation)
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Handling Exceptions
•
Java has some built in keywords and objects for
handling program problems
•
Handling exceptions well will allow you to:
•
Help users of your program realize why your
program is not functioning the way they think
it should be
•
Allow you to have your program recover from
issues that would normally make it crash
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Exception Handling
Keywords
• throw - manually throw an exception,
exits the current method you are in when it
is used
• throws - specifies that a method can
throw this type of exception, used after a
method is declared
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public class Arithmetic {
public static int quotient(int number1, int number2)
throws ArithmeticException {
if (number2 == 0)
throw new ArithmeticException("Divisor cannot be zero");
return number1 / number2;
}
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
int number1 = input.nextInt();
int number2 = input.nextInt();
}
}
try {
int result = quotient(number1, number2);
System.out.println(number1 + "/" + number2 + " is " + result);
} catch (ArithmeticException ex) {
System.out
.println("Exception: an integer cannot be divided by zero ");
} finally {
System.out.println("Execution continues ....");
}
From Pearson’s Textbook, Page 37
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public class Arithmetic {
public static int quotient(int number1, int number2)
throws ArithmeticException {
if (number2 == 0)
throw new ArithmeticException("Divisor cannot be zero");
return number1 / number2;
}
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
int number1 = input.nextInt();
int number2 = input.nextInt();
}
}
try {
int result = quotient(number1, number2);
System.out.println(number1 + "/" + number2 + " is " + result);
} catch (ArithmeticException ex) {
System.out
.println("Exception: an integer cannot be divided by zero ");
} finally {
System.out.println("Execution continues ....");
}
From Pearson’s Textbook, Page 37
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More Keywords!
• try - block of code to run that may throw
exceptions
• catch - block of code to run if a certain
type of exception is thrown in the try block
• finally - block of code to execute
whether an exception has been caught or
not
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public class Arithmetic {
public static int quotient(int number1, int number2)
throws ArithmeticException {
if (number2 == 0)
throw new ArithmeticException("Divisor cannot be zero");
return number1 / number2;
}
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
int number1 = input.nextInt();
int number2 = input.nextInt();
}
}
try {
int result = quotient(number1, number2);
System.out.println(number1 + "/" + number2 + " is " + result);
} catch (ArithmeticException ex) {
System.out
.println("Exception: an integer cannot be divided by zero ");
} finally {
System.out.println("Execution continues ....");
}
From Pearson’s Textbook, Page 37
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To the Code!
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