Quartz 5

3. Exception Handling

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3.1 Handling exceptions

Exception: an unexpected incident that stops the normal execution of the program

NOTE: scnr.nextInt accepts inputs that begin with + or - to indicate sign and accepts inputs that optionally use commas to separate every third digit

Try block: surrounds normal code; exits immediately if a statement throws an exception
Catch block (exception handler): catches an exception that matches the parameter type thrown in preceding try block

3.2 Throwing exceptions

NOTE: Java throws an ArithmeticException for in integer division by zero; floating-point division by zero results in NaN or infinity

throw: throws a throwable object during program execution

NOTE: list most specific (subclass) exceptions first, followed by general (superclass) exceptions becuase JVM evaluates catch blocks sequentially

3.3 Exceptions with methods

Throw clause: specifies the types of exceptions that a method may throw and callers should handle

Checked and unchecked exceptions
Checked exception: expect a programmer to handle within the program
Unchecked exception: Not expect a programmer to catch and handle at runtime

Java’s catch or specify requirements requires methods to either catch the checked exception or specify that methods throws the checked exception

NOTE: RuntimeException and Error and thier subclasses are the only unchecked exceptions in Java

3.4 User-defined exceptions

Define custom exception by extending an existing exception classes, with option to add custom constructors, methods, and variables

3.5 Exceptions with files

Closing files:

import java.util.Scanner;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.PrintWriter;
// need to catch checked exception: FileNotFoundException
import java.io.FileNotFoundException;
try {
    // open input file
    FileInputStream in = new FileInputStream("input.txt");
    Scanner scnr = new Scanner(in);
    // open output file
    FileOutputStream out = new FileOutputStream("output.txt");
    PrintWriter writer = new PrintWriter(out);
    ...
} catch (FileNotFoundException e) {
    ...
}
// close both files
scnr.close();
writer.close();

Closing files with try-with-resources:

import java.util.Scanner;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.PrintWriter;
// need to catch checked exception: FileNotFoundException
import java.io.FileNotFoundException;
try (
    // open input file
    FileInputStream in = new FileInputStream("input.txt");
    Scanner scnr = new Scanner(in);
    // open output file
    FileOutputStream out = new FileOutputStream("output.txt");
    PrintWriter writer = new PrintWriter(out);
) {
    ...
} catch (FileNotFoundException e) {
    ...
}
// automatically close both files after the try block exits

Finally block: always executes when a try block exits

NOTE: A try-with-resources variable must be newly declared, final, or effectively final

3.6 Java example: Generate number format exception (optional)

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