Throwable

All exceptions descend from the Throwable class, which extends Object.

• Only objects that descend from Throwable can be thrown and caught.

A Throwable encapsulates a String message, and provides methods like getMessage and printStackTrace.

• Throwable defines a constructor that takes a String for the message, as do almost all descendant classes.

Java defines three distinct categories of Throwables: * Exception * RuntimeException * Error

Exception

Exception extends Throwable and is the ancestor of all checked exceptions.

Packages define application-specific Exception subclasses and descendants whose names describe the condition causing the exception to be thrown.

public class IOException extends Exception { /*...*/ }

public class FileNotFoundException extends IOException { /*...*/ }

public class FileReader extends InputStreamReader {
    public FileReader(String fileName) throws FileNotFoundException { /*...*/ }
    //...
}

• By convention, names of checked exceptions end in Exception.

• Subclasses can add new, application-specific fields and methods if they choose.

remember...

Checked exceptions represent conditions external to a program that well-written code can anticipate and recover from.

RuntimeException

RuntimeException extends Exception, but is special: it and its subclasses are unchecked exceptions.

public class RuntimeException extends Exception { /*...*/ }

class IllegalArgumentException extends RuntimeException { /*...*/ }

public class SecurityException extends RuntimeException { /*...*/ }

Callers of methods that throw these exceptions can, but are not required to, catch or declare them.

SecurityManager sm = new SecurityManager();
sm.checkListen(80);             // throws SecurityException, don't have to catch.
try {
  sm.checkRead("/etc/sudoers"); // throws SecurityException, can catch if we want.
}
catch (SecurityException e) {
  System.out.println(e);
}

remember...

Unchecked exceptions typically represent conditions internal to a program that well-written code can anticipate and either prevent or recover from.

Error

Error extends Throwable, so is a sibling (not descendant of) Exception.

public class Error extends Throwable { /*...*/ }

• By convention, error class names end in Error, not Exception.

Errors are also unchecked by the compiler - callers of methods that may throw Error do not have to catch or declare them.

• This is because even the most well-written program is not expected to recover from an Error and continue executing.

Most errors are generated by the JVM itself.

abstract public class VirtualMachineError extends Error { /*...*/ }

public class StackOverflowError extends VirtualMachineError { /*...*/ }

Errors represent conditions from which recovery is likely impossible.

• These include conditions like running out of memory, overflowing the method call stack, trying to run a program with no main, or even a bug in the code of the JVM.

• You don't write try/catch blocks for Errors.

remember...

Errors represent conditions from which recovery is likely impossible. You should not attempt to handle them.

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Drill

Exceptions/com.example.exceptions.drills.ErrorDrill * Open ErrorDrill.java and add code to the callMe method as the comments describe. * What happens when you run ErrorDrill? * Right-click on the Console view and select Preferences. Un-check Limit console output and run ErrorDrill again.

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