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Exceptions in Java

The full story of exceptions in the Java language and virtual machine

By Bill Venners, JavaWorld.com, 07/01/98

Page 4 of 5

Exceptions and the method invocation stack

Code inside a try block is in a sense surrounded by the catch clauses associated with the try block. When an exception is thrown, the surrounding catch clauses are examined in inside-out order. You can nest try blocks inside try blocks, in effect building up more and more layers of catch clauses that surround the code. When a method is invoked from within a try block, the catch clauses associated with that try block surround the code in the invoked method as well. If that method has try blocks and catch clauses, they are added as inner surrounding layers. What this means is that an exception may be thrown far up the method invocation stack before landing in a catch clause that can handle it.

As an example, consider the following exception classes, which are simpler versions of exceptions introduced in examples above:

// In Source Packet in file except/ex7/TemperatureException.java
class TemperatureException extends Exception {
}
// In Source Packet in file except/ex7/TooColdException.java
class TooColdException extends TemperatureException {
}
// In Source Packet in file except/ex7/TooHotException.java
class TooHotException extends TemperatureException {
}
// In Source Packet in file except/ex7/UnusualTasteException.java
class UnusualTasteException extends Exception {
}

When the drinkCoffee() method of class VirtualPerson is invoked, it throws one of these four exceptions, chosen at random:

// In Source Packet in file except/ex7/VirtualPerson.java
class VirtualPerson {
    public void drinkCoffee(CoffeeCup cup) throws TooColdException,
        TemperatureException, UnusualTasteException {
        try {
            int i = (int) (Math.random() * 4.0);
            switch (i) {
            case 0:
                throw new TooHotException();
            case 1:
                throw new TooColdException();
            case 2:
                throw new UnusualTasteException();
            default:
                throw new TemperatureException();
            }
        }
        catch (TooHotException e) {
            System.out.println("This coffee is too hot.");
            // Customer will wait until it cools to an
            // acceptable temperature.
        }
    }
    //...
}

If variable i in the drinkCoffee() method above happens to be set to a value of 0 the switch statement will instantiate and throw a TooHotException. Because the switch statement itself is enclosed within a try block that has a catch clause for TooHotException, execution continues at that catch clause. The program prints out:

This coffee is too hot.

If variable i in the drinkCoffee() method above happens to be set to the value of 1, the switch statement will instantiate and throw a TooColdException. When this exception is thrown, the Java virtual machine will first check the catch clauses of the try block that surrounds the switch statement. In this case, however, no catch clause matches the thrown exception.

Because the TooColdException is not caught by the drinkCoffee() method, the Java virtual machine throws the exception up the method invocation stack to the method that invoked drinkCoffee(). As used here, a method invocation stack (or call stack) is a list of the methods that have been invoked by a thread, starting with the first method the thread invoked and ending with the current method. A method invocation stack shows the path of method invocations a thread took to arrive at the current method.