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Exceptions in Java
Exceptions in the Java Language and Virtual Machine
by Bill Venners
First Published in JavaWorld, June 1998

<<  Page 3 of 9  >>

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Catching exceptions
To catch an exception in Java, you write a try block with one or more catch clauses. Each catch clause specifies one exception type that it is prepared to handle. The try block places a fence around a bit of code that is under the watchful eye of the associated catchers. If the bit of code delimited by the try block throws an exception, the associated catch clauses will be examined by the Java virtual machine. If the virtual machine finds a catch clause that is prepared to handle the thrown exception, the program continues execution starting with the first statement of that catch clause.

As an example, consider a program that requires one argument on the command line, a string that can be parsed into an integer. When you have a String and want an int, you can invoke the parseInt() method of the Integer class. If the string you pass represents an integer, parseInt() will return the value. If the string doesn't represent an integer, parseInt() throws NumberFormatException. Here is how you might parse an int from a command-line argument:

// In Source Packet in file except/ex1/Example1.java
class Example1 {
    public static void main(String[] args) {

        int temperature = 0;
        if (args.length > 0) {
            try {
                temperature = Integer.parseInt(args[0]);
            }
            catch(NumberFormatException e) {
                System.out.println(
                    "Must enter integer as first argument.");
                return;
            }
        }
        else {
            System.out.println(
                "Must enter temperature as first argument.");
            return;
        }

        // Create a new coffee cup and set the temperature of
        // its coffee.
        CoffeeCup cup = new CoffeeCup();
        cup.setTemperature(temperature);

        // Create and serve a virtual customer.
        VirtualPerson cust = new VirtualPerson();
        VirtualCafe.serveCustomer(cust, cup);
    }
}

Here, the invocation of parseInt() sits inside a try block. Attached to the try block is a catch clause that catches NumberFormatException:

catch(NumberFormatException e) {
    System.out.println(
        "Must enter integer as first argument.");
    return;
}

The lowercase character e is a reference to the thrown (and caught) NumberFormatException object. This reference could have been used inside the catch clause, although in this case it isn't. (Examples of catch clauses that use the reference are shown later in this article.)

If the user types Harumph as the first argument to the Example1 program, parseInt() will throw a NumberFormatException exception and the catch clause will catch it. The program will print:

Must enter integer as first argument.

Although the above example had only one catch clause, you can have many catch clauses associated with a single try block. Here's an example:

// In Source Packet in file except/ex1/VirtualCafe.java
class VirtualCafe {

    public static void serveCustomer(VirtualPerson cust,
        CoffeeCup cup) {

        try {
            cust.drinkCoffee(cup);
            System.out.println("Coffee is just right.");
        }
        catch (TooColdException e) {
            System.out.println("Coffee is too cold.");
            // Deal with an irate customer...
        }
        catch (TooHotException e) {
            System.out.println("Coffee is too hot.");
            // Deal with an irate customer...
        }
    }
}

If any code inside a try block throws an exception, its catch clauses are examined in their order of appearance in the source file. For example, if the try block in the above example throws an exception, the catch clause for TooColdException will be examined first, then the catch clause for TooHotException. During this examination process, the first catch clause encountered that handles the thrown object's class gets to "catch" the exception. The ordering of catch-clause examination matters because it is possible that multiple catch clauses of a try block could handle the same exception.

catch clauses indicate the type of abnormal condition they handle by the type of exception reference they declare. In the example above, the catch clauses declare exception type TooColdException and TooHotException. Had a single catch clause declared a TemperatureException, a thrown TooColdException or TooHotException still would have been caught, because TemperatureException is the superclass of both these classes. In the object-oriented way of thinking, a TooColdException is a TemperatureException, therefore, a catch clause for TemperatureException also will catch a thrown TooColdException. An example of this is shown below:

// In Source Packet in file except/ex2/VirtualCafe.java
class VirtualCafe {

    public static void serveCustomer(VirtualPerson cust,
        CoffeeCup cup) {

        try {
            cust.drinkCoffee(cup);
            System.out.println("Coffee is just right.");
        }
        catch (TemperatureException e) {
            // This catches TooColdException, TooHotException,
            // as well as TemperatureException.
            System.out.println("Coffee is too cold or too hot.");
            // Deal with an irate customer...
        }
    }
}

Multiple catch clauses could handle the same exception because you may, for example, declare two catch clauses, one for TooColdException and another for TemperatureException. In this case, however, you must place the catch clause for TooColdException above the one for TemperatureException, or the source file won't compile. If a catch clause for TemperatureException could be declared before a catch clause for TooColdException, the first catch clause would catch all TooColdExceptions, leaving nothing for the second catch clause to do. The second catch clause would never be reached. The general rule is: subclass catch clauses must precede superclass catch clauses. Here's an example of both orders, only one of which compiles:

// In Source Packet in file except/ex3/VirtualCafe.java
class VirtualCafe {

    public static void serveCustomer(VirtualPerson cust,
        CoffeeCup cup) {

        try {
            cust.drinkCoffee(cup);
            System.out.println("Coffee is just right.");
        }
        catch (TemperatureException e) {
            // This catches TooColdException, TooHotException,
            // as well as TemperatureException.
            System.out.println("Coffee is too cold or too hot.");
            // Deal with an irate customer...
        }
        // THIS WON'T COMPILE, BECAUSE THIS catch clause
        // WILL NEVER BE REACHED.
        catch (TooColdException e) {
            System.out.println("Coffee is too cold.");
        }
    }
}

// In Source Packet in file except/ex4/VirtualCafe.java
// This class compiles fine.
class VirtualCafe {

    public static void serveCustomer(VirtualPerson cust,
        CoffeeCup cup) {

        try {
            cust.drinkCoffee(cup);
            System.out.println("Coffee is just right.");
        }
        catch (TooColdException e) {
            System.out.println("Coffee is too cold.");
            // Deal with an irate customer...
        }
        catch (TemperatureException e) {
            // This catches TooHotException as well
            // as TemperatureException.
            System.out.println(
                "There's temperature trouble in this coffee.");
            // Deal with an irate customer...
        }
    }
}

<<  Page 3 of 9  >>


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