Java’s Exception Hierarchy

Section 10.3 Java’s Exception Hierarchy

Subsection 10.3.1 Java Predefined Exceptions

The Java class library contains a number of predefined exceptions, some of which are shown in Figure 10.3.1. The most general type of exception, java.lang.Exception, is located in the java.lang package, but most of its subclasses are contained in other packages. Some of the various IOException classes are contained in the java.io package, while others are contained in the java.net package. In general, exception classes are placed in the package that contains the methods that throw those exceptions.

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Figure 10.3.1. Part of Java’s exception hierarchy. All subclasses of `RuntimeException` are known as *unchecked* exceptions. Java programs are not required to catch these exceptions.
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Each of the classes in Figure 10.3.1 identifies a particular type of exception, and each is a subclass of the Exception class. Obviously a subclass defines a more specific exception than its superclass. Thus, both ArrayIndexOutOfBoundsException and StringIndexOutOfBoundsException are more specific than IndexOutOfBoundsException.

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Table 10.3.2. Some of Java’s important exceptions.

Class	Description
`ArithmeticException`	Division by zero or some other kind of arithmetic problem
`ArrayIndexOutOfBoundsException`	An array index is less than zero or greater than or equal to the array’s length
`FileNotFoundException`	Reference to a file that cannot be found
`IllegalArgumentException`	Calling a method with an improper argument
`IndexOutOfBoundsException`	An array or string index is out of bounds
`NullPointerException`	Reference to an object that has not been instantiated
`NumberFormatException`	Use of an illegal number format, such as when calling a method
`StringIndexOutOfBoundsException`	A `String` index is less than zero or greater than or equal to the `String`’s length

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Table 10.3.2 gives a brief summary of some of the most important exceptions. You’ve undoubtedly encountered some of these exceptions, because they are thrown by methods we have used repeatedly in programming examples. Table 10.3.3 summarizes the exceptions raised by some of the methods we’ve used most frequently.

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Table 10.3.3. Some of Java’s important exceptions by method.

Class	Method	Exception Raised	Description
`Double`	`valueOf(String)`	`NumberFormatException`	The `String` is not a `double`
`Integer`	`parseInt(String)`	`NumberFormatException`	The `String` is not a `int`
`String`	`String(String)`	`NullPointerException`	The `String` is `null`
	`indexOf(String)`	`NullPointerException`	The `String` is `null`
	`lastIndexOf(String)`	`NullPointerException`	The `String` is `null`
	`charAt(int)`	`StringIndexOutOfBoundsException`	The `int` is not a valid index
	`substring(int)`	`StringIndexOutOfBoundsException`	The `int` is not a valid index
	`substring(int,int)`	`StringIndexOutOfBoundsException`	An `int` is not a valid index

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Exercises Self-Study Exercise

1. Exceptions Matching.

What type of exception would be thrown for the following statements?

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Integer.parseInt("26.2");
NumberFormatException
String s; s.indexOf(’a’);
NullPointerException
String s = "hello"; s.charAt(5);
StringIndexOutOfBoundsException

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Subsection 10.3.2 Checked and Unchecked Exceptions

Java’s exception hierarchy is divided into two types of exceptions. A checked exception is one that can be analyzed by the Java compiler. Checked exceptions are thrown by methods such as the BufferedReader.readLine() method, in which there is a substantial likelihood that something might go wrong. When the compiler encounters one of these method calls, it checks whether the program either handles or declares the exception. Compile-time checking for these exceptions is designed to reduce the number of exceptions that are not properly handled within a program. This improves the security of Java programs.

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Principle 10.3.4. Checked Exceptions.

A checked exception, such as an IOException, must either be handled or declared within the program.

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Subsection 10.3.3 The `throws` Clause

The IOException, which we encountered in Chapter 4 , is a checked exception. The Java compiler knows that readLine() is a method that can throw an IOException. A method that contains an expression that might throw a checked exception must either handle the exception or declare it. Otherwise, the compiler would generate a syntax error. The simplest way to avoid such a syntax error is to declare the exception, in our case that means qualifying the method header with the expression throws IOException.

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In general, any method that contains an expression that might throw a checked expression must declare the exception. However, because one method can call another method, declaring exceptions can get a little tricky. If a method calls another method that contains an expression that might throw an unchecked exception, then both methods must have a throws clause. For example, consider the following program:

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In this case, the doRead() method contains a readLine() expression, which might throw an IOException. Therefore, the doRead() method must declare that it

throws
IOException

. However, because doRead() is called by main(), the main() method must also declare the IOException.

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Principle 10.3.5. Where to Use `throws`.

Unless a checked exception, such as an IOException, is caught and handled by a method, it must be declared with a throws clause within the method and within any method that calls that method.

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The alternative approach would be to catch the IOException within the body of the method. We will discuss this approach in the next section.

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Subsection 10.3.4 Unchecked Exceptions

An unchecked exception is any exception belonging to a subclass of RuntimeException( Figure 10.3.1). Unchecked exceptions are not checked by the compiler. The possibility that some statement or expression will lead to an ArithmeticException or NullPointerException is extremely difficult to detect at compile time. The designers of Java decided that forcing programmers to declare such exceptions would not significantly improve the correctness of Java programs.

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Therefore, unchecked exceptions do not have to be handled within a program. And they do not have to be declared in a throws clause. As shown in the chapter’s early divide-by-zero exception example, unchecked exceptions are handled by Java’s default exception handlers, unless your program takes specific steps to handle them directly. In many cases leaving the handling of such exceptions up to Java may be the best course of action, as we will see Section 10.5.

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Principle 10.3.6. Unchecked Exceptions.

An unchecked exception—one belonging to some subclass of RunTimeException—does not have to be caught within your program.

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Subsection 10.3.5 The `Exception`Class

The java.lang.Exception class itself is very simple, consisting of just two constructor methods (Figure 10.3.7). The Throwable class, from which Exception is derived, is the root class of Java’s exception and error hierarchy. It contains definitions for the getMessage() and printStackTrace() methods, which are two methods that we will use frequently in our error-handling routines.

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Subsection 10.3.6 Self-Study Exercises

Exercise 10.3.8. Unchecked Exceptions.

Which of the following are examples of unchecked exceptions?

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IOException
IOException is unchecked.
IndexOutOfBoundsException
IndexOutOfBoundsException is unchecked because it is a subclass of RuntimeException.
NullPointerException
NullPointerException is unchecked because it is a subclass of RuntimeException.
ClassNotFoundException
This is a checked exception
NumberFormatException
NumberFormatException is unchecked because it is a subclass of RuntimeException.

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