The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
This section describes how to use the three exception handler components the try
, catch
, and finally
blocks to write an exception handler. Then, the try-
with-resources statement, introduced in Java SE 7, is explained. The try-
with-resources statement is particularly suited to situations that use Closeable
resources, such as streams.
The last part of this section walks through an example and analyzes what occurs during various scenarios.
The following example defines and implements a class named ListOfNumbers
. When constructed, ListOfNumbers
creates an ArrayList
that contains 10 Integer
elements with sequential values 0 through 9. The ListOfNumbers
class also defines a method named writeList
, which writes the list of numbers into a text file called OutFile.txt
. This example uses output classes defined in java.io
, which are covered in
Basic I/O.
// Note: This class will not compile yet. import java.io.*; import java.util.List; import java.util.ArrayList; public class ListOfNumbers { private List<Integer> list; private static final int SIZE = 10; public ListOfNumbers () { list = new ArrayList<Integer>(SIZE); for (int i = 0; i < SIZE; i++) { list.add(new Integer(i)); } } public void writeList() { // The FileWriter constructor throws IOException, which must be caught. PrintWriter out = new PrintWriter(new FileWriter("OutFile.txt")); for (int i = 0; i < SIZE; i++) { // The get(int) method throws IndexOutOfBoundsException, which must be caught. out.println("Value at: " + i + " = " + list.get(i)); } out.close(); } }
The first line in boldface is a call to a constructor. The constructor initializes an output stream on a file. If the file cannot be opened, the constructor throws an IOException
. The second boldface line is a call to the ArrayList
class's get
method, which throws an IndexOutOfBoundsException
if the value of its argument is too small (less than 0) or too large (more than the number of elements currently contained by the ArrayList
).
If you try to compile the
class, the compiler prints an error message about the exception thrown by the ListOfNumbers
FileWriter
constructor. However, it does not display an error message about the exception thrown by get
. The reason is that the exception thrown by the constructor, IOException
, is a checked exception, and the one thrown by the get
method, IndexOutOfBoundsException
, is an unchecked exception.
Now that you're familiar with the ListOfNumbers
class and where the exceptions can be thrown within it, you're ready to write exception handlers to catch and handle those exceptions.