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<TITLE>try/catch Statement (Java Developers Almanac Example)
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<h3>e1087. try/catch Statement</h3>
The <code>try</code>/<code>catch</code> statement encloses some code and is used to
handle errors and exceptions that might occur in that code. Here is
the general syntax of the <code>try</code>/<code>catch</code> statement:
<pre> try {
<font color="#0066ff"><i>body-code</i></font>
} catch (<font color="#0066ff"><i>exception-classname</i></font> <font color="#0066ff"><i>variable-name</i></font>) {
<font color="#0066ff"><i>handler-code</i></font>
}
</pre>
The <code>try</code>/<code>catch</code> statement has four parts. The
<font color="#0066ff"><i>body-code</i></font> contains code that might throw the exception that we
want to handle. The <font color="#0066ff"><i>exception-classname</i></font> is the class name of
the exception we want to handle. The <font color="#0066ff"><i>variable-name</i></font> specifies a
name for a variable that will hold the exception object if the
exception occurs. Finally, the <font color="#0066ff"><i>handler-code</i></font> contains the code
to execute if the exception occurs. After the handler-code executes,
execution of the thread continues after the <code>try</code>/<code>catch</code>
statement. Here is an example of code that tries to create a file in
a non-existent directory which results in an <code>IOException</code>.
<pre> String filename = <font color="#0066ff"><i>"/nosuchdir/myfilename"</i></font>;
try {
// Create the file
new File(filename).createNewFile();
} catch (IOException e) {
// Print out the exception that occurred
System.out.println("Unable to create "+filename+": "+e.getMessage());
}
// Execution continues here after the IOException handler is executed
</pre>
Here's the output:
<pre> Unable to create /nosuchdir/myfilename: The system cannot find the path specified
</pre>
It is possible to specify more than one exception handler in a
<code>try</code>/<code>catch</code> statement. When an exception occurs, each handler
is checked in order (i.e. top-down) and the handler that first matches
is executed. The following example registers a name at a website. The
example code can throw two kinds of exceptions -- one if the URL is
invalid and one if the web page is not accessible. This example uses
an invalid URL which results in a <code>MalformedURLException</code>.
<pre> // This URL string is deliberately missing the http: protocol to cause an exception
String urlStr = <font color="#0066ff"><i>"xeo.com:90/register.jsp?name=joe"</i></font>;
try {
// Get the image
URL url = new URL(urlStr);
InputStream is = url.openStream();
is.close();
} catch (MalformedURLException e) {
// Print out the exception that occurred
System.out.println("Invalid URL "+urlStr+": "+e.getMessage());
} catch (IOException e) {
// Print out the exception that occurred
System.out.println("Unable to execute "+urlStr+": "+e.getMessage());
}
</pre>
Here's the output:
<pre> Invalid URL xeo.com:90/register.jsp?name=joe: no protocol: xeo.com:90/register.jsp?name=joe
</pre>
Here is the same example with a valid URL. The URL refers to a
non-existant web page which results in a <code>IOException</code>.
<pre> urlStr = <font color="#0066ff"><i>"http://xeo.com:90/register.jsp?name=joe"</i></font>;
try {
URL url = new URL(urlStr);
InputStream is = url.openStream();
is.close();
} catch (MalformedURLException e) {
// Print out the exception that occurred
System.out.println("Invalid URL "+urlStr+": "+e.getMessage());
} catch (IOException e) {
// Print out the exception that occurred
System.out.println("Unable to execute "+urlStr+": "+e.getMessage());
}
</pre>
Here's the output:
<pre> Unable to execute http://xeo.com:90/register.jsp?name=joe: Connection refused: connect
</pre>
When an exception occurs, the exception is compared with the specified
exception class name using the <code>instanceof</code> comparator. This means
that if the handler specifies a class name E, then any exception whose
class either equals E or is a subclass of E, matches the handler. The
following example catches either <code>MalformedURLException</code> or
<code>IOException</code> using <code>Exception</code> since both exceptions are
subclasses of <code>Exception</code>.
<pre> urlStr = <font color="#0066ff"><i>"http://xeo.com:90/register.jsp?name=joe"</i></font>;
try {
URL url = new URL(urlStr);
InputStream is = url.openStream();
is.close();
} catch (Exception e) {
// Print out the exception that occurred
System.out.println(urlStr+": "+e.getMessage());
}
</pre>
Here's the output:
<pre> http://xeo.com:90/register.jsp?name=joe: Connection refused: connect
</pre>
<P> If an exception occurs and does not match any of the handlers,
the exception is passed to any enclosing <code>try</code>/<code>catch</code>
statements and up the call chain until it is caught by some
handler. If the exception is not handled by any handler, it becomes an
<font color="#0066ff"><i>uncaught exception</i></font> and the thread is immediately terminated. In
the following example, a <code>NullPointerException</code> is thrown, which is
not caught by <code>MalformedURLException</code> and so becomes an
uncaught exception.
<pre> // This string is deliberately set to null to cause an exception
urlStr = null;
try {
int len = urlStr.length(); // Causes a NullPointerException
URL url = new URL(urlStr);
} catch (MalformedURLException e) {
// Print out the exception that occurred
System.out.println("Invalid URL "+urlStr+": "+e.getMessage());
}
</pre>
The result of an uncaught exception is a stack trace that identifies
the line of code that threw the exception. The output looks something
like:
<pre> Exception in thread "main" java.lang.NullPointerException
at MyClass.mymethod(MyClass.java:162)
</pre>
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<b>Related Examples</b></font></td>
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e1086. <a class="eglink" href="If.html?l=rel">
if/else Statement
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e1088. <a class="eglink" href="CatchThrowable.html?l=rel">
Catching all Errors and Exceptions
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