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<!doctype html>
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<h1><i class="chapter">Part FIVE</i><br />
Exceptions and Assertions</h1>
<h1><i class="chapter">Chapter NINETEEN</i><br />
Exceptions</h1>
<p><br /></p>
<h3 style="text-align: center;"><i>Exam Objectives</i></h3>
<p style="text-align: center;"><i>Use try-catch and throw statements.<br /></i><i>Use catch, multi-catch, and finally clauses.<br /></i><i>Use Autoclose resources with a try-with-resources statement.<br /></i><i>Create custom exceptions and Auto-closeable resources.</i></p>
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<h2>Exception</h2>
<p>Errors can (and will) happen in any program. In Java, errors are represented by exceptions.</p>
<p>Basically, in Java there are three types of exception:</p>
<p><b><code>java.lang.Exception</code></b><br /> Extends from <code>java.lang.Throwable</code> and represents errors that are expected. In some cases, the program can recover itself from them. Some examples are: <code>IOException</code>, <code>ParseException</code>, <code>SQLException</code></p>
<p><b><code>java.lang.RuntimeException</code></b><br /> Extends from <code>java.lang.Exception</code> and represents unexpected errors generated at runtime. In most cases, the program cannot recover itself from them. Some examples are: <code>ArithmeticException</code>, <code>ClassCastException</code>, <code>NullPointerException</code></p>
<p><b><code>java.lang.Error</code></b><br /> Extends from <code>java.lang.Throwable</code> and represents serious problems or abnormal conditions that a program should not deal with. Some examples are: <code>AssertionError</code>, <code>IOError</code>, <code>LinkageError</code>, <code>VirtualMachineError</code></p>
<p><code>RuntimeException</code> and its subclasses are not required to be caught since they're not expected all the time. They're also called unchecked.</p>
<p>Exception and its subclasses (except for <code>RuntimeException</code>) are known as checked exceptions because the compiler has to check if they are caught at some point by a <code>try-catch</code> statement.</p>
<hr />
<h4><i>Try-Catch Block</i></h4>
<p><i>There's only one try block</i></p>
<p><i><code class="java hljs"><span class="hljs-keyword">try</span> {<br />
<span class="hljs-comment">// Code that may throw an exception</span><br />
} <span class="hljs-keyword">catch</span>(Exception e) {<br />
<span class="hljs-comment"> // Do something with the exception using</span><br />
<span class="hljs-comment"> // reference e</span><br />
}</code></i></p>
<p><i>There can be more than one catch block (one for each exception to catch)</i></p>
<hr />
<h2>Try-Catch Block</h2>
<p>A <code>try</code> block is used to enclose code that might throw an exception, it doesn't matter if it's a checked or an unchecked one.</p>
<p>A <code>catch</code> block is used to handle an exception. It defines the type of the exception and a reference to it.</p>
<p>Let's see an example:</p>
<p><code class="java hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Test</span></span> {<br />
<span class="hljs-function"><span class="hljs-keyword"> public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">main</span><span class="hljs-params">(String[] args)</span></span> {<br />
<span class="hljs-keyword"> int</span>[] arr = <span class="hljs-keyword">new</span> <span class="hljs-keyword">int</span>[<span class="hljs-number">3</span>];<br />
<span class="hljs-keyword"> for</span>(<span class="hljs-keyword">int</span> i = <span class="hljs-number">0</span>; i <= arr.length; i++) {<br />
arr[i] = i * <span class="hljs-number">2</span>;<br />
}<br />
System.out.println(<span class="hljs-string">"Done"</span>);<br />
}<br />
}</code></p>
<p>There's an error in the above program, can you see it?</p>
<p>In the last iteration of the loop, <code>i</code> will be <code>3</code>, and since arrays have zero-based indexes, an exception will be thrown at runtime:</p>
<p><code class="java hljs">Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: <span class="hljs-number">3</span> at com.example.Test.main(TestException.java:<span class="hljs-number">8</span>)</code></p>
<p>If an exception is not handled, the JVM provides a default exception handler that performs the following tasks:<br /> 1. It prints out exception description.<br /> 2. It prints the stack trace (hierarchy of methods where the exception occurred).<br /> 3. It causes the program to terminate.</p>
<p>However, if the exception is handled by in a <code>try-catch</code> block, the normal flow of the application is maintained and rest of the code is executed.</p>
<p><code class="java hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Test</span></span> {<br />
<span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">main</span><span class="hljs-params">(String[] args)</span></span> {<br />
<span class="hljs-keyword">try</span> {<br />
<span class="hljs-keyword">int</span>[] arr = <span class="hljs-keyword">new</span> <span class="hljs-keyword">int</span>[<span class="hljs-number">3</span>];<br />
<span class="hljs-keyword"> for</span>(<span class="hljs-keyword">int</span> i = <span class="hljs-number">0</span>; i <= arr.length; i++) {<br />
arr[i] = i * <span class="hljs-number">2</span>;<br />
}<br />
} <span class="hljs-keyword">catch</span>(ArrayIndexOutOfBoundsException e) {<br />
System.err.println(<span class="hljs-string">"Exception caught"</span>);<br />
}<br />
System.out.println(<span class="hljs-string">"Done"</span>);<br />
}<br />
}</code></p>
<p>The output:</p>
<p><code class="java hljs">Exception caught<br />
Done</code></p>
<p>This is an example of an unchecked exception. Again, they don't have to be caught, but catching them is certainly useful.</p>
<p>On the other hand, we have checked exceptions, which need to be surrounded by a <code>try</code> block if you don't want the compiler to complain. So this piece of code:</p>
<p><code class="java hljs">SimpleDateFormat sdf = <span class="hljs-keyword">new</span> SimpleDateFormat(<span class="hljs-string">"MM/dd"</span>);<br />
Date date = sdf.parse(<span class="hljs-string">"01-10"</span>); <span class="hljs-comment">// Compile-time error</span><br />
System.out.println(date);</code></p>
<p>Becomes this:</p>
<p><code class="java hljs"><span class="hljs-keyword">try</span> {<br />
SimpleDateFormat sdf = <span class="hljs-keyword">new</span> SimpleDateFormat(<span class="hljs-string">"MM/dd"</span>);<br />
Date date = sdf.parse(<span class="hljs-string">"01-10"</span>);<br />
System.out.println(date);<br />
} <span class="hljs-keyword">catch</span> (ParseException e) {<br />
System.err.println(<span class="hljs-string">"ParseException caught"</span>);<br />
}</code></p>
<p>Since, according to its signature, the parse method throws a <code>java.text.ParseException</code> (that extends directly from <code>java.lang.Exception</code>):</p>
<p><code class="java hljs"><span class="hljs-function"><span class="hljs-keyword">public</span> Date <span class="hljs-title">parse</span><span class="hljs-params">(String source)</span> <span class="hljs-keyword">throws</span> ParseException</span></code></p>
<p>The <code>throws</code> keyword indicates the exceptions that a method can throw. Only checked exceptions are required to be declared this way.</p>
<p>Now, remember not to confuse <code>throws</code> with <code>throw</code>. The latter will actually throw an exception:</p>
<p><code class="java hljs"><span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">myMethod</span><span class="hljs-params">()</span> <span class="hljs-keyword">throws</span> SQLException</span> {<br />
<span class="hljs-comment"> // Exceptions are created with the new operator<br />
// like any Java class</span><br />
<span class="hljs-keyword"> throw</span> <span class="hljs-keyword">new</span> SQLException();<br />
}</code></p>
<p>We can also catch the superclass directly:</p>
<p><code class="java hljs"><span class="hljs-keyword">try</span> {<br />
SimpleDateFormat sdf = <span class="hljs-keyword">new</span> SimpleDateFormat(<span class="hljs-string">"MM/dd"</span>);<br />
Date date = sdf.parse(<span class="hljs-string">"01-10"</span>); System.out.println(date);<br />
} <span class="hljs-keyword">catch</span> (Exception e) {<br />
System.err.println(<span class="hljs-string">"Exception caught"</span>);<br />
}</code></p>
<p>Although this is not recommended, since the above catch block will catch every exception (checked or unchecked) that could be possibly thrown by the code.</p>
<p>So it's better to catch both in this way:</p>
<p><code class="java hljs"><span class="hljs-keyword">try</span> {<br />
SimpleDateFormat sdf = <span class="hljs-keyword">new</span> SimpleDateFormat(<span class="hljs-string">"MM/dd"</span>);<br />
Date date = sdf.parse(<span class="hljs-string">"01-10"</span>);<br />
System.out.println(date);<br />
} <span class="hljs-keyword">catch</span> (ParseException e) {<br />
System.err.println(<span class="hljs-string">"ParseException caught"</span>);<br />
} <span class="hljs-keyword">catch</span> (Exception e) {<span class="hljs-number"><br /></span> System.err.println(<span class="hljs-string">"Exception caught"</span>);<br />
}</code></p>
<p>If an exception can be caught in more than one block, the exception will be caught in the first block defined.</p>
<p>However, we have to respect the hierarchy of the classes, if a superclass is defined before a subclass, a compile-time error is generated:</p>
<p><code class="java hljs"><span class="hljs-keyword">try</span> {<br />
SimpleDateFormat sdf = <span class="hljs-keyword">new</span> SimpleDateFormat(<span class="hljs-string">"MM/dd"</span>);<br />
Date date = sdf.parse(<span class="hljs-string">"01-10"</span>);<br />
System.out.println(date);<br />
} <span class="hljs-keyword">catch</span> (Exception e) {<br />
System.err.println(<span class="hljs-string">"Exception caught"</span>);<br />
} <span class="hljs-keyword">catch</span> (ParseException e) {<font color="#1C00CF"><br />
</font> System.err.println(<span class="hljs-string">"ParseException caught"</span>);<br />
}</code></p>
<p>An error is also generated if a <code>catch</code> block is defined for an exception that couldn't be thrown by the code in the <code>try</code> block:</p>
<p><code class="java hljs"><span class="hljs-keyword">try</span> {<br />
SimpleDateFormat sdf = <span class="hljs-keyword">new</span> SimpleDateFormat(<span class="hljs-string">"MM/dd"</span>);<br />
Date date = sdf.parse(<span class="hljs-string">"01-10"</span>);<br />
System.out.println(date);<br />
} <span class="hljs-keyword">catch</span> (SQLException e) { <span class="hljs-comment">// Compile-time error<br /></span> System.err.println(<span class="hljs-string">"ParseException caught"</span>);<br />
}</code></p>
<p>The reason of these two errors is that the code of both <code>catch</code> blocks will never be executed (it's unreachable, as the compiler says).</p>
<p>In one case, the <code>catch</code> block with the superclass will be executed for all exceptions that belong to that type and in the other case, the exception can never be possible thrown and the <code>catch</code> block can never be possible executed.</p>
<p>Finally, if the code that throws a checked exception is not inside a <code>try-catch</code> block, the method that contains that code must declare the exception in the <code>throws</code> clause.</p>
<p>In this case, the caller of the method must either catch the exception or also declare it in the <code>throws</code> clause and so on until the main method of the program is reached:</p>
<p><code class="java hljs"><span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Test</span></span> {<br />
<span class="hljs-function"><span class="hljs-keyword"> public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">main</span><span class="hljs-params">(String[] args)<font color="#000000"><br /></font></span><span class="hljs-keyword"><font color="#000000"> </font>throws</span> ParseException</span> {<br />
m1();<br />
}<br />
<span class="hljs-function"><span class="hljs-keyword"> private</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">m1</span><span class="hljs-params">()</span> <span class="hljs-keyword">throws</span> ParseException</span> {<br />
m2();<br />
}<br />
<span class="hljs-function"><span class="hljs-keyword">private</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">m2</span><span class="hljs-params">()</span> <span class="hljs-keyword">throws</span> ParseException</span> {<br />
m3();<br />
}<br />
<span class="hljs-function"><span class="hljs-keyword"> private</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">m3</span><span class="hljs-params">()</span> <span class="hljs-keyword">throws</span> ParseException</span> {<br />
m4();<br />
}<br />
<span class="hljs-function"><span class="hljs-keyword"> private</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">m4</span><span class="hljs-params">()</span> <span class="hljs-keyword">throws</span> ParseException</span> {<br />
SimpleDateFormat sdf =<br />
<span class="hljs-keyword"> new</span> SimpleDateFormat(<span class="hljs-string">"MM/dd"</span>);<br />
Date date = sdf.parse(<span class="hljs-string">"01-10"</span>);<br />
System.out.println(date);<br />
}<br />
}</code></p>
<hr />
<h4><i>Multi-Catch</i></h4>
<p><i><code class="java hljs"><span class="hljs-keyword">try</span> {<br />
<span class="hljs-comment"> // Code that may throw one or</span><br />
<span class="hljs-comment"> // two exceptions</span><br />
} <span class="hljs-keyword">catch</span>(Exception1 | Exception2 e) {<br />
<span class="hljs-comment"> // Do something with the caught</span><br />
<span class="hljs-comment"> // exception using reference e</span><br />
}</code></i></p>
<p><i>Catch either Exception1 or Exception2</i></p>
<h4><i>Finally</i></h4>
<p><i><code class="java hljs"><span class="hljs-keyword">try</span> {<br />
<span class="hljs-comment"> // Code that may throw an</span><br />
<span class="hljs-comment"> // exception</span> }<br />
<span class="hljs-keyword">finally</span> {<br />
<span class="hljs-comment"> // Block that is always executed</span><br />
}</code></i></p>
<p><i>The catch block is optional. You can have both or either a catch block or a finally block</i></p>
<p><i>The finally block is always executed, no matter if an exception is thrown in the try block, re-thrown inside the catch block, or not caught at all</i></p>
<hr />
<h2>Multi-Catch and Finally</h2>
<p>Consider something like the following code:</p>
<p><code class="java hljs"><span class="hljs-keyword">int</span> res = <span class="hljs-number">0</span>;<br />
<span class="hljs-keyword">try</span> {<br />
<span class="hljs-keyword"> int</span>[] arr = <span class="hljs-keyword">new</span> <span class="hljs-keyword">int</span>[<span class="hljs-number">2</span>];<br />
res = (arr[<span class="hljs-number">1</span>] != <span class="hljs-number">0</span>) ? <span class="hljs-number">10</span> / arr[<span class="hljs-number">1</span>] : <span class="hljs-number">10</span> * arr[<span class="hljs-number">2</span>];<br />
} <span class="hljs-keyword">catch</span> (ArithmeticException e) {<br />
e.printStackTrace();<br />
<span class="hljs-keyword"> return</span> res;<br />
} <span class="hljs-keyword">catch</span> (IndexOutOfBoundsException e) {<br />
e.printStackTrace();<br />
<span class="hljs-keyword"> return</span> res;<br />
}<br />
<span class="hljs-keyword">return</span> res;</code></p>
<p>Isn't that ugly? I mean to have two catch blocks with the same code. Fortunately, the <code>multi-catch</code> block allows us to catch two or more exception with a single <code>catch</code> block:</p>
<p><code class="java hljs"><span class="hljs-keyword">try</span> {<br />
...<br />
} <span class="hljs-keyword">catch</span> (ArithmeticException | IndexOutOfBoundsException e) {<br />
e.printStackTrace();<br />
<span class="hljs-keyword"> return</span> res;<br />
}</code></p>
<p>Think of the pipe character as an <b>OR</b> operator. Also, notice there's only one variable at the end of the <code>catch</code> clause for all the exceptions declared. If you want to differentiate between exceptions, you can use the <code>instanceof</code> operator:</p>
<p><code class="java hljs"><span class="hljs-keyword">try</span> {<br />
...<br />
} <span class="hljs-keyword">catch</span> (ArithmeticException | IndexOutOfBoundsException e) {<br />
<span class="hljs-keyword">if</span>(e <span class="hljs-keyword">instanceof</span> ArithmeticException) {<br />
<span class="hljs-comment"> // Do something else if the exception type</span><br />
<span class="hljs-comment"> // is ArithmeticException</span><br />
}<br />
e.printStackTrace();<br />
<span class="hljs-keyword"><font color="#000000"> </font>return</span> res;<br />
}</code></p>
<p>Also, the variable is treated as final, which means that you can't reassign (why would you want anyway?):</p>
<p><code class="java hljs"><span class="hljs-keyword">try</span> {<br />
...<br />
} <span class="hljs-keyword">catch</span> (ArithmeticException | IndexOutOfBoundsException e) {<br />
<span class="hljs-keyword">if</span>(e <span class="hljs-keyword">instanceof</span> ArithmeticException) {<br />
<span class="hljs-comment">// Compile-time error</span><br />
e = <span class="hljs-keyword">new</span> ArithmethicException(<span class="hljs-string">"My Exception"</span>);<br />
}<br />
} <span class="hljs-keyword">catch</span>(Exception e) {<br />
e = <span class="hljs-keyword">new</span> Exception(<span class="hljs-string">"My Exception"</span>); <span class="hljs-comment">// It compiles!</span><br />
<span class="hljs-keyword"> throw</span> e;<br />
}</code></p>
<p>One last rule. You cannot combine subclasses and their superclasses in the same <code>multi-catch</code> block:</p>
<p><code class="java hljs"><span class="hljs-keyword">try</span> {<br />
...<br />
} <span class="hljs-keyword">catch</span> (ArithmeticException | RuntimeException e) {<br />
<span class="hljs-comment"> // The above line generates a compile-time error<br/> // because ArithmeticException is a subclass of<br/> // RuntimeException</span><br/>
e.printStackTrace();<br />
<span class="hljs-keyword"> return</span> res;<br />
}</code></p>
<p>This is similar to the case when a superclass is declared in a <code>catch</code> block before the subclass. The code is redundant, the superclass will always catch the exception.</p>
<p>Back to this piece of code:</p>
<p><code class="java hljs"><span class="hljs-keyword">int</span> res = <span class="hljs-number">0</span>;<br />
<span class="hljs-keyword">try</span> {<br />
<span class="hljs-keyword"> int</span>[] arr = <span class="hljs-keyword">new</span> <span class="hljs-keyword">int</span>[<span class="hljs-number">2</span>];<br />
res = (arr[<span class="hljs-number">1</span>] != <span class="hljs-number">0</span>) ? <span class="hljs-number">10</span> / arr[<span class="hljs-number">1</span>] : <span class="hljs-number">10</span> * arr[<span class="hljs-number">2</span>];<br />
} <span class="hljs-keyword">catch</span> (ArithmeticException | IndexOutOfBoundsException e) {<br />
e.printStackTrace();<br />
<span class="hljs-keyword"> return</span> res;<br />
}<br />
<span class="hljs-keyword">return</span> res;</code></p>
<p>Since the value of <code>res</code> is always returned, we can use a <code>finally</code> block:</p>
<p><code class="java hljs"><span class="hljs-keyword">try</span> {<br />
...<br />
} <span class="hljs-keyword">catch</span> (ArithmeticException | IndexOutOfBoundsException e) {<br />
e.printStackTrace();<br />
} <span class="hljs-keyword">finally</span> {<br />
<span class="hljs-keyword"> return</span> res;<br />
}</code></p>
<p>The <code>finally</code> block is <b>ALWAYS</b> executed, even when an exception is caught or when either the <code>try</code> or <code>catch</code> block contains a return statement. For that reason, it's commonly used to close resources like database connections or file handlers.</p>
<p>There's only one exception to this rule. If you call <code>System.exit()</code>, the program will terminate abnormally without executing the <code>finally</code> block. However, as it's considered bad practice to call <code>System.exit()</code>, this rarely happens.</p>
<hr />
<h4><i>Try-With-Resources</i></h4>
<p><i>Resource that is closed automatically</i></p>
<p><i><code class="java hljs"><span class="hljs-keyword">try</span> (AutoCloseableResource r = <span class="hljs-keyword">new</span> AutoCloseableResource()) {<br />
<span class="hljs-comment"> // Code that may thrown an exception</span><br />
} <span class="hljs-keyword">catch</span>(Exception e) {<br />
<span class="hljs-comment"> // Handle exception</span><br />
} <span class="hljs-keyword">finally</span> {<br />
<span class="hljs-comment"> // Always executes</span><br />
}</code></i></p>
<p><i>Resource is closed after the try block finishes</i></p>
<p><i>Catch and finally blocks are both optional in a try-with-resources</i></p>
<hr />
<h2>try-with-resources</h2>
<p>As we said before, the <code>finally</code> block is generally used to close resources. Since Java 7, we have the <code>try-with-resources</code> block, in which the <code>try</code> block, one or more resources are declared so they can be closed without doing it explicitly in a <code>finally</code> block:</p>
<p><code class="java hljs"><span class="hljs-keyword">try</span> (BufferedReader br =<br />
<span class="hljs-keyword"> new</span> BufferedReader(<span class="hljs-keyword">new</span> FileReader("/file.txt"))) {<br />
<span class="hljs-keyword">int</span> value = <span class="hljs-number">0</span>;<br />
<span class="hljs-keyword"> while</span>((value = br.read()) != -<span class="hljs-number">1</span>) {<br />
System.out.println((<span class="hljs-keyword">char</span>)value);<br />
}<br />
} <span class="hljs-keyword">catch</span> (IOException e) {<br />
e.printStackTrace();<br />
}</code></p>
<p>In the example, the <code>BufferedReader</code> is closed after the <code>try</code> block finishes its execution. This would be equivalent to:</p>
<p><code class="java hljs">BufferedReader br = <span class="hljs-keyword">null</span>;<br />
<span class="hljs-keyword">try</span> {<br />
<span class="hljs-keyword"> int</span> value = <span class="hljs-number">0</span>;<br />
br = <span class="hljs-keyword">new</span> BufferedReader(<span class="hljs-keyword">new</span> FileReader("/file.txt"));<br />
<span class="hljs-keyword"> while</span>((value = br.read()) != -<span class="hljs-number">1</span>) {<br />
System.out.println((<span class="hljs-keyword">char</span>)value);<br />
}<br />
} <span class="hljs-keyword">catch</span> (IOException e) {<br />
e.printStackTrace();<br />
} <span class="hljs-keyword">finally</span> {<br />
<span class="hljs-keyword"> try</span> {<br />
<span class="hljs-keyword"> if</span> (br != <span class="hljs-keyword">null</span>) br.close();<br />
} <span class="hljs-keyword">catch</span> (IOException e) {<br />
e.printStackTrace();<br />
}<br />
}</code></p>
<p>If you declare more than one resource, they have to be separated by a semicolon:</p>
<p><code class="java hljs"><span class="hljs-keyword">try</span> (FileReader fr = <span class="hljs-keyword">new</span> FileReader("/file.txt");<br />
BufferedReader br = <span class="hljs-keyword">new</span> BufferedReader(fr)) {<br />
...<br />
}</code></p>
<p>Also, resources declared inside a <code>try-with-resources</code> cannot be used outside this block (first reason, they're out of scope, second reason, they're closed after the <code>try</code> block ends):</p>
<p><code class="java hljs"><span class="hljs-keyword">try</span> (BufferedReader br = <br />
<span class="hljs-keyword"><font color="#000000"> </font>new</span> BufferedReader(<span class="hljs-keyword">new</span> FileReader("/file.txt"))) {<br />
...<br />
}<br />
String line = br.readLine(); <span class="hljs-comment">// Compile-time error</span></code></p>
<p>Now, don't think any class will work in a <code>try-with-resources</code>.</p>
<p><code class="java hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyResource</span></span> {<br />
<span class="hljs-function"><span class="hljs-keyword"> void</span> <span class="hljs-title">useResource</span><span class="hljs-params">()</span></span> { }<br />
}<br />
...<br />
<span class="hljs-keyword">try</span> (MyResource r = <span class="hljs-keyword">new</span> MyResource()) { <span class="hljs-comment">// Compile-time error</span><br />
r.useResource()<br />
}</code></p>
<p>The class(es) used in a <code>try-with-resources</code> block must implement one of the following interfaces:</p>
<ul>
<li><code>java.lang.AutoClosable</code></li>
<li><code>java.io.Closable</code></li>
</ul>
<p>They both declare a <code>close()</code> method, and the only practical difference between these two interfaces is that the close method of the <code>Closeable</code> interface only throws exceptions of type <code>IOException</code>:</p>
<p><code class="java hljs"><span class="hljs-function"><span class="hljs-keyword">void</span> <span class="hljs-title">close</span><span class="hljs-params">()</span> <span class="hljs-keyword">throws</span> IOException</span>;</code></p>
<p>While the <code>close()</code> method of the <code>AutoCloseable</code> interface throws exceptions of type <code>Exception</code> (in other words, it can throw almost any kind of exception):</p>
<p><code class="java hljs"><span class="hljs-function"><span class="hljs-keyword">void</span> <span class="hljs-title">close</span><span class="hljs-params">()</span> <span class="hljs-keyword">throws</span> Exception</span>;</code></p>
<p>So the <code>close()</code> method is called automatically, and if this method actually throws and exception, we can catch it in the <code>catch</code> block.</p>
<p><code class="java hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MyResource</span> <span class="hljs-keyword">implements</span> <span class="hljs-title">AutoCloseable</span></span> {<br />
<span class="hljs-function"><span class="hljs-keyword"> public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">close</span><span class="hljs-params">()</span> <span class="hljs-keyword">throws</span> Exception</span> {<br />
<span class="hljs-keyword"> int</span> x = <span class="hljs-number">0</span>;<br />
<span class="hljs-comment"> //...</span><br />
<span class="hljs-keyword"> if</span>(x == <span class="hljs-number">1</span>) <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> Exception(<span class="hljs-string">"Close Exception"</span>);<br />
}<br />
<span class="hljs-function"><span class="hljs-keyword">void</span> <span class="hljs-title">useResource</span><span class="hljs-params">()</span></span> {}<br />
}<br />
...<br />
<span class="hljs-keyword">try</span> (MyResource r = <span class="hljs-keyword">new</span> MyResource()) { <span class="hljs-comment">// Problem gone!</span><br />
r.useResource();<br />
} <span class="hljs-keyword">catch</span> (Exception e) {<br />
e.printStackTrace();<br />
}</code></p>
<p>But what happens if the <code>try</code> block also throws an exception?</p>
<p>Well, the result is that the exception from the <code>try</code> block "wins" and the exceptions from the <code>close()</code> method are "suppressed".</p>
<p>In fact, you can retrieve these suppressed exceptions by calling the <code>Throwable[] java.lang.Throwable.getSuppressed()</code> method from the exception thrown by the <code>try</code> block.</p>
<p><code class="java hljs"><span class="hljs-keyword">try</span> (Resource r = <span class="hljs-keyword">new</span> Resource()) {<br />
r.useResource();<br />
<span class="hljs-keyword"> throw</span> <span class="hljs-keyword">new</span> Exception(<span class="hljs-string">"Exception inside try"</span>);<br />
} <span class="hljs-keyword">catch</span> (Exception e) {<br />
System.err.println(e.getMessage());<br />
Stream.of(e.getSuppressed())<br />
.forEach(t -> System.err.println(t.getMessage()));<br />
}</code></p>
<p>The output (assuming the <code>close()</code> method throws an exception):</p>
<p><code class="java hljs">Exception inside try<br />
Close Exception</code></p>
<h2>Custom exceptions</h2>
<p>Since exceptions are classes, we can just extend any exception of the language to create our own exceptions.</p>
<p>If you want to force the catching of your exception, extend from <code>Exception</code> or one of its subclasses. If you don't want to force it, extend from <code>RuntimeException</code> or one of its subclasses.</p>
<p><code class="java hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">TooHardException</span> <span class="hljs-keyword">extends</span> <span class="hljs-title">Exception</span></span> {<br />
<span class="hljs-function"><span class="hljs-keyword"> public</span> <span class="hljs-title">TooHardException</span><span class="hljs-params">(Exception e)</span></span> {<br />
<span class="hljs-keyword"> super</span>(e);<br />
}<br />
}<br />
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">TooEasyException</span> <span class="hljs-keyword">extends</span> <span class="hljs-title">RuntimeException</span></span> { }</code></p>
<p>As you can see, it's a convention to add <code>Exception</code> to your classes' name. The <code>Error</code> and <code>Throwable</code> classes are not actually used for custom exceptions.</p>
<p>The main members of the <code>Exception</code> class that you'd want to know are:</p>
<table border="1" width="100%">
<tr>
<td><b>Description</b></td>
<td><b>Constructor/Method</b></td>
</tr>
<tr>
<td>Default constructor</td>
<td><code>Exception()</code></td>
</tr>
<tr>
<td>Constructor that takes a message</td>
<td><code>Exception(String)</code></td>
</tr>
<tr>
<td>Constructor that takes another exception (that represents the cause)</td>
<td><code>Exception(Throwable)</code></td>
</tr>
<tr>
<td>Returns exception's message</td>
<td><code>String getMessage()</code></td>
</tr>
<tr>
<td>Returns (if any) the exception's cause</td>
<td><code>Throwable getCause()</code></td>
</tr>
<tr>
<td>Returns the list of suppressed exceptions</td>
<td><code>Throwable[] getSuppressed()</code></td>
</tr>
<tr>
<td>Prints the stack trace (cause and suppressed exceptions included)</td>
<td><code>void printStackTrace()</code></td>
</tr>
</table>
<h2>Key Points</h2>
<ul>
<li>In Java, there are three types of exception
<ul>
<li><code>java.lang.Exception</code></li>
<li><code>java.lang.RuntimeException</code></li>
<li><code>java.lang.Error</code></li>
</ul>
</li>
<li><code>RuntimeException</code> and its subclasses are not required to be caught since they're not expected all the time. They're also called unchecked.</li>
<li><code>Exception</code> and its subclasses (except for <code>RuntimeException</code>) are known as checked exceptions because the compiler has to check if they are caught at some point by a <code>try-catch</code> statement.</li>
<li>If an exception can be caught in more than one block, the exception will be caught in the first block defined.</li>
<li>However, we have to respect the hierarchy of the classes, if a superclass is defined before a subclass, a compile-time error is generated.</li>
<li>If the code that throws a checked exception is not inside a <code>try-catch</code> block, the method that contains that code must declare the exception in the <code>throws</code> clause.</li>
<li>In this case, the caller of the method must either catch the exception or also declare it in the <code>throws</code> clause and so on until the main method of the program is reached.</li>
<li>The <code>multi-catch</code> block allows us to catch two or more unrelated exceptions with a single <code>catch</code> block: <code class="java hljs"><span class="hljs-keyword">try</span> {<br />
<span class="hljs-comment"> // ...</span><br />
} <span class="hljs-keyword">catch</span>(Exception1 | Exception2 e) {<br />
<span class="hljs-comment"> // ...</span><br />
}</code></li>
<li>The finally block is <b>ALWAYS</b> executed, even when an exception is caught or when either the try or catch block contains a <code>return</code> statement.</li>
<li>In a <code>try-with-resources</code> block, one or more resources are declared so they can be closed automatically after the <code>try</code> block ends just by implementing <code>java.lang.AutoCloseable</code> or <code>java.io.Closeable</code>: <code class="java hljs"><span class="hljs-keyword">try</span> (Resource r = <span class="hljs-keyword">new</span> Resource()) {<br />
<span class="hljs-comment"> //...</span><br />
} <span class="hljs-keyword">catch</span>(Exception e) { }</code></li>
<li>When using a <code>try-with-resources</code> block, <code>catch</code> and <code>finally</code> are optional. You can create your own exceptions just by extending from <code>java.lang.Exception</code> (for checked exceptions) or <code>java.lang.RuntimeException</code> (for unchecked exceptions).</li>
</ul>
<h2>Self Test</h2>
<p>1. Given:</p>
<p><code class="java hljs"><span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Question_19_1</span></span> {<br />
<span class="hljs-function"><span class="hljs-keyword"> protected</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">int</span> <span class="hljs-title">m1</span><span class="hljs-params">()</span></span> {<br />
<span class="hljs-keyword">try</span> {<br />
<span class="hljs-keyword"> throw</span> <span class="hljs-keyword">new</span> RuntimeException();<br />
} <span class="hljs-keyword">catch</span>(RuntimeException e) {<br />
<span class="hljs-keyword"> return</span> <span class="hljs-number">1</span>;<br />
} <span class="hljs-keyword">finally</span> {<br />
<span class="hljs-keyword"> return</span> <span class="hljs-number">2</span>;<br />
}<br />
}<br />
<span class="hljs-function"><span class="hljs-keyword"> public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">main</span><span class="hljs-params">(String[] args)</span></span> {<br />
System.out.println(m1());<br />
}<br />
}</code></p>
<p>What is the result?<br /> A. <code>1</code><br /> B. <code>2</code><br /> C. Compilation fails<br /> D. An exception occurs at runtime</p>
<p>2. Given:</p>
<p><code class="java hljs"><span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Question_19_2</span></span> {<br />
<span class="hljs-function"><span class="hljs-keyword"> public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">main</span><span class="hljs-params">(String[] args)</span></span> {<br />
<span class="hljs-keyword"> try</span> {<br />
<span class="hljs-comment"> // Do nothing</span><br />
} <span class="hljs-keyword">finally</span> {<br />
<span class="hljs-comment"> // Do nothing</span><br />
}<br />
}<br />
}</code></p>
<p>What of the following is true?<br /> A. The code doesn't compile correctly<br /> B. The code would compile correctly if we add a <code>catch</code> block<br /> C. The code would compile correctly if we remove the <code>finally</code> block<br /> D. The code compiles correctly as it is</p>
<p>3. Which of the following statements are true?<br /> A. In a <code>try-with-resources</code>, the <code>catch</code> block is required.<br /> B. The <code>throw</code>s keyword is used to throw an exception.<br /> C. In a <code>try-with-resources</code> block, if you declare more than one resource, they have to be separated by a semicolon.<br /> D. If a <code>catch</code> block is defined for an exception that couldn't be thrown by the code in the <code>try</code> block, a compile-time error is generated.</p>
<p>4. Given:</p>
<p><code class="java hljs"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Connection</span> <span class="hljs-keyword">implements</span> <span class="hljs-title">java</span>.<span class="hljs-title">io</span>.<span class="hljs-title">Closeable</span></span> {<br />
<span class="hljs-function"><span class="hljs-keyword"> public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">close</span><span class="hljs-params">()</span> <span class="hljs-keyword">throws</span> IOException</span> {<br />
<span class="hljs-keyword"> throw</span> <span class="hljs-keyword">new</span> IOException(<span class="hljs-string">"Close Exception"</span>);<br />
}<br />
}<br />
<br />
<span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Question_19_4</span></span> {<br />
<span class="hljs-function"><span class="hljs-keyword"> public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">main</span><span class="hljs-params">(String[] args)</span></span> {<br />
<span class="hljs-keyword"> try</span> (Connection c = <span class="hljs-keyword">new</span> Connection()) {<br />
<span class="hljs-keyword"> throw</span> <span class="hljs-keyword">new</span> RuntimeException(<span class="hljs-string">"RuntimeException"</span>);<br />
} <span class="hljs-keyword">catch</span> (IOException e) {<br />
System.err.println(e.getMessage());<br />
}<br />
}<br />
}</code></p>
<p>What is the result?<br /> A. Close <code>Exception</code><br /> B. <code>RuntimeException</code><br /> C. <code>RuntimeException</code> and then <code>CloseException</code><br /> D. Compilation fails<br /> E. The stack trace of an uncaught exception is printed</p>
<p>5. Which of the following exceptions are direct subclasses of <code>RuntimeException</code>?<br /> A. <code>java.io.FileNotFoundException</code><br /> B. <code>java.lang.ArithmeticException</code><br /> C. <code>java.lang.ClassCastException</code><br /> D. <code>java.lang.InterruptedException</code></p>
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