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    <p>To define a <strong>checked</strong> exception you create a subclass (or hierarchy of subclasses) of <a href="http://java.sun.com/javase/7/docs/api/java/lang/Exception.html" rel="noreferrer"><code>java.lang.Exception</code></a>. For example:</p> <pre><code>public class FooException extends Exception { public FooException() { super(); } public FooException(String message) { super(message); } public FooException(String message, Throwable cause) { super(message, cause); } public FooException(Throwable cause) { super(cause); } } </code></pre> <p>Methods that can potentially throw or propagate this exception must declare it:</p> <pre><code>public void calculate(int i) throws FooException, IOException; </code></pre> <p>... and code calling this method must either handle or propagate this exception (or both):</p> <pre><code>try { int i = 5; myObject.calculate(5); } catch(FooException ex) { // Print error and terminate application. ex.printStackTrace(); System.exit(1); } catch(IOException ex) { // Rethrow as FooException. throw new FooException(ex); } </code></pre> <p>You'll notice in the above example that <a href="https://docs.oracle.com/javase/7/docs/api/java/io/IOException.html" rel="noreferrer"><code>IOException</code></a> is caught and rethrown as <code>FooException</code>. This is a common technique used to encapsulate exceptions (typically when implementing an API).</p> <p>Sometimes there will be situations where you don't want to force every method to declare your exception implementation in its throws clause. In this case you can create an <strong>unchecked</strong> exception. An unchecked exception is any exception that extends <a href="http://java.sun.com/javase/7/docs/api/java/lang/RuntimeException.html" rel="noreferrer"><code>java.lang.RuntimeException</code></a> (which itself is a subclass of <a href="https://docs.oracle.com/javase/7/docs/api/java/lang/Exception.html" rel="noreferrer"><code>java.lang.Exception</code></a>):</p> <pre><code>public class FooRuntimeException extends RuntimeException { ... } </code></pre> <p>Methods can throw or propagate <code>FooRuntimeException</code> exception without declaring it; e.g.</p> <pre><code>public void calculate(int i) { if (i &lt; 0) { throw new FooRuntimeException("i &lt; 0: " + i); } } </code></pre> <p>Unchecked exceptions are typically used to denote a programmer error, for example passing an invalid argument to a method or attempting to breach an array index bounds.</p> <p>The <a href="http://java.sun.com/javase/7/docs/api/java/lang/Throwable.html" rel="noreferrer"><code>java.lang.Throwable</code></a> class is the root of all errors and exceptions that can be thrown within Java. <a href="http://java.sun.com/javase/7/docs/api/java/lang/Exception.html" rel="noreferrer"><code>java.lang.Exception</code></a> and <a href="http://java.sun.com/javase/7/docs/api/java/lang/Error.html" rel="noreferrer"><code>java.lang.Error</code></a> are both subclasses of <a href="http://java.sun.com/javase/7/docs/api/java/lang/Throwable.html" rel="noreferrer"><code>Throwable</code></a>. Anything that subclasses <a href="http://java.sun.com/javase/7/docs/api/java/lang/Throwable.html" rel="noreferrer"><code>Throwable</code></a> may be thrown or caught. However, it is typically bad practice to catch or throw <a href="http://java.sun.com/javase/7/docs/api/java/lang/Error.html" rel="noreferrer"><code>Error</code></a> as this is used to denote errors internal to the JVM that cannot usually be "handled" by the programmer (e.g. <a href="http://java.sun.com/javase/7/docs/api/java/lang/OutOfMemoryError.html" rel="noreferrer"><code>OutOfMemoryError</code></a>). Likewise you should avoid catching <a href="http://java.sun.com/javase/7/docs/api/java/lang/Throwable.html" rel="noreferrer"><code>Throwable</code></a>, which could result in you catching <a href="http://java.sun.com/javase/7/docs/api/java/lang/Error.html" rel="noreferrer"><code>Error</code></a>s in addition to <a href="http://java.sun.com/javase/7/docs/api/java/lang/Exception.html" rel="noreferrer"><code>Exception</code></a>s.</p>
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