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copied!<h3>Overview of <code>Scanner.hasNextXXX</code> methods</h3> <p><a href="http://java.sun.com/javase/6/docs/api/java/util/Scanner.html" rel="noreferrer"><code>java.util.Scanner</code></a> has many <code>hasNextXXX</code> methods that can be used to validate input. Here's a brief overview of all of them:</p> <ul> <li><a href="http://java.sun.com/javase/6/docs/api/java/util/Scanner.html#hasNext%28%29" rel="noreferrer"><code>hasNext()</code></a> - does it have <em>any</em> token at all?</li> <li><a href="http://java.sun.com/javase/6/docs/api/java/util/Scanner.html#hasNextLine%28%29" rel="noreferrer"><code>hasNextLine()</code></a> - does it have another line of input?</li> <li>For Java primitives <ul> <li><a href="http://java.sun.com/javase/6/docs/api/java/util/Scanner.html#hasNextInt%28%29" rel="noreferrer"><code>hasNextInt()</code></a> - does it have a token that can be parsed into an <code>int</code>?</li> <li>Also available are <a href="http://java.sun.com/javase/6/docs/api/java/util/Scanner.html#hasNextDouble%28%29" rel="noreferrer"><code>hasNextDouble()</code></a>, <a href="http://java.sun.com/javase/6/docs/api/java/util/Scanner.html#hasNextFloat%28%29" rel="noreferrer"><code>hasNextFloat()</code></a>, <a href="http://java.sun.com/javase/6/docs/api/java/util/Scanner.html#hasNextByte%28%29" rel="noreferrer"><code>hasNextByte()</code></a>, <a href="http://java.sun.com/javase/6/docs/api/java/util/Scanner.html#hasNextShort%28%29" rel="noreferrer"><code>hasNextShort()</code></a>, <a href="http://java.sun.com/javase/6/docs/api/java/util/Scanner.html#hasNextLong%28%29" rel="noreferrer"><code>hasNextLong()</code></a>, and <a href="http://java.sun.com/javase/6/docs/api/java/util/Scanner.html#hasNextBoolean%28%29" rel="noreferrer"><code>hasNextBoolean()</code></a></li> <li>As bonus, there's also <a href="http://java.sun.com/javase/6/docs/api/java/util/Scanner.html#hasNextBigInteger%28%29" rel="noreferrer"><code>hasNextBigInteger()</code></a> and <a href="http://java.sun.com/javase/6/docs/api/java/util/Scanner.html#hasNextBigDecimal%28%29" rel="noreferrer"><code>hasNextBigDecimal()</code></a></li> <li>The integral types also has overloads to specify radix (for e.g. hexadecimal)</li> </ul></li> <li>Regular expression-based <ul> <li><a href="http://java.sun.com/javase/6/docs/api/java/util/Scanner.html#hasNext%28java.lang.String%29" rel="noreferrer"><code>hasNext(String pattern)</code></a></li> <li><a href="http://java.sun.com/javase/6/docs/api/java/util/Scanner.html#hasNext%28java.lang.String%29" rel="noreferrer"><code>hasNext(Pattern pattern)</code></a> is the <a href="http://java.sun.com/javase/6/docs/api/java/util/regex/Pattern.html#compile%28java.lang.String%29" rel="noreferrer"><code>Pattern.compile</code></a> overload</li> </ul></li> </ul> <p><code>Scanner</code> is capable of more, enabled by the fact that it's regex-based. One important feature is <a href="http://java.sun.com/javase/6/docs/api/java/util/Scanner.html#useDelimiter%28java.lang.String%29" rel="noreferrer"><code>useDelimiter(String pattern)</code></a>, which lets you define what <em>pattern</em> separates your tokens. There are also <code>find</code> and <code>skip</code> methods that <em>ignores</em> delimiters.</p> <p>The following discussion will keep the regex as simple as possible, so the focus remains on <code>Scanner</code>.</p> <hr> <h3>Example 1: Validating positive ints</h3> <p>Here's a simple example of using <code>hasNextInt()</code> to validate positive <code>int</code> from the input.</p> <pre><code>Scanner sc = new Scanner(System.in); int number; do { System.out.println("Please enter a positive number!"); while (!sc.hasNextInt()) { System.out.println("That's not a number!"); sc.next(); // this is important! } number = sc.nextInt(); } while (number <= 0); System.out.println("Thank you! Got " + number); </code></pre> <p>Here's an example session:</p> <blockquote> <p><strong><em>Please enter a positive number!</em></strong><br> five<br> <strong><em>That's not a number!</em></strong><br> -3<br> <strong><em>Please enter a positive number!</em></strong><br> 5<br> <strong><em>Thank you! Got 5</em></strong></p> </blockquote> <p>Note how much easier <code>Scanner.hasNextInt()</code> is to use compared to the more verbose <code>try/catch</code> <a href="http://java.sun.com/javase/6/docs/api/java/lang/Integer.html#parseInt%28java.lang.String%29" rel="noreferrer"><code>Integer.parseInt</code></a>/<a href="http://java.sun.com/javase/6/docs/api/java/lang/NumberFormatException.html" rel="noreferrer"><code>NumberFormatException</code></a> combo. By contract, a <code>Scanner</code> <em>guarantees</em> that if it <code>hasNextInt()</code>, then <code>nextInt()</code> will peacefully give you that <code>int</code>, and will <em>not</em> throw any <code>NumberFormatException</code>/<code>InputMismatchException</code>/<code>NoSuchElementException</code>.</p> <h3>Related questions</h3> <ul> <li><a href="https://stackoverflow.com/questions/2912817/how-to-use-scanner-to-accept-only-valid-int-as-input">How to use Scanner to accept only valid int as input </a></li> <li><a href="https://stackoverflow.com/questions/2496239/how-do-i-keep-a-scanner-from-throwing-exceptions-when-the-wrong-type-is-entered">How do I keep a scanner from throwing exceptions when the wrong type is entered? (java) </a></li> </ul> <hr> <h3>Example 2: Multiple <code>hasNextXXX</code> on the same token</h3> <p>Note that the snippet above contains a <code>sc.next()</code> statement to advance the <code>Scanner</code> until it <code>hasNextInt()</code>. It's important to realize that <strong><em>none</em> of the</strong> <code>hasNextXXX</code> <strong>methods advance the</strong> <code>Scanner</code> <strong>past any input!</strong> You will find that if you omit this line from the snippet, then it'd go into an infinite loop on an invalid input!</p> <p>This has two consequences:</p> <ul> <li>If you need to skip the "garbage" input that fails your <code>hasNextXXX</code> test, then you need to advance the <code>Scanner</code> one way or another (e.g. <code>next()</code>, <code>nextLine()</code>, <code>skip</code>, etc).</li> <li>If one <code>hasNextXXX</code> test fails, you can <em>still</em> test if it perhaps <code>hasNextYYY</code>!</li> </ul> <p>Here's an example of performing multiple <code>hasNextXXX</code> tests.</p> <pre><code>Scanner sc = new Scanner(System.in); while (!sc.hasNext("exit")) { System.out.println( sc.hasNextInt() ? "(int) " + sc.nextInt() : sc.hasNextLong() ? "(long) " + sc.nextLong() : sc.hasNextDouble() ? "(double) " + sc.nextDouble() : sc.hasNextBoolean() ? "(boolean) " + sc.nextBoolean() : "(String) " + sc.next() ); } </code></pre> <p>Here's an example session:</p> <blockquote> <p>5<br> <strong><em>(int) 5</em></strong><br> false<br> <strong><em>(boolean) false</em></strong><br> blah<br> <strong><em>(String) blah</em></strong><br> 1.1<br> <strong><em>(double) 1.1</em></strong><br> 100000000000<br> <strong><em>(long) 100000000000</em></strong><br> exit </p> </blockquote> <p>Note that the order of the tests matters. If a <code>Scanner</code> <code>hasNextInt()</code>, then it also <code>hasNextLong()</code>, but it's not necessarily <code>true</code> the other way around. More often than not you'd want to do the more specific test before the more general test.</p> <hr> <h3>Example 3 : Validating vowels</h3> <p><code>Scanner</code> has many advanced features supported by regular expressions. Here's an example of using it to validate vowels.</p> <pre><code>Scanner sc = new Scanner(System.in); System.out.println("Please enter a vowel, lowercase!"); while (!sc.hasNext("[aeiou]")) { System.out.println("That's not a vowel!"); sc.next(); } String vowel = sc.next(); System.out.println("Thank you! Got " + vowel); </code></pre> <p>Here's an example session:</p> <blockquote> <p><strong><em>Please enter a vowel, lowercase!</em></strong><br> 5<br> <strong><em>That's not a vowel!</em></strong><br> z<br> <strong><em>That's not a vowel!</em></strong><br> e<br> <strong><em>Thank you! Got e</em></strong></p> </blockquote> <p>In regex, as a Java string literal, the pattern <code>"[aeiou]"</code> is what is called a "character class"; it matches any of the letters <code>a</code>, <code>e</code>, <code>i</code>, <code>o</code>, <code>u</code>. Note that it's trivial to make the above test case-insensitive: just provide such regex pattern to the <code>Scanner</code>.</p> <h3>API links</h3> <ul> <li><a href="http://java.sun.com/javase/6/docs/api/java/util/Scanner.html#hasNext%28java.lang.String%29" rel="noreferrer"><code>hasNext(String pattern)</code></a> - Returns <code>true</code> if the next token matches the pattern constructed from the specified string.</li> <li><a href="http://java.sun.com/javase/6/docs/api/java/util/regex/Pattern.html" rel="noreferrer"><code>java.util.regex.Pattern</code></a></li> </ul> <h3>Related questions</h3> <ul> <li><a href="https://stackoverflow.com/questions/3043306/reading-a-single-char-in-java/">Reading a single char in Java</a></li> </ul> <h3>References</h3> <ul> <li><a href="http://java.sun.com/docs/books/tutorial/essential/regex/" rel="noreferrer">Java Tutorials/Essential Classes/Regular Expressions</a></li> <li><a href="http://www.regular-expressions.info/charclass.html" rel="noreferrer">regular-expressions.info/Character Classes</a></li> </ul> <hr> <h3>Example 4: Using two <code>Scanner</code> at once</h3> <p>Sometimes you need to scan line-by-line, with multiple tokens on a line. The easiest way to accomplish this is to use <em>two</em> <code>Scanner</code>, where the second <code>Scanner</code> takes the <code>nextLine()</code> from the first <code>Scanner</code> as input. Here's an example:</p> <pre><code>Scanner sc = new Scanner(System.in); System.out.println("Give me a bunch of numbers in a line (or 'exit')"); while (!sc.hasNext("exit")) { Scanner lineSc = new Scanner(sc.nextLine()); int sum = 0; while (lineSc.hasNextInt()) { sum += lineSc.nextInt(); } System.out.println("Sum is " + sum); } </code></pre> <p>Here's an example session:</p> <blockquote> <p><strong><em>Give me a bunch of numbers in a line (or 'exit')</em></strong><br> 3 4 5<br> <strong><em>Sum is 12</em></strong><br> 10 100 a million dollar<br> <strong><em>Sum is 110</em></strong><br> wait what?<br> <strong><em>Sum is 0</em></strong><br> exit</p> </blockquote> <p>In addition to <a href="http://java.sun.com/javase/6/docs/api/java/util/Scanner.html#Scanner%28java.lang.String%29" rel="noreferrer"><code>Scanner(String)</code></a> constructor, there's also <a href="http://java.sun.com/javase/6/docs/api/java/util/Scanner.html#Scanner%28java.io.File%29" rel="noreferrer"><code>Scanner(java.io.File)</code></a> among others.</p> <hr> <h3>Summary</h3> <ul> <li><code>Scanner</code> provides a rich set of features, such as <code>hasNextXXX</code> methods for validation.</li> <li>Proper usage of <code>hasNextXXX/nextXXX</code> in combination means that a <code>Scanner</code> will <strong>NEVER</strong> throw an <code>InputMismatchException</code>/<code>NoSuchElementException</code>.</li> <li>Always remember that <code>hasNextXXX</code> does not advance the <code>Scanner</code> past any input.</li> <li>Don't be shy to create multiple <code>Scanner</code> if necessary. Two simple <code>Scanner</code> is often better than one overly complex <code>Scanner</code>.</li> <li>Finally, even if you don't have any plans to use the advanced regex features, do keep in mind which methods are regex-based and which aren't. Any <code>Scanner</code> method that takes a <code>String pattern</code> argument is regex-based. <ul> <li><em>Tip</em>: an easy way to turn any <code>String</code> into a literal pattern is to <a href="http://java.sun.com/javase/6/docs/api/java/util/regex/Pattern.html#quote%28java.lang.String%29" rel="noreferrer"><code>Pattern.quote</code></a> it.</li> </ul></li> </ul>

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