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<br /><b>Lesson:</b> Numbers and Strings
<br /><b>Section:</b> Strings
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<div class="nolinkBHEAD">Converting Between Numbers and Strings</div>
<div class="linkBHEAD"><a href="manipstrings.html">Manipulating Characters in a String</a></div>
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<div id="PageTitle"><h1>Converting Between Numbers and Strings</h1></div>
<div id="PageContent">
<!-- Converting Between Strings and Numbers-->
<h2>Converting Strings to Numbers</h2>
<p>Frequently, a program ends up with numeric data in a string object—a value entered by the user, for example.</p>
<p>The <code>Number</code> subclasses that wrap primitive numeric types (
<a class="APILink" target="_blank" href="http://docs.oracle.com/javase/8/docs/api/java/lang/Byte.html"><code>Byte</code></a>,
<a class="APILink" target="_blank" href="http://docs.oracle.com/javase/8/docs/api/java/lang/Integer.html"><code>Integer</code></a>,
<a class="APILink" target="_blank" href="http://docs.oracle.com/javase/8/docs/api/java/lang/Double.html"><code>Double</code></a>,
<a class="APILink" target="_blank" href="http://docs.oracle.com/javase/8/docs/api/java/lang/Float.html"><code>Float</code></a>,
<a class="APILink" target="_blank" href="http://docs.oracle.com/javase/8/docs/api/java/lang/Long.html"><code>Long</code></a>, and
<a class="APILink" target="_blank" href="http://docs.oracle.com/javase/8/docs/api/java/lang/Short.html"><code>Short</code></a>) each provide a class method named <code>valueOf</code> that converts a string to an object of that type. Here is an example,
<a class="SourceLink" target="_blank" href="examples/ValueOfDemo.java" onclick="showCode('../../displayCode.html', 'examples/ValueOfDemo.java'); return false;"><code>ValueOfDemo</code></a> , that gets two strings from the command line, converts them to numbers, and performs arithmetic operations on the values:</p>
<div class="codeblock"><pre>
public class ValueOfDemo {
public static void main(String[] args) {
// this program requires two
// arguments on the command line
if (args.length == 2) {
// convert strings to numbers
float a = (Float.valueOf(args[0])).floatValue();
float b = (Float.valueOf(args[1])).floatValue();
// do some arithmetic
System.out.println("a + b = " +
(a + b));
System.out.println("a - b = " +
(a - b));
System.out.println("a * b = " +
(a * b));
System.out.println("a / b = " +
(a / b));
System.out.println("a % b = " +
(a % b));
} else {
System.out.println("This program " +
"requires two command-line arguments.");
}
}
}
</pre></div>
<p>The following is the output from the program when you use <code>4.5</code> and <code>87.2</code> for the command-line arguments:</p>
<div class="codeblock"><pre>
a + b = 91.7
a - b = -82.7
a * b = 392.4
a / b = 0.0516055
a % b = 4.5
</pre></div>
<div class="note"><hr /><strong>Note:</strong> Each of the <code>Number</code> subclasses that wrap primitive numeric types also provides a <code>parseXXXX()</code> method (for example, <code>parseFloat()</code>) that can be used to convert strings to primitive numbers. Since a primitive type is returned instead of an object, the <code>parseFloat()</code> method is more direct than the <code>valueOf()</code> method. For example, in the <code>ValueOfDemo</code> program, we could use:
<div class="codeblock"><pre>
float a = Float.parseFloat(args[0]);
float b = Float.parseFloat(args[1]);
</pre></div>
<hr /></div>
<h2>Converting Numbers to Strings</h2>
<p>Sometimes you need to convert a number to a string because you need to operate on the value in its string form. There are several easy ways to convert a number to a string:</p>
<div class="codeblock"><pre>
int i;
// Concatenate "i" with an empty string; conversion is handled for you.
String s1 = "" + i;
</pre></div>
<p>or</p>
<div class="codeblock"><pre>
// The valueOf class method.
String s2 = String.valueOf(i);
</pre></div>
<p>Each of the <code>Number</code> subclasses includes a class method, <code>toString()</code>, that will convert its primitive type to a string. For example:</p>
<div class="codeblock"><pre>
int i;
double d;
String s3 = Integer.toString(i);
String s4 = Double.toString(d);
</pre></div>
<p>The
<a class="SourceLink" target="_blank" href="examples/ToStringDemo.java" onclick="showCode('../../displayCode.html', 'examples/ToStringDemo.java'); return false;"><code>ToStringDemo</code></a> example uses the <code>toString</code> method to convert a number to a string. The program then uses some string methods to compute the number of digits before and after the decimal point:</p>
<div class="codeblock"><pre>
public class ToStringDemo {
public static void main(String[] args) {
double d = 858.48;
String s = Double.toString(d);
int dot = s.indexOf('.');
System.out.println(dot + " digits " +
"before decimal point.");
System.out.println( (s.length() - dot - 1) +
" digits after decimal point.");
}
}
</pre></div>
<p>The output of this program is:</p>
<div class="codeblock"><pre>
3 digits before decimal point.
2 digits after decimal point.
</pre></div>
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