java中double类型转换成字符串自动格式化成科学计数法

在使用double类型的时候，常常使用String.valueOf(Double d)方法来将double转换成String，而String.valueOf(Double)调用的是Double自身的toString()方法。

/**
     * Returns the string representation of the <code>double</code> argument.
     * <p>
     * The representation is exactly the one returned by the
     * <code>Double.toString</code> method of one argument.
     *
     * @param   d   a <code>double</code>.
     * @return  a  string representation of the <code>double</code> argument.
     * @see     java.lang.Double#toString(double)
     */
    public static String valueOf(double d) {
	return Double.toString(d);
    }

而Double自身的toString方法使用FloatingDecimal来对数字进行格式化，代码如下：

    public static String toString(double d) {
	return new FloatingDecimal(d).toJavaFormatString();
    }

该方法注释中写到：

  * <li>If <i>m</i> is less than 10<sup>-3</sup> or greater than or
     * equal to 10<sup>7</sup>, then it is represented in so-called
     * "computerized scientific notation." Let <i>n</i> be the unique
     * integer such that 10<sup><i>n</i></sup> &lt;= <i>m</i> &lt;
     * 10<sup><i>n</i>+1</sup>; then let <i>a</i> be the
     * mathematically exact quotient of <i>m</i> and
     * 10<sup><i>n</i></sup> so that 1 &lt;= <i>a</i> &lt; 10. The
     * magnitude is then represented as the integer part of <i>a</i>,
     * as a single decimal digit, followed by '<code>.</code>'
     * (<code>'&#92;u002E'</code>), followed by decimal digits
     * representing the fractional part of <i>a</i>, followed by the
     * letter '<code>E</code>' (<code>'&#92;u0045'</code>), followed
     * by a representation of <i>n</i> as a decimal integer, as
     * produced by the method {@link Integer#toString(int)}.

可以很明确的看到，如果数字大于10的7次方或者小于10的-3次方，就会使用科学计数法。这个在很多时候很适用，同时在很多地方也让很多人很头疼。想要避免这个问题，就要自己使用格式化类去重新格式化。最简单的使用DecimalFormat类去格式化，用这个可以很容易的得到不转换成科学计数法的字符串。

 DecimalFormat df = new DecimalFormat("###0.0#");//最多保留几位小数，就用几个#，最少位就用0来确定
String s=df.format(d);

这样就可以了~