Java中提供了三個特殊的浮點數值：正無窮大、負無窮大、非數，用于表示溢出和出錯。

正無窮大：用一個正數除以0將得到一個正無窮大，通過Double或Float的POSITIVE_INFINITY表示。

負無窮大：用一個負數除以0將得到一個負無窮大，通過Double或Float的NEGATIVE_INFINITY表示。

非數：0.0除以0.0或對一個負數開放將得到一個非數，通過Double或Float的NaN表示。

可以點開jdk源碼，看看java都是如何表示的：

/**

* The {@code Double} class wraps a value of the primitive type

* {@code double} in an object. An object of type

* {@code Double} contains a single field whose type is

* {@code double}.

*

*

In addition, this class provides several methods for converting a

* {@code double} to a {@code String} and a

* {@code String} to a {@code double}, as well as other

* constants and methods useful when dealing with a

* {@code double}.

*

* @author Lee Boynton

* @author Arthur van Hoff

* @author Joseph D. Darcy

* @since JDK1.0

*/

public final class Double extends Number implements Comparable {

/**

* A constant holding the positive infinity of type

* {@code double}. It is equal to the value returned by

* {@code Double.longBitsToDouble(0x7ff0000000000000L)}.

*/

public static final double POSITIVE_INFINITY = 1.0 / 0.0;

/**

* A constant holding the negative infinity of type

* {@code double}. It is equal to the value returned by

* {@code Double.longBitsToDouble(0xfff0000000000000L)}.

*/

public static final double NEGATIVE_INFINITY = -1.0 / 0.0;

/**

* A constant holding a Not-a-Number (NaN) value of type

* {@code double}. It is equivalent to the value returned by

* {@code Double.longBitsToDouble(0x7ff8000000000000L)}.

*/

public static final double NaN = 0.0d / 0.0;

...

}

(實際上在字節碼里正無窮和負無窮還有NaN都有一個特定的值用來表示

0x7f800000表示正無窮

0xff800000表示負無窮

在0x7f800001~0x7fffffff 和?0xff80001~0xffffffff兩個的范圍內的值表示NaN.

)

所有的正無窮大的數值都是相等的，所有的負無窮大的數值都是相等；而NaN不與任何數值相等，甚至和NaN都不等。

舉例

public class javaLesson5

{

public static void main(String[] args)

{

float af = 5.2325556f;

//下面將看到af的值已經發生改變,顯示結果為5.2325554.

System.out.println(af);

double a = 0.0;

double c = Double.NEGATIVE_INFINITY;

float d = Float.NEGATIVE_INFINITY;

//將看到float和double的負無窮大是相等的。顯示結果為：true。

System.out.println(c == d);

//0.0除以0.0將出現非數。顯示結果為：NaN。

System.out.println(a / a);

//兩個非數之間是不相等的。顯示結果為：false。

System.out.println(a == Float.NaN);

//所有正無窮大都是相等的。顯示結果為：true。

System.out.println(6.0 / 0 == 555.0/0);

//負數除以0.0將得到負無窮大。顯示結果為：-Infinity

System.out.println(-8 / a);

//下面代碼將拋出除以0的異常。

//System.out.pintln(0 / 0);

}

}

部分內容轉自(http://blog.csdn.net/ml863606/article/details/50853555)