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title description
Floating-point cheat sheet for Java
Tips for using floating-point and decimal numbers in Java

Floating-Point Types

Java has IEEE 754 single and double precision types supported by keywords:

	float f = 0.1f; // 32 bit float, note f suffix
	double d = 0.1d; // 64 bit float, suffix optional

The strictfp keyword on classes, interfaces and methods forces all intermediate results of floating-point calculations to be IEEE 754 values as well, guaranteeing identical results on all platforms. Without that keyword, implementations can use an extended exponent range where available, resulting in more precise results and faster execution on many common CPUs.

Decimal Types

Java has an arbitrary-precision decimal type named java.math.BigDecimal, which also allows to choose the rounding mode.

	BigDecimal a = new BigDecimal("0.1");
	BigDecimal b = new BigDecimal("0.2");
	BigDecimal c = a.add(b); // returns a BigDecimal representing exactly 0.3

How to Round

To get a String:

	String.format("%.2f", 1.2399) // returns "1.24"
	String.format("%.3f", 1.2399) // returns "1.240"
	String.format("%.2f", 1.2) // returns "1.20"

To print to standard output (or any PrintStream):

	System.out.printf("%.2f", 1.2399) // same syntax as String.format()

If you don't want trailing zeroes:

	new DecimalFormat("0.00").format(1.2)// returns "1.20"
	new DecimalFormat("0.##").format(1.2)// returns "1.2"

If you need a specific rounding mode:

	new BigDecimal("1.25").setScale(1, RoundingMode.HALF_EVEN); // returns 1.2

Resources

title: 为 Java 准备的浮点小抄 description: Tips for using floating-point and decimal numbers in Java

浮点类型

Java 用关键字来支持 IEEE 754 的单精度和双精度类型：

	float f = 0.1f; // 32 位浮点，注意 f 后缀
	double d = 0.1d; // 64 位浮点，后缀可选

用在类、接口和方法上的 strictfp 关键字会强制所有浮点计算的中间结果也使用 IEEE 754 的值，以保证在所有平台上的结果一致。不用那个关键字，在许多通用CPU上，Java 实现能使用更多的可用扩展范围，从而使结果更精确，执行速度更快。

小数类型

Java 有 arbitrary-precision 小数类型，称为 java.math.BigDecimal，它同样允许选择 取整模式.

	BigDecimal a = new BigDecimal("0.1");
	BigDecimal b = new BigDecimal("0.2");
	BigDecimal c = a.add(b); // 返回一个 BigDecimal 表示的精确的 0.3

如何取整

得到字符串：

	String.format("%.2f", 1.2399) // 返回 "1.24"
	String.format("%.3f", 1.2399) // 返回 "1.240"
	String.format("%.2f", 1.2) // 返回 "1.20"

打印到标准输出（或者任何 PrintStream）：

	System.out.printf("%.2f", 1.2399) // 和 String.format() 语法一样

如果你不想要小数末尾的零：

	new DecimalFormat("0.00").format(1.2)// 返回 "1.20"
	new DecimalFormat("0.##").format(1.2)// 返回 "1.2"

如果需要特殊的取整模式:

	new BigDecimal("1.25").setScale(1, RoundingMode.HALF_EVEN); // 返回 1.2

资源