一个JDK7的四舍五入的bug引发的思考

一个JDK7的四舍五入的bug引发的思考

1.背景:

今天我的 feilong-core 项目使用 jdk8 进行maven install 的时候,有一个测试类报错, 但是原先使用jdk7 进行maven install的时候却是正常通过,

issue 参见 venusdrogon/feilong-core#165

2.测试类代码如下:

    @Test
    public void testFormat32(){
        assertEquals("1.2", NumberFormatUtil.format(1.15, "#####.#", RoundingMode.HALF_EVEN));
        assertEquals("1.2", NumberFormatUtil.format(1.25, "#####.#", RoundingMode.HALF_EVEN));
        assertEquals("1.3", NumberFormatUtil.format(1.251, "#####.#", RoundingMode.HALF_EVEN));

        assertEquals("-1.2", NumberFormatUtil.format(-1.15, "#####.#", RoundingMode.HALF_EVEN));
        assertEquals("-1.2", NumberFormatUtil.format(-1.25, "#####.#", RoundingMode.HALF_EVEN));
        assertEquals("-1.3", NumberFormatUtil.format(-1.251, "#####.#", RoundingMode.HALF_EVEN));
    }


    @Test
    public void testFormat321(){
        assertEquals("1.2", NumberFormatUtil.format(1.15, "#####.#", null));
        assertEquals("1.3", NumberFormatUtil.format(1.25, "#####.#", null));
        assertEquals("1.3", NumberFormatUtil.format(1.251, "#####.#", null));

        assertEquals("-1.2", NumberFormatUtil.format(-1.15, "#####.#", null));
        assertEquals("-1.3", NumberFormatUtil.format(-1.25, "#####.#", null));
        assertEquals("-1.3", NumberFormatUtil.format(-1.251, "#####.#", null));
    }


    @Test
    public void testFormat111(){
        assertEquals("1.2", NumberFormatUtil.format(1.15, "#####.#"));
        assertEquals("1.3", NumberFormatUtil.format(1.25, "#####.#"));
        assertEquals("1.3", NumberFormatUtil.format(1.251, "#####.#"));

        assertEquals("-1.2", NumberFormatUtil.format(-1.15, "#####.#"));
        assertEquals("-1.3", NumberFormatUtil.format(-1.25, "#####.#"));
        assertEquals("-1.3", NumberFormatUtil.format(-1.251, "#####.#"));
    }

3.报错信息

    Tests run: 568, Failures: 3, Errors: 0, Skipped: 2, Time elapsed: 2.194 sec <<< FAILURE! - in com.feilong.core.FeiLongSuiteTests
    testFormat111(com.feilong.core.text.NumberFormatUtilTest)  Time elapsed: 0.009 sec  <<< FAILURE!
    org.junit.ComparisonFailure: expected:<1.[2]> but was:<1.[1]>
        at com.feilong.core.text.NumberFormatUtilTest.testFormat111(NumberFormatUtilTest.java:149)

    testFormat321(com.feilong.core.text.NumberFormatUtilTest)  Time elapsed: 0 sec  <<< FAILURE!
    org.junit.ComparisonFailure: expected:<1.[2]> but was:<1.[1]>
        at com.feilong.core.text.NumberFormatUtilTest.testFormat321(NumberFormatUtilTest.java:135)

    testFormat32(com.feilong.core.text.NumberFormatUtilTest)  Time elapsed: 0.001 sec  <<< FAILURE!
    org.junit.ComparisonFailure: expected:<1.[2]> but was:<1.[1]>
        at com.feilong.core.text.NumberFormatUtilTest.testFormat32(NumberFormatUtilTest.java:121)


    Results :

    Failed tests: 
      NumberFormatUtilTest.testFormat111:149 expected:<1.[2]> but was:<1.[1]>
      NumberFormatUtilTest.testFormat32:121 expected:<1.[2]> but was:<1.[1]>
      NumberFormatUtilTest.testFormat321:135 expected:<1.[2]> but was:<1.[1]>

    Tests run: 568, Failures: 3, Errors: 0, Skipped: 2

    [INFO] ------------------------------------------------------------------------
    [INFO] BUILD FAILURE

4.原因

经过搜索,发现

https://bugs.openjdk.java.net/browse/JDK-7131459
https://bugs.openjdk.java.net/browse/JDK-8029896
http://stackoverflow.com/questions/22797964/is-inconsistency-in-rounding-between-java-7-and-java-8-a-bug

jdk7上是bug , jdk8 修复了

5.思考

• 不要使用 float 或者 double 浮点数来表述货币这样的精确数量的值,会导致舍入误差。使用浮点数来进行元和分计算会得到灾难性的后果。
• 浮点数最好用来表示象测量值这类数值，这类值从一开始就不怎么精确。(PS:一般情况在你的工作中理论上是用不到的)
• 尽量不要用 float 或者 double ,来做 +-*/ 运算,以及format,小数请使用 BigDecimal

• BigDecimal 的构造函数有

  • java.math.BigDecimal.BigDecimal(double)
  • java.math.BigDecimal.BigDecimal(String)

  请使用 java.math.BigDecimal.BigDecimal(String))(PS:如果你使用 sonar 进行代码扫描的话,它会给你提示和建议)

• 比较两个 BigDecimal 大小,请使用 java.math.BigDecimal.compareTo(BigDecimal) 方法,而不要使用 java.math.BigDecimal.equals(Object)方法,

  因为

  equals()方法认为，两个表示同一个数但换算值不同（例如， 100.00 和 100.000 ）的 BigDecimal 值是不相等的。
  然而， compareTo() 方法会认为这两个数是相等的，所以在从数值上比较两个 BigDecimal 值时，应该使用 compareTo() 而不是 equals() 。

  参见源码:

6.参考

• https://bugs.openjdk.java.net/browse/JDK-7131459
• https://bugs.openjdk.java.net/browse/JDK-8029896
• http://stackoverflow.com/questions/22797964/is-inconsistency-in-rounding-between-java-7-and-java-8-a-bug
• http://www.ibm.com/developerworks/cn/java/j-jtp0114/index.html#4