how does Java convert floats to strings

Question

Here\'s what I\'m looking at:

float p=1.15f;
BigDecimal bdp=new BigDecimal(p);
float q=1.1499999f;
float r=1.14999999f;

System.out.println(p);   //1.15
Syst         


        

Answer 1

Thanks mttdbrd ... that helps. From your first paragraph then I think that the answer to my question is: yes, Java does this rounding internally in accordance with the IEEE spec.

Here's the output of a program that let's me see a little bit of that rounding in action:

-------------------------------------------
string            1.49999
bigdec of float   1.499989986419677734375
float back to str 1.49999
-------------------------------------------
string            1.4999991
bigdec of float   1.49999904632568359375
float back to str 1.499999
-------------------------------------------
string            1.4999992
bigdec of float   1.49999916553497314453125
float back to str 1.4999992
-------------------------------------------
string            1.4999993
bigdec of float   1.4999992847442626953125
float back to str 1.4999993
-------------------------------------------
string            1.4999994
bigdec of float   1.49999940395355224609375
float back to str 1.4999994
-------------------------------------------
string            1.4999995
bigdec of float   1.499999523162841796875
float back to str 1.4999995
-------------------------------------------
string            1.4999996
bigdec of float   1.49999964237213134765625
float back to str 1.4999996
-------------------------------------------
string            1.4999997
bigdec of float   1.49999964237213134765625
float back to str 1.4999996
-------------------------------------------
string            1.4999998
bigdec of float   1.4999997615814208984375
float back to str 1.4999998
-------------------------------------------
string            1.4999999
bigdec of float   1.49999988079071044921875
float back to str 1.4999999
-------------------------------------------
string            1.15
bigdec of float   1.14999997615814208984375
float back to str 1.15
-------------------------------------------
string            1.49999964237213134765626
bigdec of float   1.49999964237213134765625
float back to str 1.4999996

And here's the program if anyone wants it:

public static void floatAccuracy3()
{
    printFloatAndBigDec("1.49999");
    for (int i = 1; i < 10; i++) {
        String s="1.499999";
        s+=i;
        printFloatAndBigDec(s);         
    }
    printFloatAndBigDec("1.15");
    printFloatAndBigDec("1.49999964237213134765626");
}

public static void printFloatAndBigDec(String s)
{
    Float f=new Float(s);
    BigDecimal bdf=new BigDecimal(f);
    System.out.println("-------------------------------------------");
    System.out.println("string            "+s);
    System.out.println("bigdec of float   "+bdf);
    System.out.println("float back to str "+f);
}

And some other links in case they're helpful to anyone else researching this stuff:

• floating point basics
• princeton cs intro floating points
• What Every Computer Scientist Should Know About Floating-Point Arithmetic
• Why 0.1 Does Not Exist In Floating-Point
• JLS 4.2.4 Floating-Point Operations
• IEEE round to nearest