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

I think this is the relevant part of the Javadoc that describes the behavior you're looking at (from the static String toString(float) method):

How many digits must be printed for the fractional part of m or a? There must be at least one digit to represent the fractional part, and beyond that as many, but only as many, more digits as are needed to uniquely distinguish the argument value from adjacent values of type float.

To paraphrase: the toString methods for floating-point types will generally produce the shortest decimal representation that can unabmiguously identify the true value of the floating-point number.

Example program to illustrate:

import java.math.BigDecimal;

public class FloatTest {

  static void show(float f) {
    BigDecimal f_exact = new BigDecimal(f);
    System.out.println("---");
    System.out.println("String value: " + f);
    System.out.println("Exact value:  " + f_exact);
    System.out.println("Delta:        " + 
        new BigDecimal("1.15").subtract(f_exact));
  }

  public static void main(String[] args) {
    show(1.15f);
    show(Math.nextUp(1.15f));
  }
}

Output:

---
String value: 1.15
Exact value:  1.14999997615814208984375
Delta:        2.384185791015625E-8
---
String value: 1.1500001
Exact value:  1.150000095367431640625
Delta:        -9.5367431640625E-8