java - Why 0.1 represented in float correctly? (I know why not in result of 2.0-1.9)

Question

I've read a lot about float representation recently (including this: How To Represent 0.1 In Floating Point Arithmetic And Decimal). Now I understand that 0.1 can't be represented correctly, and when I do this:

System.out.println(2.0f - 1.9f);

I'll never get precise result.

So the question is: How represented 0.1f in the following code in order to print 0.1 correctly? Is that some kind of syntatic sugar? In the article that I above mentioned says: 0.1 represented in memory as 0.100000001490116119384765625. So why I don't get this output for this code:

System.out.println(0.1f);

How does Java deal with this?

Answer 1

System.out.println performs some rounding for floats and doubles. It uses Float.toString(), which itself (in the oraclew JDK) delegates to the FloatingDecimal class - you can have a look at the source of FloatingDecimal#toJavaFormatString() for gory details.

If you try:

BigDecimal bd = new BigDecimal(0.1f);
System.out.println(bd);

You will see the real value of 0.1f: 0.100000001490116119384765625.