比较从文本文件中读取的双打

Question

I am comparing one double from a text file with a hard-coded double, the value read from a text file is put into a double variable using Double.parseDouble() in Java. Then, I'm using the following line to assert their equality. Is it possible that this assert fails? Does it depend on the value?

assertEquals(17353820.450663020000, Double.parseDouble(input.next()), 0.0);

Note that 0.0 is the value of epsilon used for comparing doubles.

Text file contents:

17353820.450663020000

Answer 1

Is it possible that this assert fails?

No.

Does it depend on the value?

No.

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Javadoc of parseDouble says:

Returns a new double initialized to the value represented by the specified String , as performed by the valueOf method of class Double .

The Java Language Specification, section 3.10.2. Floating-Point Literals says:

The details of proper input conversion from a Unicode string representation of a floating-point number to the internal IEEE 754 binary floating-point representation are described for the methods valueOf of class Float and class Double of the package java.lang .

As you can see, the compiler uses he same logic for parsing doubles as the parseDouble method, so the result will always be the same.

Answer 2

Floats and doubles in any language have a limit on precision, so can be approximations to the string format. See https://floating-point-gui.de for some information. That's what the epsilon argument is for, it says how close the value being tested has to be to be considered "correct" when it can't be an exact match.