Compare two strings meaningfully in Java

Question

I've two strings which I need to compare, but I want to compare them meaningfully such that when they are numbers their actual value should be compared. So far I've tried the following solution:

String str1 = "-0.6";
String str2 = "-.6";
if (NumberUtils.isNumber(str1) && NumberUtils.isNumber(str2)) {
    Number num1 = NumberUtils.createNumber(str1);
    Number num2 = NumberUtils.createNumber(str2);
    System.out.println(num1.equals(num2));
} else {
    System.out.println(str1.equals(str2));
}

This works as both are converted to doubles.
But this won't work in this case where:

String str1 = "6";
String str2 = "6.0";

Is there any easy way to do this, or will I have to write my own Comparator?

And how do you act when one string is not a number of some sort? — fge
– fge, Commented Jul 21, 2015 at 6:31
Can't you make the createNumber() method to always return Double for numbers? — Codebender
– Codebender, Commented Jul 21, 2015 at 6:32
This means that string is not meant for numerical comparisons such as this. You should go with convrting to double and compare them as you have mentioned. — sakthisundar
– sakthisundar, Commented Jul 21, 2015 at 6:32
Does it solve using something like Number num1 = 0. + NumberUtils.createNumber(str1);? The same for num2. — skypjack
– skypjack, Commented Jul 21, 2015 at 6:36

Mureinik · Accepted Answer · 2015-07-21 06:37:10Z

3

Instead of using the general-purpose createNumber(String), force them to doubles using createDouble(String):

String str1 = "-0.6";
String str2 = "-.6";
if (NumberUtils.isNumber(str1) && NumberUtils.isNumber(str2)) {
    Double d1 = NumberUtils.createDouble(str1);
    Double d2 = NumberUtils.createDouble(str2);
    System.out.println(d1.equals(d2));
} else {
    System.out.println(str1.equals(str2));
}

edited Jul 21, 2015 at 6:37

answered Jul 21, 2015 at 6:34

Mureinik

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Comments

fge · Accepted Answer · 2015-07-21 06:32:31Z

2

You can probably use BigDecimal for this:

final BigDecimal b1 = new BigDecimal(str1);
final BigDecimal b2 = new BigDecimal(str2);
return b1.compareTo(b2) == 0;

answered Jul 21, 2015 at 6:32

fge

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4 Comments

sakthisundar Over a year ago

Why big decimal for such smaller numbers that are mentioned in the code snippet ?

dsharew Over a year ago

what happens if one the strings is not a number?

fge Over a year ago

@DegenSharew I mentioned that first thing in the comments

fge Over a year ago

@sakthisundar because precision; at least, with a BigDecimal you don't get any surprises!

milez · Accepted Answer · 2015-07-21 06:36:38Z

2

You could use Double.parseDouble(String) instead. It throws the NumberFormatException, which can be caught to inform you that you should not compare them as numbers.

try {
    Double d1 = Double.parseDouble(str1);
    Double d2 = Double.parseDouble(str2);

    //Now you have your doubles
} catch (NumberFormatException e)
{
    //Could not compare as numbers, do something else
}

answered Jul 21, 2015 at 6:36

milez

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Comments

dsharew · Accepted Answer · 2015-07-21 06:38:45Z

2

As suggested on the comment one of the string could be a number and the other not or vice versa or both could be a string so instead of checking if the strings are number strings I will go using try catch block.

Like this:

try{
    Double d1 = Double.parseDouble(str1);
    Double d2 = Double.parseDouble(str2);
    System.out.println(d1.equals(d2));
}catch(NumberFormatException ex){

    System.out.println(num1.equals(num2));

}

This first tries to compare them as numbers but if at least one of them is not a number string it will fallback to string comparison in the catch block.

answered Jul 21, 2015 at 6:38

dsharew

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