How to determine whether a string is a number in java

preface

Numbers are often represented and conveyed in strings in some fields. So how do we determine whether a string is a number? Today we will discuss this topic.

Empty characters and null

First of all, we can clearly know the empty characters""andnull Definitely not numbers. In fact, we write other logic the same. Give priority to some of the most extreme and easily identifiable logical judgments. This is a small tip.

toCharArray

Strings that exclude previous situations can be passed throughtoCharArray() Method to convert to a char array. andCharacter.isDigit(int) It's easy to determine whether the char element is a number (don't ask why char is an int!). So will this method work? Let's operate a wave of various situations:

public class Main {
    public static void main(String[] args) {
        // false
        System.out.println("\"\"  是不是数字： "+isNumeric(""));
        // false
        System.out.println("\" \"  是不是数字： "+isNumeric(" "));
        // false
        System.out.println("null  是不是数字： "+isNumeric(null));
        // false
        System.out.println("1,200  是不是数字： "+isNumeric("1,200"));
        // true
        System.out.println("1  是不是数字： "+isNumeric("1"));
        // 预期是负数  却为 false
        System.out.println("-1  是不是数字： "+isNumeric("-1"));
        // true
        System.out.println("200  是不是数字： "+isNumeric("200"));
        // 预期是保留两位的浮点数  却为false
        System.out.println("3000.00  是不是数字： "+isNumeric("3000.00"));
        // 二进制
        System.out.println("0b11001  是不是数字： "+isNumeric("0b11001"));
        // 八进制  true
        System.out.println("012  是不是数字： "+isNumeric("012"));
        // 十六进制 false
        System.out.println("0x12  是不是数字： "+isNumeric("0x12"));
        //  A-F 代表十六进制中的 10-15   false
        System.out.println("0xAF  是不是数字： "+isNumeric("0xAF"));
        // double false
        System.out.println("12.12d  是不是数字： "+isNumeric("12.12d"));
        // double 科学计数法   false
        System.out.println("12E4  是不是数字： "+isNumeric("12E4"));
        // float  false
        System.out.println("12.123f  是不是数字： "+isNumeric("12.123f"));
        // 分隔符   jdk1.7      false
        System.out.println("1_000_000  是不是数字： "+isNumeric("1_000_000"));

    }


    public static boolean isNumeric(final String str) {
        // null or empty
        if (str == null || str.length() == 0) {
            return false;
        }
        return str.chars().allMatch(Character::isDigit);
    }
}

As can be seen from the above, there are not many problems with decimal positive integers that are moderate to moderate. Once it is floating point numbers, decimal numbers, negative numbers, binary, hexadecimal, scientific notation, and separators, this method becomes less easy to use. It suddenly occurred to me that there were some methods available in packaging.

parse

Numbers have corresponding packaging categoriesparsemethod. Throw if the string does not meet the rules for the corresponding number typeNumberFormatException abnormal. So let's change our judgment here:

   public static boolean isNumeric(final String str) {
        // null or empty
        if (str == null || str.length() == 0) {
            return false;
        }

        try {
            Integer.parseInt(str);
            return true;
        } catch (NumberFormatException e) {
            try {
                Double.parseDouble(str);
                return true;
            } catch (NumberFormatException ex) {
                try {
                    Float.parseFloat(str);
                    return true;
                } catch (NumberFormatException exx) {
                    return false;
                }
            }
        }
    }

Implement it again, and the following results are obtained:

""  Is it a number: false
" "  Is it a number: false
null  Is it a number: false
1,200  Is it a number: false
1  Is it a number: true
-1  Is it a number: true
200  Is it a number: true
3000.00  Is it a number: true
0b11001  Is it a number: false
012  Is it a number: true
0x12  Is it a number: false
0xAF  Is it a number: false
12.12d  Is it a number: true
12E4  Is it a number: true
12.123f  Is it a number: true
1_000_000  Is it a number: false

Starting from the fifth line above, the representation of numbers is supported by java. Judging from the execution results, everything except binary, hexadecimal, and separators met expectations. Although this method is not perfect, we can also learn some rules corresponding to the parse method. That's the point.

third-party class libraries

In other words, the api provided by jdk does not have silver bullets. So is there a third-party library to detect it? We used the common lang3 library (version 3.9)NumberUtilsTool class to process, IisParsable 、isDigits、isCreatable Methods were tested separately and found thatisCreatable The method works best, onlydelimiterWe did not meet our expectations. If you don't consider this situation, it should beisCreatable Basically enough to meet your needs.

regular expression

In fact, using regular expressions is the most perfect solution to the problem.

summary

Today's verification of whether a string is a number type in java allows us to review how numbers are represented and converted in java. Some rules were discovered. I believe this article will make you have a deeper impression on the numbers in java, and I hope it will be helpful to your work.