java 判断是否为数字

java

老外很厉害, 茴香豆有好多中写法, 是目前看到最全的.

https://www.baeldung.com/java-check-string-number

https://javarevisited.blogspot.com/2016/10/how-to-check-if-string-is-numeric-in-Java.html

 Using Plain Java

 

Perhaps the easiest and the most reliable way to check whether a String is numeric or not is by parsing it using Java's built-in methods:

1. Integer.parseInt(String)
2. Float.parseFloat(String)
3. Double.parseDouble(String)
4. Long.parseLong(String)
5. new BigInteger(String)

If these methods don't throw any NumberFormatException, then it means that the parsing was successful and the String is numeric:

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public static boolean isNumeric(String strNum) {      if (strNum == null ) {          return false ;      }      try {          double d = Double.parseDouble(strNum);      } catch (NumberFormatException nfe) {          return false ;      }      return true ; }

Let's see this method in action:

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assertThat(isNumeric( "22" )).isTrue(); assertThat(isNumeric( "5.05" )).isTrue(); assertThat(isNumeric( "-200" )).isTrue(); assertThat(isNumeric( "10.0d" )).isTrue(); assertThat(isNumeric( "   22   " )).isTrue();    assertThat(isNumeric( null )).isFalse(); assertThat(isNumeric( "" )).isFalse(); assertThat(isNumeric( "abc" )).isFalse();

In our isNumeric() method, we're just checking for values that are of type Double, but this method can also be modified to check for Integer, Float, Long and large numbers by using any of the parse methods that we have enlisted earlier.

These methods are also discussed in the Java String Conversions article.

4. Using Regular Expressions

 

Now let's use regex -?\d+(\.\d+)? to match numeric Strings consisting of the positive or negative integer and floats.

But this goes without saying, that we can definitely modify this regex to identify and handle a wide range of rules. Here, we'll keep it simple.

Let’s break down this regex and see how it works:

• -? – this part identifies if the given number is negative, the dash “–” searches for dash literally and the question mark “?” marks its presence as an optional one
• \d+ – this searches for one or more digits
• (\.\d+)? – this part of regex is to identify float numbers. Here we're searching for one or more digits followed by a period. The question mark, in the end, signifies that this complete group is optional

Regular expressions are a very broad topic. To get a brief overview, check our tutorial on the Java regular expressions API.

For now, let's create a method using the above regular expression:

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private Pattern pattern = Pattern.compile( "-?\\d+(\\.\\d+)?" );   public boolean isNumeric(String strNum) {      if (strNum == null ) {          return false ;      }      return pattern.matcher(strNum).matches(); }

Let's now look at some assertions for the above method:

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assertThat(isNumeric( "22" )).isTrue(); assertThat(isNumeric( "5.05" )).isTrue(); assertThat(isNumeric( "-200" )).isTrue();   assertThat(isNumeric( null )).isFalse(); assertThat(isNumeric( "abc" )).isFalse();

 5. Using Apache Commons

 

In this section, we'll discuss various methods available in the Apache Commons library.

5.1. NumberUtils.isCreatable(String) 

 

NumberUtils from Apache Commons provides a static method NumberUtils.isCreatable(String) which checks whether a String is a valid Java number or not.

This method accepts:

1. Hexadecimal numbers starting with 0x or 0X
2. Octal numbers starting with a leading 0
3. Scientific notation (for example 1.05e-10)
4. Numbers marked with a type qualifier (for example 1L or 2.2d)

If the supplied string is null or empty/blank, then it's not considered a number and the method will return false.

Let's run some tests using this method:

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assertThat(NumberUtils.isCreatable( "22" )).isTrue(); assertThat(NumberUtils.isCreatable( "5.05" )).isTrue(); assertThat(NumberUtils.isCreatable( "-200" )).isTrue(); assertThat(NumberUtils.isCreatable( "10.0d" )).isTrue(); assertThat(NumberUtils.isCreatable( "1000L" )).isTrue(); assertThat(NumberUtils.isCreatable( "0xFF" )).isTrue(); assertThat(NumberUtils.isCreatable( "07" )).isTrue(); assertThat(NumberUtils.isCreatable( "2.99e+8" )).isTrue();    assertThat(NumberUtils.isCreatable( null )).isFalse(); assertThat(NumberUtils.isCreatable( "" )).isFalse(); assertThat(NumberUtils.isCreatable( "abc" )).isFalse(); assertThat(NumberUtils.isCreatable( " 22 " )).isFalse(); assertThat(NumberUtils.isCreatable( "09" )).isFalse();

Note how we're getting true assertions for hexadecimal numbers, octal numbers and scientific notations in lines 6, 7 and 8 respectively.

 

Also, on line 14, the string “09” returns false because the preceding “0” indicates that this is an octal number and “09” is not a valid octal number.

For every input that returns true with this method, we can use NumberUtils.createNumber(String) which will give us the valid number.

5.2. NumberUtils.isParsable(String) 

 

The NumberUtils.isParsable(String) method checks whether the given String is parsable or not.

Parsable numbers are those that are parsed successfully by any parse method like Integer.parseInt(String), Long.parseLong(String), Float.parseFloat(String) or Double.parseDouble(String).

Unlike NumberUtils.isCreatable(), this method won't accept hexadecimal numbers, scientific notations or strings ending with any type qualifier, that is, ‘f', ‘F', ‘d' ,'D' ,'l'or‘L'.

Let's look at some affirmations:

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assertThat(NumberUtils.isParsable( "22" )).isTrue(); assertThat(NumberUtils.isParsable( "-23" )).isTrue(); assertThat(NumberUtils.isParsable( "2.2" )).isTrue(); assertThat(NumberUtils.isParsable( "09" )).isTrue();   assertThat(NumberUtils.isParsable( null )).isFalse(); assertThat(NumberUtils.isParsable( "" )).isFalse(); assertThat(NumberUtils.isParsable( "6.2f" )).isFalse(); assertThat(NumberUtils.isParsable( "9.8d" )).isFalse(); assertThat(NumberUtils.isParsable( "22L" )).isFalse(); assertThat(NumberUtils.isParsable( "0xFF" )).isFalse(); assertThat(NumberUtils.isParsable( "2.99e+8" )).isFalse();

On line 4, unlike NumberUtils.isCreatable(), the number starting with string “0” isn't considered as an octal number, but a normal decimal number and hence it returns true.

We can use this method as a replacement for what we did in section 3, where we’re trying to parse a number and checking for an error.

5.3. StringUtils.isNumeric(CharSequence) 

 

The method StringUtils.isNumeric(CharSequence) checks strictly for Unicode digits. This means:

1. Any digits from any language that is a Unicode digit is acceptable
2. Since a decimal point is not considered as a Unicode digit, it's not valid
3. Leading signs (either positive or negative) are also not acceptable

Let's now see this method in action:

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assertThat(StringUtils.isNumeric( "123" )).isTrue(); assertThat(StringUtils.isNumeric( "١٢٣" )).isTrue(); assertThat(StringUtils.isNumeric( "१२३" )).isTrue();    assertThat(StringUtils.isNumeric( null )).isFalse(); assertThat(StringUtils.isNumeric( "" )).isFalse(); assertThat(StringUtils.isNumeric( "  " )).isFalse(); assertThat(StringUtils.isNumeric( "12 3" )).isFalse(); assertThat(StringUtils.isNumeric( "ab2c" )).isFalse(); assertThat(StringUtils.isNumeric( "12.3" )).isFalse(); assertThat(StringUtils.isNumeric( "-123" )).isFalse();

Note that the input parameters in lines 2 and 3 are representing numbers 123 in Arabic and Devanagari respectively. Since they're valid Unicode digits, this method returns true on them.

5.4.  StringUtils.isNumericSpace(CharSequence)

 

The StringUtils.isNumericSpace(CharSequence) checks strictly for Unicode digits and/or space. This is same as StringUtils.isNumeric() with the only difference being that it accepts spaces as well, not only leading and trailing spaces but also if they're in between numbers:

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assertThat(StringUtils.isNumericSpace( "123" )).isTrue(); assertThat(StringUtils.isNumericSpace( "١٢٣" )).isTrue(); assertThat(StringUtils.isNumericSpace( "" )).isTrue(); assertThat(StringUtils.isNumericSpace( "  " )).isTrue(); assertThat(StringUtils.isNumericSpace( "12 3" )).isTrue();    assertThat(StringUtils.isNumericSpace( null )).isFalse(); assertThat(StringUtils.isNumericSpace( "ab2c" )).isFalse(); assertThat(StringUtils.isNumericSpace( "12.3" )).isFalse(); assertThat(StringUtils.isNumericSpace( "-123" )).isFalse();