java regular expression returning false

Question

I am newbie to java regular expression. I wrote following code for validating the non digit number. If we enter any non digit number it should return false. for me the below code always return false. whats the wrong here?

package regularexpression;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class NumberValidator {

    private static final String NUMBER_PATTERN = "\\d";
    Pattern pattern;

    public NumberValidator() {
        pattern = Pattern.compile(NUMBER_PATTERN);
    }

    public boolean validate(String line){
        Matcher matcher = pattern.matcher(line);
        return matcher.matches();
    }

    public static void main(String[] args) {

        NumberValidator validator = new NumberValidator();

        boolean validate = validator.validate("123");

        System.out.println("validate:: "+validate);
    }

}

Answer 1

From Java documentation:

The matches method attempts to match the entire input sequence against the pattern.

Your regular expression matches a single digit, not a number. Add + after \\d to matchone or more digits:

private static final String NUMBER_PATTERN = "\\d+";

As a side note, you can combine initialization and declaration of pattern, making the constructor unnecessary:

Pattern pattern = Pattern.compile(NUMBER_PATTERN);

Answer 2

matches "returns true if, and only if, the entire region sequence matches this matcher's pattern."

The string is 3 digits, which doesn't match the pattern \d, meaning 'a digit'.

Instead you want the pattern \d+, meaning 'one or more digits.' This is expressed in a string as "\\d+"