How to check if a String is numeric in Java

Question

How would you check if a String was a number before parsing it?

Answer 1

Here was my answer to the problem.

A catch all convenience method which you can use to parse any String with any type of parser: isParsable(Object parser, String str). The parser can be a Class or an object. This will also allows you to use custom parsers you've written and should work for ever scenario, eg:

isParsable(Integer.class, "11");
isParsable(Double.class, "11.11");
Object dateFormater = new java.text.SimpleDateFormat("yyyy.MM.dd G 'at' HH:mm:ss z");
isParsable(dateFormater, "2001.07.04 AD at 12:08:56 PDT");

Here's my code complete with method descriptions.

import java.lang.reflect.*;

/**
 * METHOD: isParsable

 * 
 * This method will look through the methods of the specified from parameter
 * looking for a public method name starting with "parse" which has only one String
 * parameter.

 * 
 * The parser parameter can be a class or an instantiated object, eg:
 * Integer.class or new Integer(1). If you use a
 * Class type then only static methods are considered.

 * 
 * When looping through potential methods, it first looks at the Class associated
 * with the parser parameter, then looks through the methods of the parent's class
 * followed by subsequent ancestors, using the first method that matches the criteria specified
 * above.

 * 
 * This method will hide any normal parse exceptions, but throws any exceptions due to
 * programmatic errors, eg: NullPointerExceptions, etc. If you specify a parser
 * parameter which has no matching parse methods, a NoSuchMethodException will be thrown
 * embedded within a RuntimeException.

 * 
 * Example:

 * isParsable(Boolean.class, "true");

 * isParsable(Integer.class, "11");

 * isParsable(Double.class, "11.11");

 * Object dateFormater = new java.text.SimpleDateFormat("yyyy.MM.dd G 'at' HH:mm:ss z");

 * isParsable(dateFormater, "2001.07.04 AD at 12:08:56 PDT");

 * 

 * 
 * @param parser    The Class type or instantiated Object to find a parse method in.
 * @param str   The String you want to parse
 * 
 * @return true if a parse method was found and completed without exception
 * @throws java.lang.NoSuchMethodException If no such method is accessible 
 */
public static boolean isParsable(Object parser, String str) {
    Class theClass = (parser instanceof Class? (Class)parser: parser.getClass());
    boolean staticOnly = (parser == theClass), foundAtLeastOne = false;
    Method[] methods = theClass.getMethods();

    // Loop over methods
    for (int index = 0; index < methods.length; index++) {
        Method method = methods[index];

        // If method starts with parse, is public and has one String parameter.
        // If the parser parameter was a Class, then also ensure the method is static. 
        if(method.getName().startsWith("parse") &&
            (!staticOnly || Modifier.isStatic(method.getModifiers())) &&
            Modifier.isPublic(method.getModifiers()) &&
            method.getGenericParameterTypes().length == 1 &&
            method.getGenericParameterTypes()[0] == String.class)
        {
            try {
                foundAtLeastOne = true;
                method.invoke(parser, str);
                return true; // Successfully parsed without exception
            } catch (Exception exception) {
                // If invoke problem, try a different method
                /*if(!(exception instanceof IllegalArgumentException) &&
                   !(exception instanceof IllegalAccessException) &&
                   !(exception instanceof InvocationTargetException))
                        continue; // Look for other parse methods*/

                // Parse method refuses to parse, look for another different method
                continue; // Look for other parse methods
            }
        }
    }

    // No more accessible parse method could be found.
    if(foundAtLeastOne) return false;
    else throw new RuntimeException(new NoSuchMethodException());
}


/**
 * METHOD: willParse

 * 
 * A convienence method which calls the isParseable method, but does not throw any exceptions
 * which could be thrown through programatic errors.

 * 
 * Use of {@link #isParseable(Object, String) isParseable} is recommended for use so programatic
 * errors can be caught in development, unless the value of the parser parameter is
 * unpredictable, or normal programtic exceptions should be ignored.

 * 
 * See {@link #isParseable(Object, String) isParseable} for full description of method
 * usability.

 * 
 * @param parser    The Class type or instantiated Object to find a parse method in.
 * @param str   The String you want to parse
 * 
 * @return true if a parse method was found and completed without exception
 * @see #isParseable(Object, String) for full description of method usability 
 */
public static boolean willParse(Object parser, String str) {
    try {
        return isParsable(parser, str);
    } catch(Throwable exception) {
        return false;
    }
}