java定义IP地址形式输入的规则

以下的源码来出自《JAVA核心编程I：基本知识（原书第7版）》

import java.text.ParseException;

/**
 * A formatter for 4-byte IP addresses of the form a.b.c.d
 *
 */
public class IPAddressFormatter extends DefaultFormatter {
    @Override
    public String valueToString(Object value) throws ParseException {
        if (!(value instanceof byte[]))
            throw new ParseException("Not a byte[]", 0);
        byte[] a = (byte[]) value;
        if (a.length != 4)
            throw new ParseException("Length != 4", 0);
        StringBuilder builder = new StringBuilder();
        for (int i = 0; i < 4; i++) {
            int b = a[i];
            if (b < 0)
                b += 256;
            builder.append(String.valueOf(b));
            if (i < 3)
                builder.append(".");
        }
        return builder.toString();
    }

    @Override
    public Object stringToValue(String text) throws ParseException {
        StringTokenizer tokenizer = new StringTokenizer(text, ".") ;
        byte[] a = new byte[4] ;
        for(int i=0; i<4 ;i++){
            int b = 0 ;
            if(!tokenizer.hasMoreTokens())
                throw new ParseException("Too few bytes", 0) ;
            try{
                b = Integer.parseInt(tokenizer.nextToken()) ;
            }catch (NumberFormatException e){
                throw new ParseException("Not an integer", 0) ;
            }
            if(b<0 || b>=256)
                throw new ParseException("Byte out of range", 0) ;
            a[i] = (byte) b ;
        }
        if(tokenizer.hasMoreTokens())
            throw new ParseException("Too many bytes", 0 ) ;
        return a ;        
    }
}