Over a million developers have joined DZone.

A Regular Expression HashMap Implementation in Java

DZone 's Guide to

A Regular Expression HashMap Implementation in Java

· Java Zone ·
Free Resource

Below is an implementation of a Regular Expression HashMap. It works with key-value pairs which the key is a regular expression. It compiles the key (regular expression) while adding (i.e. putting), so there is no compile time while getting. Once getting an element, you don't give regular expression; you give any possible value of a regular expression. 

As a result, this behaviour provides to map numerous values of a regular expression into the same value. The class does not depend to any external libraries, uses only default java.util. So, it will be used simply when a behaviour like that is required.

import java.util.ArrayList;
import java.util.HashMap;
import java.util.regex.Pattern;
 * This class is an extended version of Java HashMap
 * and includes pattern-value lists which are used to
 * evaluate regular expression values. If given item
 * is a regular expression, it is saved in regexp lists.
 * If requested item matches with a regular expression,
 * its value is get from regexp lists.
 * @author cb
 * @param <K> : Key of the map item.
 * @param <V> : Value of the map item.
public class RegExHashMap<K, V> extends HashMap<K, V> {
    // list of regular expression patterns
    private ArrayList<Pattern> regExPatterns = new ArrayList<Pattern>();
    // list of regular expression values which match patterns
    private ArrayList<V> regExValues = new ArrayList<V>();
     * Compile regular expression and add it to the regexp list as key.
    public V put(K key, V value) {
        return value;
     * If requested value matches with a regular expression,
     * returns it from regexp lists.
    public V get(Object key) {
        CharSequence cs = new String(key.toString());
        for (int i = 0; i < regExPatterns.size(); i++) {
            if (regExPatterns.get(i).matcher(cs).matches()) {
                return regExValues.get(i);
        return super.get(key);



Published at DZone with permission of

Opinions expressed by DZone contributors are their own.

{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}