package org.apidesign.javamonitorflaws;

import java.util.HashMap;

import java.util.Map;

/** Classical caching support class that makes sure there is

 * always one "To" value for each "From" one returned from the {@link #get}

 * method. However it does not prevent multiple threads to call

 * {@link #createItem} multiple times for the same "From" value.

 * <p>

 *

 *

 * @author Jaroslav Tulach <jtulach@netbeans.org>

 */

// BEGIN: monitor.pitfalls.Cache

public abstract class Cache<From,To> {

    private Map<From,To> cache;

    protected abstract To createItem(From f);

    public final To get(From f) {

        To t = inspectValue(f);

        if (t != null) {

            return t;

        }

        To newT = createItem(f);

        return registerValue(f, newT);

    }

    private synchronized To inspectValue(From f) {

        if (cache == null) {

            cache = new HashMap<From, To>();

        }

        return cache.get(f);

    }

    private synchronized To registerValue(From f, To newT) {

        To t = cache.get(f);

        if (t == null) {

            cache.put(f, newT);

            return newT;

        } else {

            return t;

        }

    }

}

// END: monitor.pitfalls.Cache