MicroMap.java

/* 
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */

package org.drools.chance.core.util;


import java.io.Serializable;
import java.util.AbstractList;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

/**
 * An implementation of the java.util.Map interface which can only hold a single
 * object. This is particularly useful to control memory usage in Wicket because
 * many containers hold only a single component.
 *
 * @author Jonathan Locke
 */
public final class MicroMap<K,T> implements Map<K,T>, Serializable
{
    private static final long serialVersionUID = 1L;

    /** The maximum number of entries this map supports. */
    public static final int MAX_ENTRIES = 1;

    /** The one and only key in this tiny map */
    private K key;

    /** The value for the only key in this tiny map */
    private T value;

    /**
     * Constructor
     */
    public MicroMap()
    {
    }

    /**
     * Constructs map with a single key and value pair.
     *
     * @param key
     *            The key
     * @param value
     *            The value
     */
    public MicroMap(final K key, final T value)
    {
        this.key = key;
        this.value = value;
    }

    /**
     * @return True if this MicroMap is full
     */
    public boolean isFull()
    {
        return size() == MAX_ENTRIES;
    }

    /**
     * @see java.util.Map#size()
     */
    public int size()
    {
        return (key != null) ? 1 : 0;
    }

    /**
     * @see java.util.Map#isEmpty()
     */
    public boolean isEmpty()
    {
        return size() == 0;
    }

    /**
     * @see java.util.Map#containsKey(java.lang.Object)
     */
    public boolean containsKey(final Object key)
    {
        return key.equals(this.key);
    }

    /**
     * @see java.util.Map#containsValue(java.lang.Object)
     */
    public boolean containsValue(final Object value)
    {
        return value.equals(this.value);
    }

    /**
     * @see java.util.Map#get(java.lang.Object)
     */
    public T get(final Object key)
    {
        if (key.equals(this.key))
        {
            return value;
        }

        return null;
    }

    /**
     * @see java.util.Map#put(java.lang.Object, java.lang.Object)
     */
    public T put(final K key, final T value)
    {
        // Replace?
        if (key.equals(this.key))
        {
            final T oldValue = this.value;

            this.value = value;

            return oldValue;
        }
        else
        {
            // Is there room for a new entry?
            if (size() < MAX_ENTRIES)
            {
                // Store
                this.key = key;
                this.value = value;

                return null;
            }
            else
            {
                throw new IllegalStateException("Map full");
            }
        }
    }

    /**
     * @see java.util.Map#remove(java.lang.Object)
     */
    public T remove(final Object key)
    {
        if (key.equals(this.key))
        {
            final T oldValue = this.value;

            this.key = null;
            this.value = null;

            return oldValue;
        }

        return null;
    }

    /**
     * @see java.util.Map#putAll(java.util.Map)
     */
    public void putAll(final Map<? extends K,? extends T> map)
    {
        if (map.size() <= MAX_ENTRIES)
        {
            final Entry<? extends K, ? extends T> e = map.entrySet().iterator().next();

            put(e.getKey(), e.getValue());
        }
        else
        {
            throw new IllegalStateException("Map full.  Cannot add " + map.size() + " entries");
        }
    }

    /**
     * @see java.util.Map#clear()
     */
    public void clear()
    {
        key = null;
        value = null;
    }

    /**
     * @see java.util.Map#keySet()
     */
    public Set<K> keySet()
    {
        return new AbstractSet<K>()
        {
            public Iterator<K> iterator()
            {
                return new Iterator<K>()
                {
                    public boolean hasNext()
                    {
                        return index < MicroMap.this.size();
                    }

                    public K next()
                    {
                        index++;

                        return key;
                    }

                    public void remove()
                    {
                        MicroMap.this.clear();
                    }

                    int index;
                };
            }

            public int size()
            {
                return MicroMap.this.size();
            }
        };
    }

    /**
     * @see java.util.Map#values()
     */
    public Collection<T> values()
    {
        return new AbstractList<T>()
        {
            public T get(final int index)
            {
                return value;
            }

            public int size()
            {
                return MicroMap.this.size();
            }
        };
    }

    /**
     * @see java.util.Map#entrySet()
     */
    public Set<Entry<K,T>> entrySet()
    {
        return new AbstractSet<Entry<K,T>>()
        {
            public Iterator<Entry<K,T>> iterator()
            {
                return new Iterator<Entry<K,T>>()
                {
                    public boolean hasNext()
                    {
                        return index < MicroMap.this.size();
                    }

                    public Entry<K,T> next()
                    {
                        index++;

                        return new Map.Entry<K,T>()
                        {
                            public K getKey()
                            {
                                return key;
                            }

                            public T getValue()
                            {
                                return value;
                            }

                            public T setValue(final T value)
                            {
                                final T oldValue = MicroMap.this.value;

                                MicroMap.this.value = value;

                                return oldValue;
                            }
                        };
                    }

                    public void remove()
                    {
                        clear();
                    }

                    int index = 0;
                };
            }

            public int size()
            {
                return MicroMap.this.size();
            }
        };
    }
}