commons-collections 之 LazyMap

作用：只有真正调用的时候才去加载

注：关注 public V get(final Object key) 方法

• 内部实现：通过 Transformer<? super K, ? extends V> factory; 映射key到value,通过key查找的时候，如果有就直接返回value,如果没有就会调用Transformer去读取相关数据
  下一章会说，Transformer是如何实现获取数据的

package org.apache.commons.collections4.map;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.Map;

import org.apache.commons.collections4.Factory;
import org.apache.commons.collections4.Transformer;
import org.apache.commons.collections4.functors.FactoryTransformer;


public class LazyMap<K, V> extends AbstractMapDecorator<K, V> implements Serializable {

    /** Serialization version */
    private static final long serialVersionUID = 7990956402564206740L;

    /** The factory to use to construct elements */
    protected final Transformer<? super K, ? extends V> factory;

    /**
     * Factory method to create a lazily instantiated map.
     *
     * @param <K>  the key type
     * @param <V>  the value type
     * @param map  the map to decorate, must not be null
     * @param factory  the factory to use, must not be null
     * @return a new lazy map
     * @throws NullPointerException if map or factory is null
     * @since 4.0
     */
    public static <K, V> LazyMap<K, V> lazyMap(final Map<K, V> map, final Factory< ? extends V> factory) {
        return new LazyMap<K,V>(map, factory);
    }

    /**
     * Factory method to create a lazily instantiated map.
     *
     * @param <K>  the key type
     * @param <V>  the value type
     * @param map  the map to decorate, must not be null
     * @param factory  the factory to use, must not be null
     * @return a new lazy map
     * @throws NullPointerException if map or factory is null
     * @since 4.0
     */
    public static <V, K> LazyMap<K, V> lazyMap(final Map<K, V> map, final Transformer<? super K, ? extends V> factory) {
        return new LazyMap<K,V>(map, factory);
    }

    //-----------------------------------------------------------------------
    /**
     * Constructor that wraps (not copies).
     *
     * @param map  the map to decorate, must not be null
     * @param factory  the factory to use, must not be null
     * @throws NullPointerException if map or factory is null
     */
    protected LazyMap(final Map<K,V> map, final Factory<? extends V> factory) {
        super(map);
        if (factory == null) {
            throw new NullPointerException("Factory must not be null");
        }
        this.factory = FactoryTransformer.factoryTransformer(factory);
    }

    /**
     * Constructor that wraps (not copies).
     *
     * @param map  the map to decorate, must not be null
     * @param factory  the factory to use, must not be null
     * @throws NullPointerException if map or factory is null
     */
    protected LazyMap(final Map<K,V> map, final Transformer<? super K, ? extends V> factory) {
        super(map);
        if (factory == null) {
            throw new NullPointerException("Factory must not be null");
        }
        this.factory = factory;
    }


    @Override
    public V get(final Object key) {
        // create value for key if key is not currently in the map
        if (map.containsKey(key) == false) {
            @SuppressWarnings("unchecked")
            final K castKey = (K) key;
            final V value = factory.transform(castKey);
            map.put(castKey, value);
            return value;
        }
        return map.get(key);
    }

    // no need to wrap keySet, entrySet or values as they are views of
    // existing map entries - you can't do a map-style get on them.
}

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