guava:java:java.util.Map和java.util.Set的Key类型转换

package net.gdface.facelog;

import static com.google.common.base.Preconditions.checkNotNull;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;

import org.junit.Test;

import com.google.common.base.Function;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.Iterators;
import com.google.common.collect.Maps;
import com.google.common.collect.Maps.EntryTransformer;

public class TestTransform {
    /** 
     * convert {@code Map<K1,V>} to {@code Map<K2,V>}   
     * @return {@linkplain ImmutableMap}
     */
    public static final <K1,K2,V>Map<K2,V> transform(Map<K1,V>fromMap,final Function<K1,K2>transformer){
        checkNotNull(fromMap,"fromMap is null");
        checkNotNull(transformer,"transformer is null");
        // 新的Map对象Key类型已经是K2了，只是Value类型成了Entry
        ImmutableMap<K2, Entry<K1, V>> k2Entry = Maps.uniqueIndex(fromMap.entrySet(), new Function<Entry<K1, V>,K2>(){
            @Override
            public K2 apply(Entry<K1, V> input) {               
                return transformer.apply(input.getKey());
            }});
        // 再做一次转换将Entry<K1, V>转换成V,这个过程并没有创建新的Map,只是创建了k2Entry的代理对象
        Map<K2, V> k2V = Maps.transformEntries(k2Entry, new EntryTransformer<K2,Entry<K1,V>,V>(){
            @Override
            public V transformEntry(K2 key, Entry<K1, V> value) {
                return value.getValue();
            }});
        return k2V;
    }
    /** 
     * convert {@code Set<E1>} to {@code Set<E2>}
     * @return {@link ImmutableSet} 
     */
    public static final <E1,E2>Set<E2> transform(final Set<E1>fromSet,final Function<E1,E2>transformer){
        checkNotNull(fromSet,"fromMap is null");
        checkNotNull(transformer,"transformer is null");
        return ImmutableSet.copyOf(Iterators.transform(fromSet.iterator(), transformer));
    }
    @Test
    public void test() {
        Map<String, String> m1 = Maps.<String,String>newLinkedHashMap();
        m1.put("1", "apple");
        m1.put("2", "orangle");
        m1.put("3", "banana");
        System.out.println(m1);
        Map<Integer, String> m2 = transform(m1, new Function<String,Integer>(){

            @Override
            public Integer apply(String input) {
                return Integer.valueOf(input);
            }});
        System.out.println(m2.toString());
    }

}