java中实现list或set转map的方法

java中实现list或set转map的方法

在开发中我们有时需要将list或set转换为map（比如对象属性中的唯一键作为map的key，对象作为map的value），一般的想法就是new一个map，然后把list或set中的值一个个push到map中。

类似下面的代码：

list<string> stringlist = lists.newarraylist("t1", "t2", "t3"); 
map<string, string> map = maps.newhashmapwithexpectedsize(stringlist.size()); 
for (string str : stringlist) { 
  map.put(str, str); 
} 

是否还有更优雅的写法呢？答案是有的。

guava提供了集合（实现了iterables接口或iterator接口）转map的方法，方法定义如下：

/** 

* returns an immutable map for which the {@link map#values} are the given 
 * elements in the given order, and each key is the product of invoking a 
 * supplied function on its corresponding value. 
 * 
 * @param values the values to use when constructing the {@code map} 
 * @param keyfunction the function used to produce the key for each value 
 * @return a map mapping the result of evaluating the function {@code 
 *     keyfunction} on each value in the input collection to that value 
 * @throws illegalargumentexception if {@code keyfunction} produces the same 
 *     key for more than one value in the input collection 
 * @throws nullpointerexception if any elements of {@code values} is null, or 
 *     if {@code keyfunction} produces {@code null} for any value 
 */ 
public static <k, v> immutablemap<k, v> uniqueindex( 
  iterable<v> values, function<? super v, k> keyfunction) { 
 return uniqueindex(values.iterator(), keyfunction); 
} 
 
/** 
 * returns an immutable map for which the {@link map#values} are the given 
 * elements in the given order, and each key is the product of invoking a 
 * supplied function on its corresponding value. 
 * 
 * @param values the values to use when constructing the {@code map} 
 * @param keyfunction the function used to produce the key for each value 
 * @return a map mapping the result of evaluating the function {@code 
 *     keyfunction} on each value in the input collection to that value 
 * @throws illegalargumentexception if {@code keyfunction} produces the same 
 *     key for more than one value in the input collection 
 * @throws nullpointerexception if any elements of {@code values} is null, or 
 *     if {@code keyfunction} produces {@code null} for any value 
 * @since 10.0 
 */ 
public static <k, v> immutablemap<k, v> uniqueindex( 
  iterator<v> values, function<? super v, k> keyfunction) { 
 checknotnull(keyfunction); 
 immutablemap.builder<k, v> builder = immutablemap.builder(); 
 while (values.hasnext()) { 
  v value = values.next(); 
  builder.put(keyfunction.apply(value), value); 
 } 
 return builder.build(); 
} 

这样我们就可以很方便的进行转换了，如下：

list<string> stringlist = lists.newarraylist("t1", "t2", "t3"); 
map<string, string> map = maps.uniqueindex(stringlist, new function<string, string>() { 
  @override 
  public string apply(string input) { 
    return input; 
  } 
}); 

需要注意的是，如接口注释所说，如果function返回的结果产生了重复的key，将会抛出异常。

java8也提供了转换的方法，这里直接照搬别人博客的代码：

@test  
public void convert_list_to_map_with_java8_lambda () {  
    
  list<movie> movies = new arraylist<movie>();  
  movies.add(new movie(1, "the shawshank redemption"));  
  movies.add(new movie(2, "the godfather"));  
  
  map<integer, movie> mappedmovies = movies.stream().collect(  
      collectors.tomap(movie::getrank, (p) -> p));  
  
  logger.info(mappedmovies);  
  
  asserttrue(mappedmovies.size() == 2);  
  assertequals("the shawshank redemption", mappedmovies.get(1).getdescription());  
}  

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