Java 8收集器方法toMap

柔情只为你懂 2022-11-15 03:40 206阅读 0赞

# 1.概述 #

Collectors类的toMap（）方法。 我们将使用它来将Streams收集到Map实例中。

# 2.List转Map #

从最简单的情况开始，将List转换为Map。

定义Book类：

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public class Book {
        private String name;
        private int releaseYear;
        private String isbn;
    }

创建书清单

List<Book> bookList = new ArrayList<>();
    bookList.add(new Book("java8实战", 2013, "0395489318"));
    bookList.add(new Book("spring实战", 2013, "0345339711"));
    bookList.add(new Book("springboot实战", 2015, "0618129111"));

在这种情况下，将使用toMap()方法的以下重载：

Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,
      Function<? super T, ? extends U> valueMapper)

使用toMap，可以指出如何获Map 的key和value的策略：

public Map<String, String> listToMap(List<Book> books) { 
            return books.stream().collect(Collectors.toMap(Book::getIsbn, Book::getName));
        }
    
        @Test
        public void whenConvertFromListToMap() { 
            Map<String, String>  map=listToMap(bookList);
            System.out.println(map);//{0395489318=java8实战, 0618129111=springboot实战, 0345339711=spring实战}
            assertEquals(3, listToMap(bookList).size());
        }

# 3.解决Key冲突 #

假设按每本书的发行年份确定了Map的key：

public Map<Integer, Book> listToMapWithDupKeyError(List<Book> books) { 
            return books.stream().collect(
                    Collectors.toMap(Book::getReleaseYear, Function.identity()));
        }
    
        @Test(expected = IllegalStateException.class)
        public void whenMapHasDuplicateKey() { 
            Map<Integer, Book> integerBookMap = listToMapWithDupKeyError(bookList);
            System.out.println(integerBookMap);
        }

要解决此问题，需要使用带有附加参数mergeFunction的其他方法：

Collector<T, ?, M> toMap(Function<? super T, ? extends K> keyMapper,
      Function<? super T, ? extends U> valueMapper,
      BinaryOperator<U> mergeFunction)

介绍一个合并函数，该函数指示在发生冲突的情况下，保留现有条目：

public Map<Integer, Book> listToMapWithDupKey(List<Book> books) { 
            return books.stream().collect(Collectors.toMap(Book::getReleaseYear, Function.identity(),
                    (existing, replacement) -> existing));
        }
    
        @Test
        public void whenMapHasDuplicateKeyThenMergeFunctionHandlesCollision() { 
            Map<Integer, Book> booksByYear = listToMapWithDupKey(bookList);
            System.out.println(booksByYear);//{2013=Book(name=java8实战, releaseYear=2013, isbn=0395489318), 2015=Book(name=springboot实战, releaseYear=2015, isbn=0618129111)}
            assertEquals(2, booksByYear.size());
            assertEquals("0395489318", booksByYear.get(2013).getIsbn());
        }

# 4.其他Map类型 #

默认情况下，toMap()方法将返回HashMap。

但是可以返回不同的Map实现：

Collector<T, ?, M> toMap(Function<? super T, ? extends K> keyMapper,
      Function<? super T, ? extends U> valueMapper,
      BinaryOperator<U> mergeFunction,
      Supplier<M> mapSupplier)

其中的mapSupplier是一个函数，该函数返回带有结果的新的空Map。

## 4.1. List 转ConcurrentMap ##

public Map<Integer, Book> listToConcurrentMap(List<Book> books) { 
            return books.stream().collect(Collectors.toMap(Book::getReleaseYear, Function.identity(),
                    (o1, o2) -> o1, ConcurrentHashMap::new));
        }
        @Test
        public void whenCreateConcurrentHashMap() { 
            assertTrue(listToConcurrentMap(bookList) instanceof ConcurrentHashMap);
        }

## 4.2Sorted Map ##

如何返回排序后的Map。 将使用TreeMap作为mapSupplier参数。

public TreeMap<String, Book> listToSortedMap(List<Book> books) { 
            return books.stream()
                    .collect(
                            Collectors.toMap(Book::getName, Function.identity(), (o1, o2) -> o1, TreeMap::new));
        }
    
        @Test
        public void whenMapisSorted() { 
            TreeMap<String, Book> treeMap = listToSortedMap(bookList);
            System.out.println(treeMap);
            //{java8实战=Book(name=java8实战, releaseYear=2013, isbn=0395489318), springboot实战=Book(name=springboot实战, releaseYear=2015, isbn=0618129111), spring实战=Book(name=spring实战, releaseYear=2013, isbn=0345339711)}
            assertEquals("java8实战", treeMap.firstKey());
        }

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