list互轉map java8

 list轉map

/**
     * If the specified key is not already associated with a value or is
     * associated with null, associates it with the given non-null value.
     * Otherwise, replaces the associated value with the results of the given
     * remapping function, or removes if the result is {@code null}. This
     * method may be of use when combining multiple mapped values for a key.
     * For example, to either create or append a {@code String msg} to a
     * value mapping:
     *
     * <pre> {@code
     * map.merge(key, msg, String::concat)
     * }</pre>
     *
     * <p>If the function returns {@code null} the mapping is removed.  If the
     * function itself throws an (unchecked) exception, the exception is
     * rethrown, and the current mapping is left unchanged.
     *
     * @implSpec
     * The default implementation is equivalent to performing the following
     * steps for this {@code map}, then returning the current value or
     * {@code null} if absent:
     *
     * <pre> {@code
     * V oldValue = map.get(key);
     * V newValue = (oldValue == null) ? value :
     *              remappingFunction.apply(oldValue, value);
     * if (newValue == null)
     *     map.remove(key);
     * else
     *     map.put(key, newValue);
     * }</pre>
     *
     * <p>The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties. In particular, all implementations of
     * subinterface {@link java.util.concurrent.ConcurrentMap} must document
     * whether the function is applied once atomically only if the value is not
     * present.
     *
     * @param key key with which the resulting value is to be associated
     * @param value the non-null value to be merged with the existing value
     *        associated with the key or, if no existing value or a null value
     *        is associated with the key, to be associated with the key
     * @param remappingFunction the function to recompute a value if present
     * @return the new value associated with the specified key, or null if no
     *         value is associated with the key
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified key is null and this map
     *         does not support null keys or the value or remappingFunction is
     *         null
     * @since 1.8
     */
    default V merge(K key, V value,
            BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
        Objects.requireNonNull(remappingFunction);
        Objects.requireNonNull(value);
        V oldValue = get(key);
        V newValue = (oldValue == null) ? value :
                   remappingFunction.apply(oldValue, value);
        if(newValue == null) {
            remove(key);
        } else {
            put(key, newValue);
        }
        return newValue;
    }

/**
     * Returns a {@code Collector} that accumulates elements into a
     * {@code Map} whose keys and values are the result of applying the provided
     * mapping functions to the input elements.
     *
     * <p>If the mapped
     * keys contains duplicates (according to {@link Object#equals(Object)}),
     * the value mapping function is applied to each equal element, and the
     * results are merged using the provided merging function.  The {@code Map}
     * is created by a provided supplier function.
     *
     * @implNote
     * The returned {@code Collector} is not concurrent.  For parallel stream
     * pipelines, the {@code combiner} function operates by merging the keys
     * from one map into another, which can be an expensive operation.  If it is
     * not required that results are merged into the {@code Map} in encounter
     * order, using {@link #toConcurrentMap(Function, Function, BinaryOperator, Supplier)}
     * may offer better parallel performance.
     *
     * @param <T> the type of the input elements
     * @param <K> the output type of the key mapping function
     * @param <U> the output type of the value mapping function
     * @param <M> the type of the resulting {@code Map}
     * @param keyMapper a mapping function to produce keys
     * @param valueMapper a mapping function to produce values
     * @param mergeFunction a merge function, used to resolve collisions between
     *                      values associated with the same key, as supplied
     *                      to {@link Map#merge(Object, Object, BiFunction)}
     * @param mapSupplier a function which returns a new, empty {@code Map} into
     *                    which the results will be inserted
     * @return a {@code Collector} which collects elements into a {@code Map}
     * whose keys are the result of applying a key mapping function to the input
     * elements, and whose values are the result of applying a value mapping
     * function to all input elements equal to the key and combining them
     * using the merge function
     *
     * @see #toMap(Function, Function)
     * @see #toMap(Function, Function, BinaryOperator)
     * @see #toConcurrentMap(Function, Function, BinaryOperator, Supplier)
     */
    public static <T, K, U, M extends Map<K, U>>
    Collector<T, ?, M> toMap(Function<? super T, ? extends K> keyMapper,
                                Function<? super T, ? extends U> valueMapper,
                                BinaryOperator<U> mergeFunction,
                                Supplier<M> mapSupplier) {
        BiConsumer<M, T> accumulator
                = (map, element) -> map.merge(keyMapper.apply(element),
                                              valueMapper.apply(element), mergeFunction);
        return new CollectorImpl<>(mapSupplier, accumulator, mapMerger(mergeFunction), CH_ID);
    }

public Map<Long, String> getIdNameMap(List<Account> accounts) {
 return accounts.stream().collect(Collectors.toMap(Account::getId, Account::getUsername));

2.收集對象實體本身 
- 在開發過程中我們也需要有時候對自己的list中的實體按照其中的一個字段進行分組（比如 id ->List），這時候要設置map的value值是實體本身。
 
public Map<Long, Account> getIdAccountMap(List<Account> accounts) {
 return accounts.stream().collect(Collectors.toMap(Account::getId, account -> account));

account -> account是一個返回本身的lambda表達式，其實還可以使用Function接口中的一個默認方法 Function.identity()，這個方法返回自身對象，更加簡潔
 
重複key的情況。 
在list轉爲map時，作爲key的值有可能重複，這時候流的處理會拋出個異常：Java.lang.IllegalStateException:Duplicate key。這時候就要在toMap方法中指定當key衝突時key的選擇。(這裏是選擇第二個key覆蓋第一個key)
public Map<String, Account> getNameAccountMap(List<Account> accounts) {
 return accounts.stream().collect(Collectors.toMap(Account::getUsername, Function.identity(), (key1, key2) -> key2));

可參考的blog：https://www.jianshu.com/p/09803ae6323c

用groupingBy 或者 partitioningBy進行分組 
根據一個字段或者屬性分組也可以直接用groupingBy方法，很方便。
Map<Integer, List<Person>> personGroups = Stream.generate(new PersonSupplier()).
 limit(100).
 collect(Collectors.groupingBy(Person::getAge));
Iterator it = personGroups.entrySet().iterator();
while (it.hasNext()) {
 Map.Entry<Integer, List<Person>> persons = (Map.Entry) it.next();
 System.out.println("Age " + persons.getKey() + " = " + persons.getValue().size());

分組排序可參考的blog：https://www.cnblogs.com/fengkunangel/p/10434735.html
https://www.jianshu.com/p/d171e86071f7

partitioningBy可以理解爲特殊的groupingBy，key值爲true和false，當然此時方法中的參數爲一個判斷語句（用於判斷的函數式接口）
Map<Boolean, List<Person>> children = Stream.generate(new PersonSupplier()).
 limit(100).
 collect(Collectors.partitioningBy(p -> p.getAge() < 18));
System.out.println("Children number: " + children.get(true).size());
System.out.println("Adult number: " + children.get(false).size());

 使用集合中的某一個元素轉化成list

            List<String> skuList = productList.stream().map(ProductList::getSku).collect(Collectors.toList());