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一行代码, Java 怎样把List 转成 Map 的方法( Java 8 中的Stream API )

List<EmployeeJobDTO> subs = getDirectSubordinates(workNo);
// 一行代码, 把List 转成 Map
val subMap = subs.stream().collect(Collectors.toMap(EmployeeJobDTO::getWorkNo, it -> it));

代码解析

public static <T, K, U> Collector<T, ?, Map<K,U>> toMap(
Function<? super T, ? extends K> keyMapper, // Key 映射器
Function<? super T, ? extends U> valueMapper // Value 映射器
) {
        return toMap(keyMapper, valueMapper, throwingMerger(), HashMap::new);
}


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);
    }

<R, A> R collect(Collector<? super T, A, R> collector);

/**
     * 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)}), an {@code IllegalStateException} is
     * thrown when the collection operation is performed.  If the mapped keys
     * may have duplicates, use {@link #toMap(Function, Function, BinaryOperator)}
     * instead.
     *
     * @apiNote
     * It is common for either the key or the value to be the input elements.
     * In this case, the utility method
     * {@link java.util.function.Function#identity()} may be helpful.
     * For example, the following produces a {@code Map} mapping
     * students to their grade point average:
     * <pre>{@code
     *     Map<Student, Double> studentToGPA
     *         students.stream().collect(toMap(Functions.identity(),
     *                                         student -> computeGPA(student)));
     * }</pre>
     * And the following produces a {@code Map} mapping a unique identifier to
     * students:
     * <pre>{@code
     *     Map<String, Student> studentIdToStudent
     *         students.stream().collect(toMap(Student::getId,
     *                                         Functions.identity());
     * }</pre>
     *
     * @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 inserted into the {@code Map} in encounter
     * order, using {@link #toConcurrentMap(Function, Function)}
     * 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 keyMapper a mapping function to produce keys
     * @param valueMapper a mapping function to produce values
     * @return a {@code Collector} which collects elements into a {@code Map}
     * whose keys and values are the result of applying mapping functions to
     * the input elements
     *
     * @see #toMap(Function, Function, BinaryOperator)
     * @see #toMap(Function, Function, BinaryOperator, Supplier)
     * @see #toConcurrentMap(Function, Function)
     */
    public static <T, K, U>
    Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,
                                    Function<? super T, ? extends U> valueMapper) {
        return toMap(keyMapper, valueMapper, throwingMerger(), HashMap::new);
    }

Collectors

一行代码, Java 怎样把List 转成 Map 的方法( Java 8 中的Stream API )

Implementations of {@link Collector} that implement various useful reduction
* operations, such as accumulating elements into collections, summarizing
* elements according to various criteria, etc.

java.util.stream public interface Collector<T, A, R>

A mutable reduction operation that accumulates input elements into a mutable result container, optionally transforming the accumulated result into a final representation after all input elements have been processed. Reduction operations can be performed either sequentially or in parallel.

Examples of mutable reduction operations include: accumulating elements into a Collection; concatenating strings using a StringBuilder; computing summary information about elements such as sum, min, max, or average; computing "pivot table" summaries such as "maximum valued transaction by seller", etc. The class Collectors provides implementations of many common mutable reductions.

A Collector is specified by four functions that work together to accumulate entries into a mutable result container, and optionally perform a final transform on the result. They are:

creation of a new result container (supplier())

incorporating a new data element into a result container (accumulator())

combining two result containers into one (combiner())

performing an optional final transform on the container (finisher())

Collectors also have a set of characteristics, such as Collector.Characteristics.CONCURRENT, that provide hints that can be used by a reduction implementation to provide better performance.

A sequential implementation of a reduction using a collector would create a single result container using the supplier function, and invoke the accumulator function once for each input element. A parallel implementation would partition the input, create a result container for each partition, accumulate the contents of each partition into a subresult for that partition, and then use the combiner function to merge the subresults into a combined result.

To ensure that sequential and parallel executions produce equivalent results, the collector functions must satisfy an identity and an associativity constraints.

The identity constraint says that for any partially accumulated result, combining it with an empty result container must produce an equivalent result. That is, for a partially accumulated result a that is the result of any series of accumulator and combiner invocations, a must be equivalent to combiner.apply(a, supplier.get()).

The associativity constraint says that splitting the computation must produce an equivalent result. That is, for any input elements t1 and t2, the results r1 and r2 in the computation below must be equivalent:

A a1 = supplier.get();
    accumulator.accept(a1, t1);
    accumulator.accept(a1, t2);
    R r1 = finisher.apply(a1);  // result without splitting

    A a2 = supplier.get();
    accumulator.accept(a2, t1);
    A a3 = supplier.get();
    accumulator.accept(a3, t2);
    R r2 = finisher.apply(combiner.apply(a2, a3));  // result with splitting

For collectors that do not have the UNORDERED characteristic, two accumulated results a1 and a2 are equivalent if finisher.apply(a1).equals(finisher.apply(a2)). For unordered collectors, equivalence is relaxed to allow for non-equality related to differences in order. (For example, an unordered collector that accumulated elements to a List would consider two lists equivalent if they contained the same elements, ignoring order.)

Libraries that implement reduction based on Collector, such as Stream.collect(Collector), must adhere to the following constraints:

The first argument passed to the accumulator function, both arguments passed to the combiner function, and the argument passed to the finisher function must be the result of a previous invocation of the result supplier, accumulator, or combiner functions.

The implementation should not do anything with the result of any of the result supplier, accumulator, or combiner functions other than to pass them again to the accumulator, combiner, or finisher functions, or return them to the caller of the reduction operation.

If a result is passed to the combiner or finisher function, and the same object is not returned from that function, it is never used again.

Once a result is passed to the combiner or finisher function, it is never passed to the accumulator function again.

For non-concurrent collectors, any result returned from the result supplier, accumulator, or combiner functions must be serially thread-confined. This enables collection to occur in parallel without the Collector needing to implement any additional synchronization. The reduction implementation must manage that the input is properly partitioned, that partitions are processed in isolation, and combining happens only after accumulation is complete.

For concurrent collectors, an implementation is free to (but not required to) implement reduction concurrently. A concurrent reduction is one where the accumulator function is called concurrently from multiple threads, using the same concurrently-modifiable result container, rather than keeping the result isolated during accumulation. A concurrent reduction should only be applied if the collector has the Collector.Characteristics.UNORDERED characteristics or if the originating data is unordered.

In addition to the predefined implementations in Collectors, the static factory methods of(Supplier, BiConsumer, BinaryOperator, Characteristics...) can be used to construct collectors. For example, you could create a collector that accumulates widgets into a TreeSet with:

Collector<Widget, ?, TreeSet<Widget>> intoSet =
         Collector.of(TreeSet::new, TreeSet::add,
                      (left, right) -> { left.addAll(right); return left; });

(This behavior is also implemented by the predefined collector Collectors.toCollection(Supplier)).

apiNote:

Performing a reduction operation with a Collector should produce a result equivalent to:

R container = collector.supplier().get();
 for (T t : data)
     collector.accumulator().accept(container, t);
 return collector.finisher().apply(container);

However, the library is free to partition the input, perform the reduction on the partitions, and then use the combiner function to combine the partial results to achieve a parallel reduction. (Depending on the specific reduction operation, this may perform better or worse, depending on the relative cost of the accumulator and combiner functions.)

Collectors are designed to be composed; many of the methods in Collectors are functions that take a collector and produce a new collector. For example, given the following collector that computes the sum of the salaries of a stream of employees:

Collector<Employee, ?, Integer> summingSalaries
         = Collectors.summingInt(Employee::getSalary))

If we wanted to create a collector to tabulate the sum of salaries by department, we could reuse the "sum of salaries" logic using Collectors.groupingBy(Function, Collector):

Collector<Employee, ?, Map<Department, Integer>> summingSalariesByDept
        = Collectors.groupingBy(Employee::getDepartment, summingSalaries);

Since:

1.8

Stream

java.util.stream public interface Stream<T>
extends BaseStream<T, Stream<T>>

一行代码, Java 怎样把List 转成 Map 的方法( Java 8 中的Stream API )

A sequence of elements supporting sequential and parallel aggregate operations. The following example illustrates an aggregate operation using Stream and IntStream:

int sum = widgets.stream()
                      .filter(w -> w.getColor() == RED)
                      .mapToInt(w -> w.getWeight())
                      .sum();

In this example, widgets is a Collection<Widget>. We create a stream of Widget objects via Collection.stream(), filter it to produce a stream containing only the red widgets, and then transform it into a stream of int values representing the weight of each red widget. Then this stream is summed to produce a total weight.

In addition to Stream, which is a stream of object references, there are primitive specializations for IntStream, LongStream, and DoubleStream, all of which are referred to as "streams" and conform to the characteristics and restrictions described here.

To perform a computation, stream operations are composed into a stream pipeline. A stream pipeline consists of a source (which might be an array, a collection, a generator function, an I/O channel, etc), zero or more intermediate operations (which transform a stream into another stream, such as filter(Predicate)), and a terminal operation (which produces a result or side-effect, such as count() or forEach(Consumer)). Streams are lazy; computation on the source data is only performed when the terminal operation is initiated, and source elements are consumed only as needed.

Collections and streams, while bearing some superficial similarities, have different goals. Collections are primarily concerned with the efficient management of, and access to, their elements. By contrast, streams do not provide a means to directly access or manipulate their elements, and are instead concerned with declaratively describing their source and the computational operations which will be performed in aggregate on that source. However, if the provided stream operations do not offer the desired functionality, the iterator() and spliterator() operations can be used to perform a controlled traversal.

A stream pipeline, like the "widgets" example above, can be viewed as a query on the stream source. Unless the source was explicitly designed for concurrent modification (such as a ConcurrentHashMap), unpredictable or erroneous behavior may result from modifying the stream source while it is being queried.

Most stream operations accept parameters that describe user-specified behavior, such as the lambda expression w -> w.getWeight() passed to mapToInt in the example above. To preserve correct behavior, these behavioral parameters:

must be non-interfering (they do not modify the stream source); and

in most cases must be stateless (their result should not depend on any state that might change during execution of the stream pipeline).

Such parameters are always instances of a functional interface such as Function, and are often lambda expressions or method references. Unless otherwise specified these parameters must be non-null.

A stream should be operated on (invoking an intermediate or terminal stream operation) only once. This rules out, for example, "forked" streams, where the same source feeds two or more pipelines, or multiple traversals of the same stream. A stream implementation may throw IllegalStateException if it detects that the stream is being reused. However, since some stream operations may return their receiver rather than a new stream object, it may not be possible to detect reuse in all cases.

Streams have a close() method and implement AutoCloseable, but nearly all stream instances do not actually need to be closed after use. Generally, only streams whose source is an IO channel (such as those returned by Files.lines(Path, Charset)) will require closing. Most streams are backed by collections, arrays, or generating functions, which require no special resource management. (If a stream does require closing, it can be declared as a resource in a try-with-resources statement.)

Stream pipelines may execute either sequentially or in parallel. This execution mode is a property of the stream. Streams are created with an initial choice of sequential or parallel execution. (For example, Collection.stream() creates a sequential stream, and Collection.parallelStream() creates a parallel one.) This choice of execution mode may be modified by the sequential() or parallel() methods, and may be queried with the isParallel() method.

Since:

1.8