使用 Java Stream 實現集合排序

排序集合中的對象

1.源碼介紹

1.1 Stream sorted()

源碼查看：

/**
 * Returns a stream consisting of the elements of this stream, sorted
 * according to natural order.  If the elements of this stream are not
 * {@code Comparable}, a {@code java.lang.ClassCastException} may be thrown
 * when the terminal operation is executed.
 *
 * <p>For ordered streams, the sort is stable.  For unordered streams, no
 * stability guarantees are made.
 *
 * <p>This is a <a href="package-summary.html#StreamOps">stateful
 * intermediate operation</a>.
 *
 * @return the new stream
 */
Stream<T> sorted();

說明：T 必須是實現了 Comparable 接口的類，否則方法會拋出 ClassCastException 異常。

1.2. Stream sorted(Comparator<? super T> comparator)

源碼查看：

/**
 * Returns a stream consisting of the elements of this stream, sorted
 * according to the provided {@code Comparator}.
 *
 * <p>For ordered streams, the sort is stable.  For unordered streams, no
 * stability guarantees are made.
 *
 * <p>This is a <a href="package-summary.html#StreamOps">stateful
 * intermediate operation</a>.
 *
 * @param comparator a <a href="package-summary.html#NonInterference">non-interfering</a>,
 *                   <a href="package-summary.html#Statelessness">stateless</a>
 *                   {@code Comparator} to be used to compare stream elements
 * @return the new stream
 */
Stream<T> sorted(Comparator<? super T> comparator);

說明：根據給定的 比較器 進行排序。Comparator是一個函數式接口，其源碼如下（僅展示關鍵部分）：

@FunctionalInterface
public interface Comparator<T> {
    /**
     * Compares its two arguments for order.  Returns a negative integer,
     * zero, or a positive integer as the first argument is less than, equal
     * to, or greater than the second.<p>
     * 
     * ......（此處略去部分注釋）
     *
     * @param o1 the first object to be compared.
     * @param o2 the second object to be compared.
     * @return a negative integer, zero, or a positive integer as the
     *         first argument is less than, equal to, or greater than the
     *         second.
     * @throws NullPointerException if an argument is null and this
     *         comparator does not permit null arguments
     * @throws ClassCastException if the arguments' types prevent them from
     *         being compared by this comparator.
     */
    int compare(T o1, T o2);

說明：方法的返回值分類以及含義如下：

• 負數：o1 小於 o2
• 0：o1 等於 o2
• 正數：o1 大於 o2

3. 使用

3.1 Stream sorted()

3.2 Stream sorted(Comparator<? super T> comparator)