String.join() --Java8中String類新增方法

序言

在看別人的代碼時發現一個方法String.join()，因為之前沒有見過所以比較好奇。

跟蹤源碼發現源碼很給力，居然有用法示例，以下是源碼：

 /**
     * Returns a new String composed of copies of the
     * {@code CharSequence elements} joined together with a copy of
     * the specified {@code delimiter}.
     * //這是用法示例
     * <blockquote>For example,
     * <pre>{@code
     *     String message = String.join("-", "Java", "is", "cool");
     *     // message returned is: "Java-is-cool"
     * }</pre></blockquote>
     *
     * Note that if an element is null, then {@code "null"} is added.
     *
     * @param  delimiter the delimiter that separates each element
     * @param  elements the elements to join together.
     *
     * @return a new {@code String} that is composed of the {@code elements}
     *         separated by the {@code delimiter}
     *
     * @throws NullPointerException If {@code delimiter} or {@code elements}
     *         is {@code null}
     *
     * @see java.util.StringJoiner
     * @since 1.8
     */
    public static String join(CharSequence delimiter, CharSequence... elements) {
        Objects.requireNonNull(delimiter);
        Objects.requireNonNull(elements);
        // Number of elements not likely worth Arrays.stream overhead.
        StringJoiner joiner = new StringJoiner(delimiter);
        for (CharSequence cs: elements) {
            joiner.add(cs);
        }
        return joiner.toString();
    }

    /**
     * Returns a new {@code String} composed of copies of the
     * {@code CharSequence elements} joined together with a copy of the
     * specified {@code delimiter}.
     *
     * <blockquote>For example,
     * <pre>{@code
     *     List<String> strings = new LinkedList<>();
     *     strings.add("Java");strings.add("is");
     *     strings.add("cool");
     *     String message = String.join(" ", strings);
     *     //message returned is: "Java is cool"
     *
     *     Set<String> strings = new LinkedHashSet<>();
     *     strings.add("Java"); strings.add("is");
     *     strings.add("very"); strings.add("cool");
     *     String message = String.join("-", strings);
     *     //message returned is: "Java-is-very-cool"
     * }</pre></blockquote>
     *
     * Note that if an individual element is {@code null}, then {@code "null"} is added.
     *
     * @param  delimiter a sequence of characters that is used to separate each
     *         of the {@code elements} in the resulting {@code String}
     * @param  elements an {@code Iterable} that will have its {@code elements}
     *         joined together.
     *
     * @return a new {@code String} that is composed from the {@code elements}
     *         argument
     *
     * @throws NullPointerException If {@code delimiter} or {@code elements}
     *         is {@code null}
     *
     * @see    #join(CharSequence,CharSequence...)
     * @see    java.util.StringJoiner
     * @since 1.8
     */
    public static String join(CharSequence delimiter,
            Iterable<? extends CharSequence> elements) {
        Objects.requireNonNull(delimiter);
        Objects.requireNonNull(elements);
        StringJoiner joiner = new StringJoiner(delimiter);
        for (CharSequence cs: elements) {
            joiner.add(cs);
        }
        return joiner.toString();
    }

總結

String類中的join()方法有兩種，第一種

join(CharSequence delimiter, CharSequence... elements)

這里join()方法第二個參數表示參數的個數不確定，可以接受任意多個，就像示例中的寫法那樣。

第二種

join(CharSequence delimiter,
            Iterable<? extends CharSequence> elements)

這種寫法第二個參數中涉及到一個泛型的用法，這個寫法表示，Iterable中存放的內容需要是CharSequence這個類的子類。