Java中Optional類及orElse方法詳解

/**
 * A container object which may or may not contain a non-null value.
 * If a value is present, {@code isPresent()} will return {@code true} and
 * {@code get()} will return the value.
 */
 public final class Optional<T> {

    /**
     * Common instance for {@code empty()}.
     */
    private static final Optional<?> EMPTY = new Optional<>(); //執行Optional的無參構造
    
	//無參構造
	private Optional() {
        this.value = null;
    }
    //有參構造
    private Optional(T value) {
        this.value = Objects.requireNonNull(value);
    }
    /**
     * Returns an {@code Optional} describing the specified value, if non-null,
     * otherwise returns an empty {@code Optional}.
     * 如果value不是null, 返回它自己本身, 是空, 則執行empty(), 返回null
     *
     * @param <T> the class of the value
     * @param value the possibly-null value to describe
     * @return an {@code Optional} with a present value if the specified value
     * is non-null, otherwise an empty {@code Optional}
     */
    public static <T> Optional<T> ofNullable(T value) {
        return value == null ? empty() : of(value);
    }
    /**
     * Returns an empty {@code Optional} instance.  No value is present for this
     * Optional.
     * 當value是空時, 返回Optional<T>物件
     *
     * @apiNote Though it may be tempting to do so, avoid testing if an object
     * is empty by comparing with {@code ==} against instances returned by
     * {@code Option.empty()}. There is no guarantee that it is a singleton.
     * Instead, use {@link #isPresent()}.
     *
     * @param <T> Type of the non-existent value
     * @return an empty {@code Optional}
     */
    public static<T> Optional<T> empty() {
        @SuppressWarnings("unchecked")
        Optional<T> t = (Optional<T>) EMPTY;  //由第一行程式碼可知, EMPTY是一個無參Optional物件
        return t;
    }
   
    /**
     * Returns an {@code Optional} with the specified present non-null value.
     * 返回不等於null的value值本身
     *
     * @param <T> the class of the value
     * @param value the value to be present, which must be non-null
     * @return an {@code Optional} with the value present
     * @throws NullPointerException if value is null
     */ 
    public static <T> Optional<T> of(T value) {
        return new Optional<>(value);  //執行Optional的有參構造
    }
 }

    /**
     * Return the value if present, otherwise return {@code other}.
     * 如果值存在(即不等於空), 返回值本身, 如果等於空, 就返回orElse()方法的引數other.
     *
     * @param other the value to be returned if there is no value present, may
     * be null
     * @return the value, if present, otherwise {@code other}
     */
    public T orElse(T other) {
        return value != null ? value : other;
    }

Optional.ofNullable(scRespDTO.getMsgBody().getSuccess()).orElse(false);

    /**
     * Return the value if present, otherwise invoke {@code other} and return
     * the result of that invocation.
     * 值如果存在(即不為空), 返回值本身, 如果不存在, 則返回實現了Supplier介面的物件. 
     * Supplier介面就只有一個get()方法. 無入參，出參要和Optional的物件同型別.
     *
     * @param other a {@code Supplier} whose result is returned if no value
     * is present
     * @return the value if present otherwise the result of {@code other.get()}
     * @throws NullPointerException if value is not present and {@code other} is
     * null
     */
    public T orElseGet(Supplier<? extends T> other) {
        return value != null ? value : other.get();
    }

System.out.println(Optional.ofNullable("努力成為一名更優秀的程式媛").orElseGet(()->"你不夠優秀"));
System.out.println(Optional.ofNullable(null).orElseGet(()->"你沒有努力"));