Java可選爲什麼不是一個ifNotPresent方法？

Question

我想知道爲什麼在Java8 API的可選類有方法ifPresent(Consumer< T> consumer)而不是ifNotPresent(Runnable** consumer)？

API的意圖是什麼？它不是模仿功能模式匹配嗎？

** Java沒有零參數無效功能接口...

Answer 1

正如米沙說，這個功能will come with jdk9在ifPresentOrElse方法的形式。

/** 
* If a value is present, performs the given action with the value, 
* otherwise performs the given empty-based action. 
* 
* @param action the action to be performed, if a value is present 
* @param emptyAction the empty-based action to be performed, if no value is 
*  present 
* @throws NullPointerException if a value is present and the given action 
*   is {@code null}, or no value is present and the given empty-based 
*   action is {@code null}. 
* @since 9 
*/ 
public void ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction) { 
    if (value != null) { 
     action.accept(value); 
    } else { 
     emptyAction.run(); 
    } 
}