Java 8 流“ifPresent”
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Java 8 streams "ifPresent"
提问by Stefan B
I am trying to learn about streams and encountered a problem: I want to get the minimal value of a list and assign it to an int variable. For that I did the following:
我正在尝试了解流并遇到一个问题：我想获取列表的最小值并将其分配给一个 int 变量。为此，我做了以下工作：
List<Integer> list = new ArrayList<>();
list.add(1);
list.add(2);
list.add(3);
int smallest = list.stream().min(Integer::compareTo).get();
System.out.println(smallest);
This works well and i get 1as a result.
The issue is that the IDE gives the warning that Optional.getis called before checking for .isPresent.
To fix that i used the slightly different ifPresentmethod and tried the following:
这很有效，我得到1了结果。问题是 IDEOptional.get在检查.isPresent. 为了解决这个问题，我使用了稍微不同的ifPresent方法并尝试了以下方法：
int smallest = list.stream().min(Integer::compareTo).ifPresent(integer -> integer);
Unfortunately this doesn't work since I get the warning: Bad return type in Lambda, Integer cannot be converted to void.My question finally is: How can I assign the min value to the int smallestvariable WITH checking ifPresent?
不幸的是，这不起作用，因为我收到警告：Bad return type in Lambda, Integer cannot be converted to void.我的问题最后是：如何通过int smallest检查 ifPresent将最小值分配给变量？
采纳答案by Nazarii Bardiuk
Stream#minreturn Optionalof result because in general stream can be empty so there can be no minimal value.
Stream#minOptional结果的返回，因为通常流可以为空，因此没有最小值。
Optional<Integer> minimal = list.stream().min(Integer::compareTo);
To get value from Optional you need to have some fallback value
要从 Optional 获得价值，您需要有一些后备价值
Integer absent = Integer.MIN_VALUE;
The easiest would be to use orElse
最简单的方法是使用 orElse
Integer smallest = minimal.orElse(absent);
Little bit longer would be isPresent
长一点会 isPresent
Integer smallest = minimal.isPresent() ? minimal.get() : absent;
回答by duffymo
Here's how I'd do it:
这是我的做法：
package lambdas;
import java.util.ArrayList;
import java.util.List;
/**
* Created by Michael
* Creation date 7/31/2016.
* @link https://stackoverflow.com/questions/38688119/java-8-streams-ifpresent
*/
public class MinimumExample {
public static void main(String[] args) {
List<Integer> list = new ArrayList<>();
int smallest = list.stream().min(Integer::compareTo).orElse(Integer.MIN_VALUE);
System.out.println(smallest);
}
}
回答by Andrew Tobilko
The ifPresentmethod takes Consumer<? super T>as a parameter. Simply speaking, it should be an action without returnstatement. You could print the value if it's present, like
该ifPresent方法Consumer<? super T>作为参数。简单的说，应该是一个没有return声明的动作。如果存在，您可以打印该值，例如
[...].ifPresent(System.out::print);
But it is not what about IDEA says. I think, you simply need to save an Option<Integer>instance and then check it by isPresent:
但这不是IDEA所说的。我认为，您只需要保存一个Option<Integer>实例，然后通过isPresent以下方式检查它：
Optional<Integer> o = list.stream().min(Integer::compareTo);
if (o.isPresent()) {
smallest = o.get();
}
Of course, there are more convenient ways with orElse:
当然，还有更方便的方法orElse：
smallest = o.orElse(Integer.MIN_VALUE);
or with the ternary operator:
或使用三元运算符：
smallest = o.isPresent() ? o.get() : Integer.MIN_VALUE;
