如何使用新的 computeIfAbsent 函数? [英] How do I use the new computeIfAbsent function?

问题描述

我非常想使用Map.computeIfAbsent 但距离本科生的 lambdas 已经太久了.

I very much want to use Map.computeIfAbsent but it has been too long since lambdas in undergrad.

几乎直接来自文档:它给出了一个旧的做事方式的例子:

Almost directly from the docs: it gives an example of the old way to do things:

Map<String, Boolean> whoLetDogsOut = new ConcurrentHashMap<>();
String key = "snoop";
if (whoLetDogsOut.get(key) == null) {
  Boolean isLetOut = tryToLetOut(key);
  if (isLetOut != null)
    map.putIfAbsent(key, isLetOut);
}

还有新的方式:

map.computeIfAbsent(key, k -> new Value(f(k)));

但在他们的例子中，我认为我不太明白".我将如何转换代码以使用新的 lambda 表达方式?

But in their example, I think I'm not quite "getting it." How would I transform the code to use the new lambda way of expressing this?

推荐答案

假设你有以下代码:

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class Test {
    public static void main(String[] s) {
        Map<String, Boolean> whoLetDogsOut = new ConcurrentHashMap<>();
        whoLetDogsOut.computeIfAbsent("snoop", k -> f(k));
        whoLetDogsOut.computeIfAbsent("snoop", k -> f(k));
    }
    static boolean f(String s) {
        System.out.println("creating a value for ""+s+'"');
        return s.isEmpty();
    }
}

然后您将看到消息 creating a value for "snoop" 恰好一次，因为在第二次调用 computeIfAbsent 时，该键已经有一个值.lambda 表达式中的 k k ->f(k) 只是映射将传递给您的 lambda 以计算值的键的占位符(参数).所以在这个例子中，key 被传递给了函数调用.

Then you will see the message creating a value for "snoop" exactly once as on the second invocation of computeIfAbsent there is already a value for that key. The k in the lambda expression k -> f(k) is just a placeolder (parameter) for the key which the map will pass to your lambda for computing the value. So in the example the key is passed to the function invocation.

或者，您可以编写:whoLetDogsOut.computeIfAbsent("snoop", k -> k.isEmpty()); 以在没有辅助方法的情况下实现相同的结果(但您不会看到然后调试输出).甚至更简单，因为它是对现有方法的简单委托，您可以编写: whoLetDogsOut.computeIfAbsent("snoop", String::isEmpty); 该委托不需要编写任何参数.

Alternatively you could write: whoLetDogsOut.computeIfAbsent("snoop", k -> k.isEmpty()); to achieve the same result without a helper method (but you won’t see the debugging output then). And even simpler, as it is a simple delegation to an existing method you could write: whoLetDogsOut.computeIfAbsent("snoop", String::isEmpty); This delegation does not need any parameters to be written.

为了更接近您问题中的示例，您可以将其写为 whoLetDogsOut.computeIfAbsent("snoop", key -> tryToLetOut(key));(无论是否您将参数命名为 k 或 key).或者写成 whoLetDogsOut.computeIfAbsent("snoop", MyClass::tryToLetOut); 如果 tryToLetOut 是 static 或 whoLetDogsOut.computeIfAbsent("snoop", this::tryToLetOut); 如果 tryToLetOut 是一个实例方法.

To be closer to the example in your question, you could write it as whoLetDogsOut.computeIfAbsent("snoop", key -> tryToLetOut(key)); (it doesn’t matter whether you name the parameter k or key). Or write it as whoLetDogsOut.computeIfAbsent("snoop", MyClass::tryToLetOut); if tryToLetOut is static or whoLetDogsOut.computeIfAbsent("snoop", this::tryToLetOut); if tryToLetOut is an instance method.

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