groovy：在地图中找到一个键，并返回其值 [英] groovy: find a key in a map and return its value

问题描述

我想在给定地图中找到特定键。如果找到了键，我就想从地图中获取该键的值。

这是我到目前为止管理的：

  def mymap = [name：Gromit，likes：cheese，id：1234] 
 
 def x = mymap。 find {it.key ==likes} 
 
 if（x）
 println x.value 
  

这样工作，输出是奶酪的预期。很好，但我不想在最后做 x.value ，我不想做 if（x）。我想让x以某种方式直接包含这个值。

我无法直接将值直接写入x：

  def mymap = [name：Gromit，likes：cheese，id：1234] 
 
 def x = mymap.find {it.key ==likesZZZ} .value 
 
 println x 
  

在这种情况下，closure是null，这导致空指针异常。当然，上面的代码片段工作时 it.key ==likes，但我不确定我会在地图中总是找到目标键。 p>

什么是Groovier和在地图上安全的方式：

• 检查地图是否有给定的键


• 如果是，请获取此键的值

解决方案

使用地图的重点是直接访问。如果你确定地图中的值肯定不会是Groovy - false ，那么你可以这样做：

  def mymap = [name：Gromit，likes：cheese，id：1234] 
 def key =likes
 
 if （mymap [key]）{
 println mymap [key] 
} 
  

但是，如果值可能为Groovy - false ，则应使用：

  if（mymap.containsKey（key））{
 println mymap [key] 
} 
  

b $ b

最简单的解决方案，如果你知道这个值不是Groovy - false （或者你可以忽略）默认值如下：

  def value = mymap [key]？：default
  

这三个解决方案都明显比你的示例快，因为他们不扫描整个地图的键。他们利用 HashMap （或 LinkedHashMap ）设计，使得直接密钥访问几乎是即时的。

I want to find a specific key in a given map. If the key is found, I then want to get the value of that key from the map.

This is what I managed so far:

def mymap = [name:"Gromit", likes:"cheese", id:1234]

def x = mymap.find{ it.key == "likes" }

if(x)
    println x.value

This works, the output is "cheese" as expected. Great, but I don't want to do x.value at the end, and I don't want to do if(x). I want x to directly contain the value somehow.

I can't get the value directly into x like this:

def mymap = [name:"Gromit", likes:"cheese", id:1234]

def x = mymap.find{ it.key == "likesZZZ" }.value

println x

Because the find closure is null in this case, this results in a Null Pointer Exception. Of course, the above code snippet works when it.key == "likes", but I am not sure that I will always find the target key in the map.

What is a "Groovier" and safe way to do this on a map:

• Check if a map has a given key,
• And if so, get the value of this key

解决方案

The whole point of using Maps is direct access. If you know for sure that the value in a map will never be Groovy-false, then you can do this:

def mymap = [name:"Gromit", likes:"cheese", id:1234]
def key = "likes"

if(mymap[key]) {
    println mymap[key]
}

However, if the value could potentially be Groovy-false, you should use:

if(mymap.containsKey(key)) {
    println mymap[key]
}

The easiest solution, though, if you know the value isn't going to be Groovy-false (or you can ignore that), and want a default value, is like this:

def value = mymap[key] ?: "default"

All three of these solutions are significantly faster than your examples, because they don't scan the entire map for keys. They take advantage of the HashMap (or LinkedHashMap) design that makes direct key access nearly instantaneous.

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