java Map 中的密钥存在检查实用程序

Key existence checking utility in Map

提问by Sam

I'm using the following code for checking if a key exists in a Mapinstance:

我正在使用以下代码检查Map实例中是否存在密钥：

if (!map_instance.containsKey(key))
    throw new RuntimeException("Specified key doesn't exist in map");
else
   return map_instance.get(key);

My question is:

我的问题是：

Is there a utility or Mapimplementation to simplify the above code, such as:

是否有实用程序或Map实现来简化上述代码，例如：

custom_map.get(key,"Specified key doesn't exist in map");

My goal is: if keydoes not exist in map, the map implementation throws an exception with the passed string.

我的目标是：如果key地图中不存在，则地图实现将使用传递的字符串抛出异常。

I don't know whether or not my desire is reasonable?

不知道我的愿望是否合理？

(Sorry if I am using the wrong terminology or grammar, I am still learning the English language.)

（对不起，如果我使用了错误的术语或语法，我仍在学习英语。）

采纳答案by Francisco Spaeth

You could take a look into the configuration map from Apache commons. It doesn't implements Map, but has a similar interface with a few Helper methods, like getString, getStringArray, getShortand so on.

您可以查看 Apache commons 中的配置映射。它没有工具Map，但与一些helper方法，像类似的界面getString，getStringArray，getShort等等。

With this implementation you could use the method setThrowExceptionOnMissing(boolean throwExceptionOnMissing)and could catch it and handle as you want.

通过此实现，您可以使用该方法setThrowExceptionOnMissing(boolean throwExceptionOnMissing)并可以根据需要捕获和处理它。

Isn't exactly with a configurable message but from my point of view it doesn't make sense to throw a fixed exception just with a custom message since the exception type itself depends on the context where the getmethod is invoked. For example, if you perform a get of an user the exception would be something related to that, maybe UserNotFoundException, and not just a RuntimeExceptionwith the message: User not Found in Map!

不完全是可配置的消息，但从我的角度来看，仅使用自定义消息抛出固定异常是没有意义的，因为异常类型本身取决于get调用方法的上下文。例如，如果您对用户执行 get 操作，则异常可能与此相关UserNotFoundException，而不仅仅是RuntimeException带有消息：用户未在地图中找到！

回答by Marcin Majkowski

In Java 8 you can use computeIfAbsentfrom Map, like this:

在 Java 8 中，您可以使用computeIfAbsentfrom Map，如下所示：

map.computeIfAbsent("invalid", key -> { throw new RuntimeException(key + " not found"); });

回答by Sir Montes

I use Optional Java util class, e.g.

我使用可选的 Java util 类，例如

Optional.ofNullable(elementMap.get("not valid key"))
            .orElseThrow(() -> new ElementNotFoundException("Element not found"));

回答by mort

Such a method does not exist for a Map in Java. You could create it by extending the Map class of course or by creating a wrapper class that contains the map and a tryGetmethod.

Java 中的 Map 不存在这种方法。您当然可以通过扩展 Map 类或通过创建包含地图和tryGet方法的包装类来创建它。

C# however does have a such a method: Directory.TryGetValue()

然而 C# 确实有一个这样的方法： Directory.TryGetValue()

public class MyMap {
    public Map<Object, Object> map = new HashMap<Object, Object>();

    public Object tryGet(Object key, String msg) {
        if(!map.containsKey(key))
           throw new IllegalArgumentException(msg);
        return map.get(key); 
   }

回答by Tomer

the way you are doing it is the way to go.

你正在做的事情就是要走的路。

as a side note, your code doesn't make much sense since you will always return true, maybe you meant:

作为旁注，您的代码没有多大意义，因为您将始终返回 true，也许您的意思是：

return map_instance.get(key);

回答by Evgeny Kharitonov

There is one more nice way to achieve this:

还有一种更好的方法可以实现这一目标：

return Objects.requireNonNull(map_instance.get(key), "Specified key doesn't exist in map");

Pros:

优点：

• pure Java - no libs
• no redundant Optional - less garbage

• 纯 Java - 没有库
• 没有多余的 Optional - 更少的垃圾

Cons:

缺点：

• only NullPointerException- sometimes NoSuchElementExceptionor custom exceptions are more desirable

• only NullPointerException- 有时NoSuchElementException或自定义异常更可取

Java 8 required

需要 Java 8