如何验证HashMap中的值是否存在

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问题描述

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我有以下 HashMap，其中 key 是 String，value 由 value 表示代码>ArrayList:

I have the following HashMap where the key is a String and the value is represented by an ArrayList:

 HashMap<String, ArrayList<String>> productsMap = AsyncUpload.getFoodMap();

我还有另一个ArrayList食品在我的应用程序中实现.

I also have another ArrayList<String> foods implemented in my application.

我的问题是，找出我的 HashMap 是否包含来自我的第二个 ArrayList 的特定 String 的最佳方法是什么?

My question is, What would be the best way to find out if my HashMap contains a Specific String from my second ArrayList?

我试过没有成功:

Iterator<String> keySetIterator = productsMap.keySet().iterator();
Iterator<ArrayList<String>> valueSetIterator = productsMap.values().iterator();

    while(keySetIterator.hasNext() && valueSetIterator.hasNext()){
        String key = keySetIterator.next();
        if(mArrayList.contains(key)){
            System.out.println("Yes! its a " + key);
        }
    }

推荐答案

为什么不:

// fast-enumerating map's values
for (ArrayList<String> value: productsMap.values()) {
    // using ArrayList#contains
    System.out.println(value.contains("myString"));
}

如果你必须遍历整个 ArrayList，而不是只查找一个特定的值:

And if you have to iterate over the whole ArrayList<String>, instead of looking for one specific value only:

// fast-enumerating food's values ("food" is an ArrayList<String>)
for (String item: foods) {
    // fast-enumerating map's values
    for (ArrayList<String> value: productsMap.values()) {
        // using ArrayList#contains
        System.out.println(value.contains(item));
    }
}

编辑

过去我用一些 Java 8 习语更新了这个.

Past time I updated this with some Java 8 idioms.

Java 8 流 API 允许以更具声明性(并且可以说是优雅)的方式来处理这些类型的迭代.

The Java 8 streams API allows a more declarative (and arguably elegant) way of handling these types of iteration.

例如，这是实现相同目标的(有点过于冗长)方法:

For instance, here's a (slightly too verbose) way to achieve the same:

// iterate foods 
foods
    .stream()
    // matches any occurrence of...
    .anyMatch(
        // ... any list matching any occurrence of...
        (s) -> productsMap.values().stream().anyMatch(
            // ... the list containing the iterated item from foods
            (l) -> l.contains(s)
        )
    )

...这是实现相同目标的更简单方法，最初迭代 productsMap 值而不是 foods 的内容:

... and here's a simpler way to achieve the same, initially iterating the productsMap values instead of the contents of foods:

// iterate productsMap values
productsMap
    .values()
    .stream()
    // flattening to all list elements
    .flatMap(List::stream)
    // matching any occurrence of...
    .anyMatch(
        // ... an element contained in foods
        (s) -> foods.contains(s)
    )

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