如何在 HashMap 中检索 ArrayList 值内的元素？

Question

I'm new to Java and I'm struggling to figure out something trivial.

I have a HashMap with keys of type String and values of type ArrayList <String> , and it returns something like this:

dict = {Color=[Blue, Purple]}

I'm struggling to figure out how I can grab a specific element from the array in the value of dict. Like if I got dict.value[0] , it would return the string 'Blue'.

I've tried doing:

Object values = dict.values().toArray[0];

and I assumed that since I changed it to an array, I could do something like values.get(0) to get the first element, but that doesn't work because values is type Object . Any ideas on how to resolve this?

Answer 1

if your HashMap is like:

Map<String, List<String>> myColorsMap = new HashMap<>();

And after populating the map is like:

{Red=[firstRed, secondRed, thirdRed], Blue=[firstBlue, secondBlue, thirdBlue]}

then you can retrieve Blue 's key value (a List ) like:

String blueFirstElement = myColorsMap.get("Blue").get(0); // --> will give first element of List<String> stored against Blue key.

To get collection of all keys in your map (or "dictionary"):

myColorsMap.keySet();

will give:

[Red, Blue]

When you do:

Object values = dict.values().toArray[0];

First, this is wrong. What you were trying to do is this:

Object values = dict.values().toArray();

Which returns Object[] and it is ambiguous. No need to do that. Java's Map interface and HashMap implementation have a lot of utility methods for you to iterate and retrieve your values.

Answer 2

First, use .get() on map to get the ArrayList using key

List<String> colors = dict.get("Color");

Then use .get() on list to get the element using index

String color = colors.get(0);

You can do this in one line this way

String color = dict.get("Color").get(0);

You are trying with dict.values().toArray[0] which is wrong syntax thats the problem. dict.values() will return Collection<List<String>> , you can get List of values this way

List<List<String>> listOfValues = new ArrayList<>(colorsMap.values());

Then you can access each value by index using .get()

List<String> colors = listOfValues.get(0);

Note : HashMap don't store any order.

Answer 3

Cast to type of java.util.List , and then you can get value through index.

@SuppressWarnings("unchecked")
public static void main(String[] args) {
    Map<String, List<String>> map = new HashMap<>(2);
    map.put("color", Arrays.asList("blue", "yellow", "green", "white"));
    map.put("size", Arrays.asList("small", "medium", "large"));
    List<String> o = (List<String>) map.values().toArray()[0];
    System.out.println(o.get(0));               //output is 'blue'

    List<String> s = (List<String>) map.values().toArray()[1];
    System.out.println(s.get(1));               //output is 'medium'
}