使用字符串键和arraylist的TreeMap

Question

I am brand new to using collections, so I am confused on how to do this. I am trying to use a TreeMap to hold a word as the key and then an ArrayList to hold one or more definitions for the word.

public class Dict {
    Map<String, ArrayList<String>> dic = new TreeMap<String, ArrayList<String>>();

    public void AddCmd(String word, String def) {
        System.out.println("Add Cmd " + word);
        if(dic.get(word)==null){
            dic.put(word, new ArrayList.add(def));      
        }
    }
}

I am getting an error on " new ArrayList.add(def) ". I thought this was the correct way to do this, but I am obviously wrong. Does anyone have any ideas as to what I am doing wrong?

Answer 1

Calling ArrayList#add returns a boolean which is not the desired value for your Map , thus getting the compiler error.

You need to insert the ArrayList and then add the element. Your code should look like this:

ArrayList<String> definitions = dic.get(word);
if (definitions == null) {
    definitions = new ArrayList<String>();
    dic.put(word, definitions);
}
definitions.add(def);

Answer 2

dic.put(word, new ArrayList.add(def)); is the culprit. since you have declared map to take Arraylist of string as a value. the value to pass for map must be Arraylist of string.

but this line is adding a value as new ArrayList.add(def) since you are trying to create a list and adding element , add method returns boolean -> true if it can add false if it fails.

so it means value to the map is going as a boolean not as arraylist which is against the map declaration. so use code as below

ArrayList<String> listOfString = dic.get(word);
if (listOfString == null) {
    listOfString = new ArrayList<String>();
listOfString .add(def);
}
dic.put(word, listOfString );

Answer 3

You have to break it up, because add does not return the original ArrayList:

ArrayList<String>> NewList = new ArrayList<String>();
NewList.add(def);
dic.put(word, NewList);

Answer 4

You are not actually creating a new ArrayList. Try this:

ArrayList<String> newDef = new ArrayList<String();
newDef.add(def);
dic.put(word, newDef);