adding multiple entries to a HashMap at once in one statement

Question

I need to initialize a constant HashMap and would like to do it in one line statement. Avoiding sth like this:

  hashMap.put(\"One\", new Integer(1)); // add         


        

Answer 1

Here's a simple class that will accomplish what you want

import java.util.HashMap;

public class QuickHash extends HashMap<String,String> {
    public QuickHash(String...KeyValuePairs) {
        super(KeyValuePairs.length/2);
        for(int i=0;i<KeyValuePairs.length;i+=2)
            put(KeyValuePairs[i], KeyValuePairs[i+1]);
    }
}

And then to use it

Map<String, String> Foo=QuickHash(
    "a", "1",
    "b", "2"
);

This yields {a:1, b:2}

Answer 2

Since Java 9, it is possible to use Map.of(...), like so:

Map<String, Integer> immutableMap = Map.of("One", 1, 
                                           "Two", 2, 
                                           "Three", 3);

This map is immutable. If you want the map to be mutable, you have to add:

Map<String, Integer> hashMap = new HashMap<>(immutableMap);

If you can't use Java 9, you're stuck with writing a similar helper method yourself or using a third-party library (like Guava) to add that functionality for you.

Answer 3

You can use the Double Brace Initialization as shown below:

Map<String, Integer> hashMap = new HashMap<String, Integer>()
{{
     put("One", 1);
     put("Two", 2);
     put("Three", 3);
}};

As a piece of warning, please refer to the thread Efficiency of Java “Double Brace Initialization" for the performance implications that it might have.