adding multiple entries to a HashMap at once in on

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)); // adding value into HashMap
  hashMap.put("Two", new Integer(2));      
  hashMap.put("Three", new Integer(3));

similar to this in objective C:

[NSDictionary dictionaryWithObjectsAndKeys:
@"w",[NSNumber numberWithInt:1],
@"K",[NSNumber numberWithInt:2],
@"e",[NSNumber numberWithInt:4],
@"z",[NSNumber numberWithInt:5],
@"l",[NSNumber numberWithInt:6],
nil] 

I have not found any example that shows how to do this having looked at so many.

Answer 1

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 2

Java has no map literal, so there's no nice way to do exactly what you're asking.

If you need that type of syntax, consider some Groovy, which is Java-compatible and lets you do:

def map = [name:"Gromit", likes:"cheese", id:1234]

Answer 3

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}