adding multiple entries to a HashMap at once in one statement

Question

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.

You can use Google Guava's ImmutableMap. This works as long as you don't care about modifying the Map later (you can't call .put() on the map after constructing it using this method):

import com.google.common.collect.ImmutableMap;

// For up to five entries, use .of()
Map<String, Integer> littleMap = ImmutableMap.of(
    "One", Integer.valueOf(1),
    "Two", Integer.valueOf(2),
    "Three", Integer.valueOf(3)
);

// For more than five entries, use .builder()
Map<String, Integer> bigMap = ImmutableMap.<String, Integer>builder()
    .put("One", Integer.valueOf(1))
    .put("Two", Integer.valueOf(2))
    .put("Three", Integer.valueOf(3))
    .put("Four", Integer.valueOf(4))
    .put("Five", Integer.valueOf(5))
    .put("Six", Integer.valueOf(6))
    .build();

See also: http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/ImmutableMap.html

A somewhat related question: ImmutableMap.of() workaround for HashMap in Maps?

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.

Maps have also had factory methods added in Java 9. For up to 10 entries Maps have overloaded constructors that take pairs of keys and values. For example we could build a map of various cities and their populations (according to google in October 2016) as follow:

Map<String, Integer> cities = Map.of("Brussels", 1_139000, "Cardiff", 341_000);

The var-args case for Map is a little bit harder, you need to have both keys and values, but in Java, methods can’t have two var-args parameters. So the general case is handled by taking a var-args method of Map.Entry<K, V> objects and adding a static entry() method that constructs them. For example:

Map<String, Integer> cities = Map.ofEntries(
    entry("Brussels", 1139000), 
    entry("Cardiff", 341000)
);

Collection Factory Methods in Java 9

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]

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}

    boolean x;
    for (x = false, 
        map.put("One", new Integer(1)), 
        map.put("Two", new Integer(2)),      
        map.put("Three", new Integer(3)); x;);

Ignoring the declaration of x (which is necessary to avoid an "unreachable statement" diagnostic), technically it's only one statement.