Java: Generic keyvalue like C#

Question

I'm currently looking for a way in java to do something like c#. For example in ASP MVC you can write:

DoMethod(new {key = value, another = value2});

Is there any nice way to do this in java? I've tried just an array: new object[] {value, value...} However that way i don't get the keys just the values.

I know you can do:

HashMap<String, object> map = new HashMap();
map.put("key", "value");
map.put("anoter", "value2");
DoMethod(map);

Unfortunately is that a cumbersome way if you need to do it a lot.

Cheers!

Answer 1

No reinventing the wheel. If you're free to use 3rd-party libs, you can try Google Guava that offers strongly typed approach without static initializers and sub-classes, surrogate Entry/Pair-like classes or variadic arguments that may appear not type-safe:

Using the builder pattern:

final Map<String, Object> map = new ImmutableMap.Builder<String, Object>()
       .put("key", "value")
       .put("anoter", "value2")
       .build();
DoMethod(map);

Or even a shorter approach if the number of entries is up to 5 (see the overloads why all of those map factory methods are type-safe):

final Map<String, Object> map = ImmutableMap.<String, Object>of("key", "value", "anoter" "value2");
DoMethod(map);

Answer 2

There is nothing like that in Java itself. You would probably be able to find such features in other JVM languages, though. If you have the freedom of choice and you like dynamic typing and related features while retaining Java-like syntax, check Groovy out.

Answer 3

You can't do this sort of thing directly in Java but you can simulate it using varargs and a Pair class. First create a Pair class as follows.

public class Pair<T, U> {

    public static <T, U> Pair<T, U> of(T t, U u) {
        return new Pair<T, U>(t, u);
    }

    private final T t;
    private final U u;

    private Pair(T t, U u) {
        this.t = t; this.u = u;
    }

    public T first() { return t; }

    public U second() { return u; }
}

Then you can write methods using varargs:

static <K, V> Map<K, V> initialiseMap(Pair<K, V>... entries) {
    Map<K, V> temp = new HashMap<K, V>();
    for (Pair<K, V> pair : entries) {
        temp.put(pair.first(), pair.second());
    }
    return temp;
}

You can then initialise a Map by writing, for example

Map<String, Integer> map = initialiseMap(Pair.of("Key1", 1), Pair.of("Key2", 2));

Answer 4

A simple solution is to use pair of arguments.

public static <K, V> Map<K, V> newMap(K key, V value, Object... keysAndValues) {
    Map<K, V> temp = new LinkedHashMap<K, V>();
    temp.put(key, value);
    for (int i = 0; i < keysAndValues.length; i+=2)
        temp.put((K) keysAndValues[i], (V) keysAndValues[i+1]);
    return temp;
}

Map<String, Integer> map = newMap("Key1", 1, "Key2", 2);

Answer 5

try:

doMethod(new HashMap(){{ put("key", "value"); put("another", "value2"); }});

Explenation:

Outer {} block means that this instance is of an anonymous class extends HashMap.

Inner {} block means a code that is copied to the anonymous class's constructor.