Is there a JavaScript style Collection in Java

Question

In JavaScript I can define the following collection with keys awaiting values

var items = {
                'book':null,
                'pen':null,
                'pencil':null,
                'chicken':null,
                'wallet':null
            };

Then when I am ready to add values to my collection, I can do for instance

for(var p in items){
  if(some condition){
     items[p]=someValue;
  }
}

Is there a way to do this with the same level of efficiency in java?

I know that in old Java I can combine a Map and a List to accomplish this, but are their new data structures in Java that can handle this? I am talking about Java 7 (or 8) perhaps? I am using Google App-Engine for my Java.

Note that you don't actually have to initialize JavaScript object properties. If you make an assignment to a non-existent property, the property is added to the object. — Pointy
– Pointy, Commented Oct 14, 2015 at 17:35
I dont understand the question I guess, but are you asking for this? — Bonatti
– Bonatti, Commented Oct 14, 2015 at 17:41
So I have edited to drive the point home: I am iterating before adding any values to the collection. — Katedral Pillon
– Katedral Pillon, Commented Oct 14, 2015 at 17:57
Since the two answers are essentially the same, I will check the first response and up vote the second one. Thank you very much for helping resolve this problem. — Katedral Pillon
– Katedral Pillon, Commented Oct 14, 2015 at 18:17

Suresh Atta · Accepted Answer · 2015-10-14 17:49:51Z

6

You could try it this way, if you're looking for the same style.

 HashMap<String, String > items  = new HashMap<String, String>(){{
        put("book",null);
        put("pen",null);
    }};

Later you can put again with keys.

items.put("book", "Some Bible");

It seems you are new to Java and both Collections. I'm highly recommend you to read the about HashMap more before proceeding.

edited Oct 14, 2015 at 17:49

answered Oct 14, 2015 at 17:43

Suresh Atta

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10 Comments

Yassin Hajaj Over a year ago

This is more what OP was asking for I think

Zaphod Beeblebrox Over a year ago

Although, to be honest, I don't see the point of putting those keys with a null value. The value null will be returned anyway, by getting "book", if it's not present in the map.

Suresh Atta Over a year ago

@ZaphodBeeblebrox True. But I don't exactly know why OP is looking that way and. One benefit I can see is containsKey() method behaviour.

Brent C Over a year ago

This looks like a bad idea to me. Creating an anonymous class extending HashMap just to avoid using items. in front of put?

Katedral Pillon Over a year ago

I suppose this is closer to what I was looking for, although it does in fact reduce to a Map and a List. You may be overlooking what is actually happening in my for-loop, where I am iterating over the keys essentially. So I suppose your answer works if I use items.keySet() to get the keys. Perhaps this is as close as it gets.

|

xaviert · Accepted Answer · 2015-10-14 18:06:31Z

4

EDIT: Updated my answer based on the latest comments.

You could perfectly use a HashMap to achieve the same effect. To iterate over the existing keys, use the Map#keySet method.

Map<String, String> map = new HashMap<String, String>();
map.put("book", null);
map.put("pen", null);

for (String key : map.keySet()) {
    map.put(key, "Some Value");
}

System.out.println(map);

edited Oct 14, 2015 at 18:06

answered Oct 14, 2015 at 17:41

xaviert

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2 Comments

Zaphod Beeblebrox Over a year ago

...and, starting with Java 7, you can use the diamond notation: Map<String, String> map = new HashMap<>();

Katedral Pillon Over a year ago

The answers are the same, but this answer was first. So I up vote the other and check this one.

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