Declare and Put String array in HashMap in one step

Question

I am trying to insert static data into a HashMap in Java like this:

HashMap<String,String[]> instruments = new HashMap<String, String[]>();
instruments.put("EURUSD", {"4001","EURUSD","10000","0.00001","0.1","USD"});

But the compiler doesn't like it. The only way I found to insert that data into the HashMap is to declare the string array separately and then put it into the HashMap, like this

String[] instruDetails = {"4001","EURUSD","10000","0.00001","0.1","USD"};
instruments.put("EURUSD", instruDetails);

But it not very expressive, and hard to maintain

So my question is, is there a way to do the put() operation and string array declaration in one step/line?

Should be new String[]{ array elements}

Shashank Kadne
– Shashank Kadne

2012-10-04 17:47:13 +00:00
Commented Oct 4, 2012 at 17:47 — Shashank Kadne
– Shashank Kadne, Commented Oct 4, 2012 at 17:47

Baz · Accepted Answer · 2012-10-04 17:46:07Z

12

This will do it:

instruments.put("EURUSD", new String[]{"4001","EURUSD","10000","0.00001","0.1","USD"});

answered Oct 4, 2012 at 17:46

Baz

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2 Comments

smatthewenglish Over a year ago

can you add to that array there?

Baz Over a year ago

@S.Matthew_English Just add a new string at the end. Maybe I don't understand what you're asking?

Rohit Jain · Accepted Answer · 2012-10-04 17:46:44Z

8

To get it all in one sentence, use double-braces initialization: -

 HashMap<String,String[]> instruments = new HashMap<String, String[]>() {
     {
      put("EURUSD", new String[]{"4001","EURUSD","10000","0.00001","0.1","USD"});
      put("EUR", new String[]{"4001","EURUSD","10000","0.00001","0.1","USD"});
     }
 };

answered Oct 4, 2012 at 17:46

Rohit Jain

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5 Comments

Ted Hopp Over a year ago

Otherwise known as an instance initializer block.

Bhesh Gurung Over a year ago

It's also subclassing the HashMap.

Rohit Jain Over a year ago

@BheshGurung. Yup thats true.. But since OP wanted it in one step.. So I gave him.. :) :)

Bhesh Gurung Over a year ago

I think the actual question is why the put method call doesn't work. But still you introduced the OP with something new, I think. +1 :)

jule64 Over a year ago

@RohitJain Thanks this is useful. I have seen this construct with Listeners before but did not realise subclassing worked with HashMap too. Will look to use this construct more often.

Bhesh Gurung · Accepted Answer · 2012-10-04 18:13:36Z

6

I think you already got what works. But the reason that

instruments.put("EURUSD", {"4001","EURUSD","10000","0.00001","0.1","USD"});

doesn't work is because {"4001","EURUSD","10000","0.00001","0.1","USD"}. {} is a syntactic sugar or short-cut in Java array for initialization. It comes with a constraint that it always has to go along with the array declaration statement, otherwise it's a syntax error.

Array declaration statement like

String[] array = {"1", "2"};

That way Java knows that the array that it needs to create for you is actually of String type elements.

If you break the above statement as follows

String[] array;
array = {"1", "2"};

It doesn't compile.

And with the new String[]{"4001","EURUSD","10000","0.00001","0.1","USD"}, the compiler knows that it has to instantiate a new array which element type is String (new String[]) and initialize the newly instantiated array with values you provided ({"4001","EURUSD","10000","0.00001","0.1","USD"}).

edited Oct 4, 2012 at 18:13

answered Oct 4, 2012 at 18:06

Bhesh Gurung

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3 Comments

smatthewenglish Over a year ago

but how to add to that array once you put it there?

Bhesh Gurung Over a year ago

You can get the array with the hash map. But you can't add to the if thats what you mean.

smatthewenglish Over a year ago

i did the thing that amit on the bottom recommended here do you think that's a good solution? do you know how to print something like that by any chance?

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