how do I make an array of new objects that implement a common interface in java

Question

This is how I'm trying to do it:

interface a{} 
class b implements a{
    a[] array; 
    new b(){
        array={ new aImplementer(), new aImplementer(), new aImplementer()}; 
    } 
}

Why can't I do this? Am I just doing it wrong? Right now the error I'm getting is an illegal start of expression error at the { part of array={

Bill K · Accepted Answer · 2011-05-15 02:37:48Z

8

Your syntax is off. I believe you want something like this (Final revision--actually tested this time--then redone after my changes were overridden.)

interface A{} 

class AImplementer implements A{};

class B { 
    A[] array; 
    B(){  
        array=new A[]{ new AImplementer(), new AImplementer(), new AImplementer()}; 
    } 
 }

There, guaranteed to compile or twice your money back :)

Also threw in "classes should start with an upper case letter" for no charge.

edited May 15, 2011 at 2:37

answered May 15, 2011 at 2:11

Bill K

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5 Comments

David Over a year ago

this gives me an <identifier> expected error right after the new

Bill K Over a year ago

is it because you didn't define aImplementor? There, I think I put what you were trying for. Does that work for you?

Bill K Over a year ago

Also note that if you really intended to use b as "aImplementer" then your logic is also wrong because b will recurse in the constructor and create b's until you run out of memory.

David Over a year ago

b is not an aImplementor, it is a collection of aImplementors

Bill K Over a year ago

In that case, b should not implement a, and my answer above is correct.

Hovercraft Full Of Eels · Accepted Answer · 2011-05-15 02:42:16Z

1

You can only assign an array to an array literal (I'm not sure what you call these beasts) at declaration of the variable. So this may be OK

// array literal assigned at variable declaration
a[] array = { new aImplementer(), new aImplementer(), new aImplementer()};  
new b(){

}

But what you have where you declare it first and then assign it at a different spot isn't OK. Why? I'm not sure other than that's how it is in the JLS.

Edit: Updated compiled/tested code:

interface A {
}

class AImplementer implements A {
};

class B {
   A[] array = {new AImplementer(), new AImplementer(), new AImplementer()};

   B() {

   }
}

edited May 15, 2011 at 2:42

answered May 15, 2011 at 2:08

Hovercraft Full Of Eels

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6 Comments

Bill K Over a year ago

I think you still need the second "new" as in: a[] array = new a[]{blah} (but it wasn't me who downvoted you--that was some fly-by).

Hovercraft Full Of Eels Over a year ago

@Bill: no, the code compiles fine without the new. Try it yourself to be sure. You need the new a[] only if initiating the array when not at declaration. Either of our code will work. +1 for your great example. /Pete

Boro Over a year ago

@Bill K +1 for nicer code. BTW: Those codes both compile fine String[] str1={"", "", ""}; String[] str2=new String[]{"", "", ""}; @Hovercraft Full Of Eels +1 for being first giving 'the same' answer and being down-voted, why hell knows?

Hovercraft Full Of Eels Over a year ago

@Boro: thanks, but down votes don't really bug me as I've got as many points as I need. You can't really redeem them for nuthin' :)

Bill K Over a year ago

You're right--didn't ever notice that differentiation, good to know. +1

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