Java Array of Arraylists

RobMunfy · 19 Nov 2008 at 16:25

Hi guys,

How do you create an array of length n of arraylists in java? What would the syntax look like? I've tried:

Code:

ArrayList<>[] al = new Arraylist[3];

but it doesn't seem to like it (Illegal start of type).

Any tips?

DireEmblem · 19 Nov 2008 at 17:08

Code:

ArrayList<>[] al = new Arraylist[3];

its your <> it doesnt like

You need to have <String> or something like below

if you have various data types in the array just use

Code:

ArrayList[] al = new Arraylist[3]();

Or like you had if you want a string say use

Code:

ArrayList<String>[] al = new Arraylist<String>[3]();

RobMunfy · 19 Nov 2008 at 17:58

ah thanks a lot.

if i have created a new data type called Actor that takes two strings and did:

Code:

ArrayList<Actor>[] al = new Arraylist<Actor>[3]();

to then add an element in, for example, arraylist no 6, would i do:

Code:

al[6].add("Sylvester","Stallone");

?

Thanks

Inquisitor · 19 Nov 2008 at 18:03

Nope, you need to create an Actor object and pass that into the add method. Also, the 6th element would be at array index 5, since Java's arrays are zero-based.

Code:

al[5].add(new Actor("Sylvester", "Stallone"));

RobMunfy · 19 Nov 2008 at 18:20

ah cool,

thanks a lot Inquisitor and DireEmblem

RobMunfy · 20 Nov 2008 at 13:18

I'm getting an error from:

Code:

ArrayList<String>[] al = new ArrayList<String>[3]();

saying:

Code:

';' expected
                  ArrayList<String>[] al = new ArrayList<String>[3]();
                                                                   ^

Any ideas?

Inquisitor · 20 Nov 2008 at 14:29

When you initialise an array, you don't need to put the empty parentheses after the square brackets. For example, when initialising an array of integers, you'd do this:

Code:

int[] array = new int[3];

It's exactly the same with array lists; you specify the type of the array (in this case, ArrayList<String>) followed by square brackets specifying the length of the array:

Code:

ArrayList<String>[] al = new ArrayList<String>[3];

Remember, though, that all of the array's elements will start off as just null (since ArrayList<String> is a reference type), so you need to initialise each element of the array with a new array list:

Code:

al[0] = new ArrayList<String>();
al[1] = new ArrayList<String>();
al[2] = new ArrayList<String>();

Hope that makes sense

PaulStat · 20 Nov 2008 at 14:59

Doesn't this create an array (with 3 elements) of ArrayLists with an initial capacity of ten?

Code:

ArrayList<String>[] al = new ArrayList<String>[3]();

Here are the constructors allowed

Code:

ArrayList

public ArrayList(int initialCapacity)

    Constructs an empty list with the specified initial capacity.

Parameters:
    initialCapacity - the initial capacity of the list. 
Throws:
    IllegalArgumentException - if the specified initial capacity is negative

ArrayList

public ArrayList()

    Constructs an empty list with an initial capacity of ten.

ArrayList

public ArrayList(Collection c)

    Constructs a list containing the elements of the specified collection, in the order they are returned by the collection's iterator. The ArrayList instance has an initial capacity of 110% the size of the specified collection.

Parameters:
    c - the collection whose elements are to be placed into this list.

Edit: Actually I see, so yes it will create an array of ArrayLists but as Inquistor said you'll need to initialise them, duhhhhhh! <---- that's aimed at me

RobMunfy · 20 Nov 2008 at 15:52

Hey,

I'm still getting an error with:

Code:

ArrayList<String>[] al = new ArrayList<String>[3];
al[0] = new ArrayList<String>();
al[1] = new ArrayList<String>();
al[2] = new ArrayList<String>();

saying:

Code:

generic array creation
                  ArrayList<String>[] al = new ArrayList<String>[3];
                                           ^

Thanks for your help so far though Inquisitor

PaulStat · 20 Nov 2008 at 16:15

Try this instead

Code:

           ArrayList a1[] = new ArrayList[3];
           a1[0] = new ArrayList<String>();
           a1[1] = new ArrayList<String>();
           a1[2] = new ArrayList<String>();

This will create an array of ArrayLists each of which is limited to holding objects of type String

RobMunfy · 20 Nov 2008 at 16:27

awesome, that works, thanks a load PaulStat

Inquisitor · 20 Nov 2008 at 16:34

Yeah, I've never actually used Java; that code would've worked in C#

RobMunfy · 20 Nov 2008 at 16:53

ok guys,

What i've done now is done:

Code:

ArrayList[] al = new ArrayList[3];
al[0]=new ArrayList<Actor>();
al[1]=new ArrayList<Actor>();
al[2]=new ArrayList<Actor>();
al[0].add(new Actor("Sylvester","Stalone"));
^^^^^^Produces a warning^^^^^^^^^

How would I then go about printing out the firstname (Called fname in the Actor class) that I have just added?

daven1986 · 20 Nov 2008 at 17:12

System.out.println(al[0].get(0).fname);

if fname is protected / public. if you have a accessor method in the actor class replace fname with the method to get fname.

RobMunfy · 20 Nov 2008 at 17:21

Code:

System.out.println(al[0].get(0).fname);

Compiler is saying that it cannot find the symbol fname.

my actor class looks like this:

Code:

class Actor {
	
	public String fname;
	public String lname;
	
	public Actor(String x, String y) {
		fname=x;
		lname=y;
	}
}

Is there anything wrong here?

RobMunfy · 20 Nov 2008 at 17:23

Also, the warning says:

Code:

warning: [unchecked] unchecked call to add(E) as a member of the raw type java.util.ArrayList
                                        al[0].add(new Actor("Sylvester","Stallone");
                                                 ^

What's the warning about?

DireEmblem · 20 Nov 2008 at 18:22

You need to create a method in Actor to return what you need, e.g following should fix:-

Code:

class Actor {
	
	public String fname;
	public String lname;
	
	public Actor(String x, String y) {
		fname=x;
		lname=y;
	}
        
        public String fname(){
               return fname;
        }
}

thus

Code:

System.out.println(al[0].get(0).fname());

should then pull back the fname in that location.

RobMunfy · 20 Nov 2008 at 18:50

hmm, still doesn't work. it still "can't find symbol".

this is doing my head in!!

DireEmblem · 20 Nov 2008 at 20:10

RobMunfy said:
hmm, still doesn't work. it still "can't find symbol".

this is doing my head in!!

Can you place up your whole code to compile/ or could you give us more details to the error?

RobMunfy · 20 Nov 2008 at 20:21

ok, my code looks like this:

Code:

ArrayList[] al = new ArrayList[3];
al[0]=new ArrayList<Actor>();
al[1]=new ArrayList<Actor>();
al[2]=new ArrayList<Actor>();
al[0].add(new Actor("Sylvester","Stalone"));
System.out.println(al[0].get(0).fname());

and the Actor class looks as it does in your bit of code:

Code:

class Actor {
	
	public String fname;
	public String lname;
	
	public Actor(String x, String y) {
		fname=x;
		lname=y;
	}
        
        public String fname(){
               return fname;
        }
}

I am compiling with:

Code:

javac MyArrayListExample.java Actor.java

The errors are:

Code:

cannot find symbol
symbol  : method fname()
location: class java.lang.Object
                                        System.out.println(al[0].get(0).fname());

                                                                       ^
1 error

and

Code:

warning: [unchecked] unchecked call to add(E) as a member of the raw type java.util.ArrayList
                                        al[0].add(new Actor("Sylvester","Stalone"));
                                                 ^
1 warning

I think maybe I am adding the Actor to the ArrayList in the wrong way. Still doesn't explain why it "cannot find symbol" though.

Thanks a lot for your help