How can I store an ArrayList in a two dimensional array?
I've tried it like this, but it won't work:
ArrayList<Integer> arrList = new ArrayList<Integer>();
ArrayList<Integer>[][] arr = new ArrayList<Integer>[9][9];
but it won't even let me declare the ArrayList-Array.
Is there a way to store a list in a 2d array?
Thanks in advance!
You can't create arrays of generic types in Java. But this compiles:
ArrayList<Integer> arrList = new ArrayList<Integer>();
ArrayList<Integer>[][] arr = (ArrayList<Integer>[][]) new ArrayList[9][9];
arr[0][0] = arrList;
Why can't you create these arrays? According to the Generics FAQ, because of this problem:
Pair<Integer,Integer>[] intPairArr = new Pair<Integer,Integer>[10]; // illegal
Object[] objArr = intPairArr;
objArr[0] = new Pair<String,String>("",""); // should fail, but would succeed
Assuming you want an ArrayList inside an ArrayList inside yet another ArrayList, you can simply specify that in your type declaration:
ArrayList<ArrayList<ArrayList<Integer>>> foo = new ArrayList<ArrayList<ArrayList<Integer>>>();
Entries can be accessed via:
Integer myInt = foo.get(1).get(2).get(3);
Just be wary of boundaries - if you try to access an out of bounds index you'll see Exceptions thrown.
Related
So, this is part of a method which checks for available rooms within a date range and is meant to return the int [] array of room numbers which are available.
ArrayList roomNums = new ArrayList();
roomNums.toArray();
for (Room room: rooms){
int roomNumber = room.getRoomNumber();
if(room.getRoomType() == roomType && isAvailable(roomNumber, checkin, checkout)){ // This works fine, ignore it
roomNums.add(roomNumber);
}
}
return roomNums.toArray(); // Error here, return meant to be int [] type but it's java.lang.Obeject []
The error occurs at the end at roomNums.toArray()
I saw someone else do this one liner and it worked for them, why is it not for me?
Every element in roomNums is an integer. (I think)
What's the quickest and easiest way to print an integer array containing the available rooms? Do I need to make a loop or can I do something with this .toArray() one liner or something similar to it?
Thanks
Don't use raw types. Replace ArrayList roomNums = new ArrayList(); with
ArrayList<Integer> roomNums = new ArrayList<>();
Then return it as Integer[]
roomNums.toArray(Integer[]::new);
If you need primitive array, then it can be done with stream:
return roomNums.stream().mapToInt(Integer::valueOf).toArray();
See also How to convert an ArrayList containing Integers to primitive int array?
Unfortunately, you will need to use Integer[] if you want to use toArray
List<Integer> roomNums = new ArrayList<>();
// code here
Integer[] arr = new Integer[roomNums.size()];
return roomNums.toArray (arr);
However you can still do it manually like
List<Integer> roomNums = new ArrayList<> ();
// code here
int[] arr = new int[roomNums.size()];
for (int i = 0; i < arr.length; i++)
arr[i] = roomNums.get (i);
return arr;
As mentioned above use the ArrayList to hold your values:
ArrayList<Integer> roomNums = new ArrayList<>();
then you can use the object super class to convert to array
Object[] numbersArray = roomNums.toArray();
then just continue running the for loop and your program
I'm trying to create a two dimensional ArrayList which will store ints, strings & booleans.
I've got as far as entering the first int, but I get a red squiggle and the error "int cannot be converted to ArrayList".
ArrayList[][] qarray= new ArrayList [10][5];
qarray[0][0]= 1;
BTW, googling the phrase "int cannot be converted to ArrayList" is giving me exactly six results.
The error is correct.
Your array type is ArrayList. You can insert only ArrayLists in that array.
If you want to store int's, your declaration should be.
int[][] qarray= new int [10][5];
And also, as someone commented, you cannot store strings and booleans in this array anymore.
ArrayList[][] qarray= new ArrayList [10][5];
Basically your code creates 2 dimensional array list objects (50 array list objects).
qarray[0][0]= 1;
And you are trying to assign integer, where you need to create a ArrayList object. It expects something like
qarray[0][0]= new ArrayList();
However this would not meet your objective. The following piece of code could meet your objectives:
ArrayList[] qarray = new ArrayList[10];
qarray[0]= new ArrayList();
qarray[0].add(1);
qarray[1]= new ArrayList();
qarray[1].add(true);
qarray[2]= new ArrayList();
qarray[2].add("hello");
Try like this:
List<Integer> qarray = new ArrayList<>();
qarray.add(1);
First I believe that you need array and not ArrayList. 2d array can be created as following.
int[][] arr = new int [10][10];
Your next problem is that you tried to assign int constant 1 to variable of other type. The following example shows how to assign int to element of array
arr [0][0] = 1;
According to javadoc, you can not create arrays of ArrayList. Use 2D array instead.
If you need 2D ArrayList anyway, you should have tried this way:
ArrayList<ArrayList<Integer>> listOfLists = new ArrayList<>();
ArrayList<Integer> list1 = new ArrayList<>();
list1.add(1);
list1.add(2);
ArrayList<Integer> list2 = new ArrayList<>();
list2.add(5);
listOfLists.add(list1);
listOfLists.add(list2);
ArrayList<Integer> list = new ArrayList<>();
list.add(1);
you wan to add int,String and boolean then you can use
ArrayList<ArrayList<Object>> listOfLists = new ArrayList<ArrayList<Object>>();
it will help you
ArrayList<Integer>[][] list = new ArrayList[10][10];
list[0][0] = new ArrayList<>();
list[0][0].add(new Integer(10);
try like this.
The following code throws an ClassCastException:
ArrayList<V>[] table = (ArrayList<V>[]) new Object[tableSize];
How can I create an Array of ArrayList if this doesn't work?
EDIT:
To be shure everybody understands the problem. I need an Array that contains multiple ArrayLists.
EDIT2:
tableSize is an int.
EDIT: OK, here is what you can do:
ArrayList<String>[] table = new ArrayList[tableSize];
It compiles and works on Java 8 i'm using.
In case of Java you can not create new instance of array that type is generic.
T[] array = new T[size]; - cause compile error Cannot create a generic array of T.
The same error will be present when you will try to use it
ArrayList<T>[] array = new ArrayList<T>[size];
With the same reason. The Java runtime does not know what is T after compilation therefor will not be able to allocate valid memory space.
To pass this issue you have following options:
Remove the generic type and use a raw.
ArrayList[] array =new ArrayList[size];
Use list of a list
List<List<T>> list = new ArrayList<List<T>>();
Not the smoothest way to do it But if I got your question correctly something like this should do the trick:
int tableSize = 2;
Collection<String> listWithItems = new ArrayList<>();
listWithItems.add("Foo");
listWithItems.add("Bar");
Collection<Collection<String>> listOfList = new ArrayList<>();
listOfList.add(listWithItems);
Object[] array = listOfList.toArray(new Object[tableSize]);
System.out.println("Col: " + listOfList);
System.out.println("Array: " + Arrays.toString(array));
Print out is:
Col: [[Foo, Bar]]
Array: [[Foo, Bar], null]
Short example without extra "fluff" to get the inital array:
Object[] theArrayOfLists = new ArrayList<List<SomeClass>>().toArray(new Object[tableSize]);
I have values that I'd like to add into an ArrayList to keep track of what numbers have shown up.
The values are integers so I created an ArrayList;
ArrayList<Integer[]> list = new ArrayList<>();
int x = 5
list.add(x);
But I'm unable to add anything to the ArrayList using this method.
It works if I use Strings for the array list. Would I have to make it a String array and then somehow convert the array to integers?
EDIT: I have another question. I'd like the list to only hold 3 values. How would I do so?
List of Integer.
List<Integer> list = new ArrayList<>();
int x = 5;
list.add(x);
You are trying to add an integer into an ArrayList that takes an array of integers Integer[]. It should be
ArrayList<Integer> list = new ArrayList<>();
or better
List<Integer> list = new ArrayList<>();
you are not creating an arraylist for integers, but you are trying to create an arraylist for arrays of integers.
so if you want your code to work just put.
List<Integer> list = new ArrayList<>();
int x = 5;
list.add(x);
you should not use Integer[] array inside the list as arraylist itself is a kind of array. Just leave the [] and it should work
Actually what u did is also not wrong your declaration is right . With your declaration JVM will create a ArrayList of integer arrays i.e each entry in arraylist correspond to an integer array hence your add function should pass a integer array as a parameter.
For Ex:
list.add(new Integer[3]);
In this way first entry of ArrayList is an integer array which can hold at max 3 values.
The [] makes no sense in the moment of making an ArrayList of Integers because I imagine you just want to add Integer values.
Just use
List<Integer> list = new ArrayList<>();
to create the ArrayList and it will work.
Here there are two different concepts that are merged togather in your question.
First : Add Integer array into List. Code is as follows.
List<Integer[]> list = new ArrayList<>();
Integer[] intArray1 = new Integer[] {2, 4};
Integer[] intArray2 = new Integer[] {2, 5};
Integer[] intArray3 = new Integer[] {3, 3};
Collections.addAll(list, intArray1, intArray2, intArray3);
Second : Add integer value in list.
List<Integer> list = new ArrayList<>();
int x = 5
list.add(x);
How about creating an ArrayList of a set amount of Integers?
The below method returns an ArrayList of a set amount of Integers.
public static ArrayList<Integer> createRandomList(int sizeParameter)
{
// An ArrayList that method returns
ArrayList<Integer> setIntegerList = new ArrayList<Integer>(sizeParameter);
// Random Object helper
Random randomHelper = new Random();
for (int x = 0; x < sizeParameter; x++)
{
setIntegerList.add(randomHelper.nextInt());
} // End of the for loop
return setIntegerList;
}
When I try to create an ArrayList myArrayList from an array, using Arrays.asList(myArray), I am not getting the List of elements in myArray. Instead I get list of Array.
The size of myArrayList is 1 . When I try to do myArrayList.toArray(), I am getting a two dimensional array. What to do to get the elements of myArray in a list? Is iterating the only option??
Firstly, the asList method is the right method:
Integer[] myArray = new Integer[3];
List<Integer> myArrayList = Arrays.asList(myArray);
System.out.println(myArrayList.size()); // prints 3, as expected
The problem may be that you are calling the varargs asList method in such a way that java is interpreting your parameter as the first varargs value (and not as an array of values).
Object myArray = new Integer[3];
List<Object> myArrayList = Arrays.asList(myArray);
System.out.println(myArrayList.size()); // prints 1 - java invoked it as an array of Integer[]
To fix this problem, try casting your parameter as Object[] to force the varargs invocation, eg:
Object myArray = new Integer[3];
List<Object> myArrayList = Arrays.asList((Object[]) myArray); // Note cast here
System.out.println(myArrayList.size()); // prints 3, as desired
What is the type of myArray? You cannot use Arrays.asList with an array of primitive type (such as int[]). You need to use loop in that case.
There are different ways by which you can achieve it.3 example of converting array to arraylist and arraylist to array in java might help.
Would this work?
Object[] myArray = new Object[4]; //change this to whatever object you have
ArrayList<Object> list = new ArrayList<Object>();
for (Object thing : myArray) list.add(thing);
Try providing the generic type in the method call. The following gives me a list of 2 String elements.
String[] strings = new String[]{"1", "2"};
List<String> list = Arrays.<String>asList(strings);
System.out.println(list.size());