We Keep Coding

How do I add arrays to an Arraylist of Arrays (Arraylist<Integer[]>)

I want to save the steps pf my player-character in a Sokoban-game.
So at first i want to fill an Array of int x and y, called "pos" with the actual position of the character.
And than i want to add this array to an ArrayList of Arrays, called "moves".
Array for one player-position:
int[] pos = new int [2];
ArrayList for all steps, the player made in the level:
Arraylist<Integer[]> moves = new ArrayList<>();
"int[]" makes an error, if placed inside the pointy brackets at the ArrayList.
How do i add the Array pos to the ArrayList moves?

This works completely fine with int[]. However int[] != Integer[], Use same in both places.
int[] pos = new int[2];
ArrayList<int[]> arrayList = new ArrayList<>();
pos[0] = 1;
pos[1] = 2;
arrayList.add(pos);
System.out.println(arrayList.get(0)[0]+ " "+ arrayList.get(0)[1]);

int and Integer are two different things (I struggled with this as well when I first learned Java a long time ago.
int is a so-called primitive datatype, which means it is not an object. Integer is a wrapper class, basically a class whose only purpose is to contain an
int so that it can be handled as an object. Since Java 5, there is "autoboxing", which means you can assign int values to Integer variables and vice versa, and the compiler will handle it silently. This tends to muddle the difference, but it still is there and important - such as in this case.
So if you want to store instances of int[] in an ArrayList, you need an ArrayList<int[]>, not an ArrayList<Integer[]>.
However, it is impossible to have an ArrayList<int> because ArrayList can only store Objects (the generic types exist only for the compiler), not primitives.
Oh yes, and to add an element to the list, use
Arraylist<int[]> moves = new ArrayList<>();
arrayList.add(pos);
You could have found this in the API doc for ArrayList - you should really learn to use the API doc, you will need it all the time to answer questions such as this.

You can also make class (for example Pos) and make x and y as attributes and make constructor. The you can make the type of arraylist is Pos
ArrayList arrayList = new ArrayList<>();
arrayList.add(new Pos(3,2));

How to add values to Double[] arraylist [duplicate]

This question already has answers here:
I'm getting an error in my Java code but I can't see whats wrong with it. Help?
(5 answers)
Closed 5 years ago.
I created this arrayList:
Double[] arrayOfNumbers = new Double[List.size()];
And I try to add it numbers with this:
arrayOfNumbers.add(0.9);
This gives me an error message that says:
Cannot invoke add(double) on the array type Double[]
So, how can I add that value in this Double[] arraylist?

That is not an ArrayList. That is an array.
You can declare an arraylist of doubles as :
int initialCapacity = 20;
List<Double> doubles = new ArrayList<Double>(initialCapacity);
doubles.add(0.9);
You can add more than 20 values in an ArrayList even though the initial capacity is specified as 20.
But to declare an array and populate it:
double[] doublesArray = new doubles[20];
doubles[0] = 0.9;
doubles[1] = 0.5;
.....
doubles[19] = 0.7; // 19 is the last index for an array of size 20.
If you add more than 20 here, you will get ArrayIndexOutOfBoundsException

Double[] is not ArrayList to add to an array you can use :
Double[] arrayOfNumbers = new Double[List.size()];
arrayOfNumbers[0] = 0.9;
Instead to add to an ArrayList you can use :
List<Double> arrayOfNumbers = new ArrayList<>();
arrayOfNumbers.add(0.9);

Arrays and lists are different things. Arrays don't have an add method, but are assignable by the subscript ([]) operator:
arrayOfNumbers[0] = 0.9;

Double[] arrayOfNumbers = new Double[List.size()] is not a List
it is an array.
We declare arrays with [] and lists with <> and generics.
For example `
int[]arr=new int[3] is an array of 3 ints, but List<Integer>list=new ArrayList<>() is a list of integers(not primitive ints you CANNOTwrite code like this List<int>=new ArrayList<>()
Hope that helps!

java.util.List is a different that an array(which has limited predefined size).
you can't declare/perform as above you did in question, java compiler will complain if you do so.
you are phasing a error like,
Cannot invoke add(double) on the array type Double[]
because an array(arrayOfNumbers) do not having a such method(add) which you can execute on arrayOfNumbers.
However,
general syntax to initialize of an array of any type(here in example, Integer taken FYI) is likewise,
int intArray[] = {1,2,3}; // initialize at the time of creation
or
int arrayOfNumbers [] = new int[] {1, 2, 3};
or
intArray[index] = 1;
Moreover, you can switch from array to List & vice-versa as likewise,
List<Integer> list = java.util.Arrays.asList(arrayOfNumbers);
or
Integer [] intArray = list.toArray(new Integer[list.size()]);

A List is an Interface that extends another interface called Collection, so a List is-a Collection. An Interface defines and describes behavior, it defines a contract that another class must conform to, and one of the classes that does so is called java.util.ArrayList, and add() is one behaviour defined in the List contract, because a List must have the ability for things to be added to it. An ArrayList is one type of a List. What you have created is NOT an ArrayList, it is an Array.
An array is a primitive data structure, once created, it's size cannot be changed, If you wish to add a new element to an array you have to create a new array that is bigger, then transfer all elements from the old array to the new one. Under the covers an ArrayList does exactly that. THIS IS HOW YOU CREATE an ArrayList :
//a list of Objects of type `Double`
List<Double> listOfNumbers = new ArrayList<>();
if you wish to add an element to this list, you would do this :
listOfNumbers.add(5.2);
You do seem like you still need to read a beginners book and practice basic Java programming. I would like to suggest this playlist
This will really be helpful to you, and remember you can only learn something by doing it hands-on, NOT by just watching somebody else doing it.

Constructing a List<T> from an Enum

What it comes down to is I'm attempting to construct a List<T> from the java.util.List package based on an enumerator, but here's the catch- I don't want the first value to be in the list. The way that I've come up with doing this... Well... It can't be a good thing.
Here's the code I'm using:
Arrays.asList(Arrays.asList(MyEnum.values()).remove(0));
This effectively creates a List<> from my enum, and removes the first value, then tries to create another list with the created list.
This may actually be the correct way to do it, but just looking at it is screaming "Inefficient". I showed it to a few people on a forum that I'm a member of and they all said how ugly it was, and laughed; However not a single one of them could/would provide a better way to go about doing it.

I don't think that code is really ugly or inefficient. The bigger problem is it doesn't work because the list returned by Arrays.asList is backed by the underlying array, and so it has a fixed size.
You can use subList instead of remove to get it working:
List<MyEnum> list = Arrays.asList(MyEnum.values());
list = list.subList(1, list.size());
This means that the resulting list actually consists of two list objects (the asList and its subList), which may or may not bother you. However, you can create a flat ArrayList from the resulting list:
list = new ArrayList<>(list);
If you'd rather minimize object allocation, loop the values and add them to a single list directly:
MyEnum[] values = MyEnum.values();
List<MyEnum> list = new ArrayList<>(values.length - 1);
for (int i = 1; i < values.length; i++)
list.add(values[i]);

How about:
Arrays.asList(MyEnum.values()).subList(1, MyEnum.values().length);

Are you sure you can't (wouldn't want to) use MyEnum.values and treat your first element differently where it's used?
public enum Enums {
first,
b,
c,
d,
e,
f;
public boolean isFirst(Enums e) {
return e == first;
}
}

This would work:
List<T> enums = Arrays.asList(Arrays.copyOfRange(MyEnum.values(), 1, MyEnum.values().length - 1));
Or java 8:
Arrays.stream(MyEnum.values()).collect(Collectors.toList()).subList(1, MyEnum.values() - 1);
Hmmm... not much better .

How to store Collection.reverse back into an array?

My issue is I am trying to reverse an array. The method I have found that apparently works for this is to use
Collections.reverse(Arrays.asList(arrx));
However I want to store it back into an array for future use.
I'm not quite familiar with Collections to begin with, I did go over the documentation for it however in terms of storing this back into an array i'm a little lost.
How would you go about this?
Thanks.

Collections.toArray() is your friend. It has two methods a type safe and a non type safe one (preferred to use type safe).
Integer[] a = new Integer[] {1,2,3};
List<Integer> l = Arrays.asList(a);
Collections.reverse(l);
l.toArray(a);
If possible using Collections instead of arrays is the best practice.

The following consideration is slightly academic, however I felt that it should be pointed out:
Arrays#asList will give you a "view" on the supplied array. The JavaDoc says: "Returns a fixed-size list backed by the specified array. (Changes to the returned list "write through" to the array.)". Looking at your example:
String[] arrx = {"alpha", "beta", "gamma"};
Collections.reverse(Arrays.asList(arrx));
System.out.println(Arrays.toString(arrx)); // [gamma, beta, alpha]
... this means, that the original array arrx will be reversed. If you just need the reversed array, all is fine. However: If you still need the original array, create a copy before reversing:
String[] arrx = {"alpha", "beta", "gamma"};
List temp = new ArrayList(Arrays.asList(arrx));
Collections.reverse(temp);
Object[] reversedArrx = temp.toArray();
System.out.println(Arrays.toString(arrx)); // [alpha, beta, gamma]
System.out.println(Arrays.toString(reversedArrx)); // [gamma, beta, alpha]
(Generics omitted for brevity).

And what about a for loop ? it would be like:
static void reverse(int[] t){
for(int x=0; x<t.length / 2; x++){
int y = t[x];
t[x] = t[t.length - 1 - x];
t[t.length - 1 - x] = y;
}
}

Can someone break down this line so I can understand it?

I'm having trouble understanding how an array of ArrayLists is initialized in Java, can someone explain what's going on in this line of code?
edges = (ArrayList<Integer>[]) new ArrayList[nodeCount + 1];

Let's break it space-by-space.
edges is a variable of type ArrayList<Integer>[]
= is the assign operator which assignes the right-hand to the left-hand
(ArrayList<Integer>[]) is a cast of a variable to the type.
new ArrayList[nodeCount + 1] means we allocate space for an array of ArrayList with nodeCount+1 unknown elements.
This is a very bad way of initializing an array. What it does is it creates an array and makes the elements into Integers.
An alternative:
edges = new ArrayList<Integer>(nodeCount+1);
Explanation: The ArrayList class has a constructor which can specify its length*, this is what I use here.
Note: According to #Rohit Jain, it doesn't specify the length, but the initial capacity.

You cannot create an array whose component type is parameterized type. It's not type safe. Although you can create an array whose component type is raw type, but that won't be type safe either. Consider the following example:
List<Integer>[] list = null; // Declaration is OK
list = new ArrayList<Integer>[5]; // Compiler error: Generic array creation
list = new ArrayList[5]; // Compiles fine. But not safe. Gives warning
Suppose you created an array of raw types. Let's see what can be the implication:
List<Integer>[] list = new ArrayList[10]; // Not type safe
Object[] objArr = list; // We can assign List<Integer>[] to Object[]
// We can add even ArrayList<String> in Object[]
// This will successfully compile, and run.
objArr[0] = new ArrayList<String>() {
{
add("rohit"); add("jain");
}
};
// Here's the problem. It will compile fine, but at runtime will throw
// ClassCastException
Integer val = list[0].get(0);
Alternative is create a List of List:
List<List<Integer>> edges = new ArrayList<List<Integer>>();
Suggested Read: -
Angelika Langer Generic FAQs:
Can I create an array whose component type is a concrete parameterized type?
Can I declare a reference variable of an array type whose component type is a concrete parameterized type?

In the above line you are creating an array of ArrayList, you could replace ArrayList by a more simple type to help you to understand, e.g. an array of String:
edges = (String[]) new String[nodeCount + 1];
nodeCount + 1 corresponds to size of the array. The array can't have more than this number of elements.
Note that using an array of a parametrized ArrayList is quite strange and prone to misunderstanding and errors. I would use a List<List<Integer>> here, e.g.:
edges = new ArrayList<List<Integer>>();

this line defines an array, like any other array out there: exampe new Object[0], new String[0], ...
and just like any other array, the values will be initiated with the null value. for primitive types is that '0', for objects/classes is that null.
so you should initiate the different arraylists before using it like:
edges = new ArrayList<Integer>[nodeCount + 1];
for(int i=0; i<edges.length; i++){
edges[i] = new ArrayList<Integer>();
}

This does not initialize an ArrayList -- it initializes an array of ArrayLists:
new ArrayList[nodeCount + 1] = create an array of ArrayList objects with nodeCount + 1 slots
(ArrayList<Integer>[]) = cast it to an "array of ArrayList objects which in turn may only contain Integer objects". This is needed because the array declaration syntax of java apparently can't handle generics (just tried it -- I never needed this before).
It could be a misunderstanding, and the writer actually wanted to initialize one ArrayList with a capacity of nodeCount+ 1. The correct code for that would be
edges = new ArrayList<Integer>(nodeCount + 1);
Actually the capacity parameter is just an optimization, since ArrayList objects grow automatically as needed. But if you already know how many entries you need, the List can be created with enough capacity from the start.

new ArrayList[nodeCount + 1]
create a new array of ArrayList, its length is nodeCount + 1;
then
(ArrayList<Integer>[])
is a cast operation, it casts the array you just created into an array of ArrayList<Integer>