Cannot Create an Array of Linkedlists in Java

Cannot create an array of LinkedLists in Java...?

You can't use generic array creation. It's a flaw/ feature of java generics.

The ways without warnings are:

1. Using List of Lists instead of Array of Lists:

   List< List<IntegerNode>> nodeLists = new LinkedList< List< IntegerNode >>();
   

2. Declaring the special class for Array of Lists:

   class IntegerNodeList {
       private final List< IntegerNode > nodes;
   }
   

Can you create an array of linked lists in Java?

If I understand you right, you basicly want to make a 2D Array, but with the second half being a linked list.

import java.util.ArrayList;
import java.util.LinkedList;

public class Test
{
    public static void main(String[] args)
    {
        LinkedList one = new LinkedList();
        LinkedList two = new LinkedList();
        LinkedList three = new LinkedList();

        ArrayList<LinkedList> array = new ArrayList<LinkedList>();

        array.add(one);
        array.add(two);
        array.add(three);

        // .. do stuff
    }
}

Java doesn't care what the Objects in Arrays or Lists are, so there's nothing against putting another Array or List in.

How to properly define an array of linked list in Java ?

This is a proper way to create an array:

@SuppressWarnings("unchecked") LinkedList<Long> [] hashtable = new LinkedList[10];

Cannot Create Arrays of Parameterized Types

You cannot create arrays of parameterized types. For example, the following code does not compile:

List<Integer>[] arrayOfLists = new List<Integer>[2];  // compile-time error

The following code illustrates what happens when different types are inserted into an array:

Object[] strings = new String[2];
strings[0] = "hi";   // OK
strings[1] = 100;    // An ArrayStoreException is thrown.

If you try the same thing with a generic list, there would be a problem:

Object[] stringLists = new List<String>[];  // compiler error, but pretend it's allowed
stringLists[0] = new ArrayList<String>();   // OK
stringLists[1] = new ArrayList<Integer>();  // An ArrayStoreException should be thrown,
                                            // but the runtime can't detect it.

If arrays of parameterized lists were allowed, the previous code would fail to throw the desired ArrayStoreException.

Taken from docs.oracle.com

So what can I store in hashtable[] ?

Does it mean I am now allowed to have a linked list of string in the
hashtable[0] and a linked list of Long in hashtable1, if I do
LinkedList [] hashtable = new LinkedList[10]?

No, compiler won't allow you to store LinkedList to the hashtable array directly. Following snippet won't compile:

hashtable[0] = new LinkedList<String>();

However you can store the LinkedList without type parameters, or even a subclass of LinkedList:

@SuppressWarnings("unchecked") LinkedList<Long>[] hashtable = new LinkedList[10];

hashtable[0] = new LinkedList<Long>();
hashtable[1] = new MyLinkedList<Long>();
hashtable[2] = new LinkedList();
hashtable[3] = new MyLinkedList();

You can store the LinkedList if you cast your array to LinkedList[]. However you won't be able to store the anything else but a LinkedList:

LinkedList[] rawHashTable = hashtable;
rawHashTable[4] = new LinkedList<String>();

Object[] objectHashTable = rawHashTable;
objectHashTable[5] = "This line will throw an ArrayStoreException ";

Array of Linked Lists Java

It’s not as straightforward as it sounds, but yes, you can use java.util.LinkedList.

It requires that you declare your array an array of LinkedList or just List (not an array of String). This works:

    List<String>[] array = new List[1000];

    for (int i = 0; i < array.length; i++){
        array[i] = new LinkedList<>();
    }

However, where I instantiate the array, I get a warning: Type safety: The expression of type List[] needs unchecked conversion to conform to List<String>[]. One would have expected new List<String>[1000] to work, but it doesn’t. It gives an error: Cannot create a generic array of List<String>. The element type of an array cannot be a generic type. It’s a peculiarity with historic reasons, we’ll just have to live with it.

How can I create an array of linked lists in java?

LinkedList<Integer>[] vertex = new LinkedList[5];
int i = 0, m = 6;
while(i!=m){
  int temp = sc.nextInt();
  int temp2 = sc.nextInt();

  // Make sure the list is initialized before adding to it
  if (vertex[temp] == null) {
     vertex[temp] = new LinkedList<Integer>();
  }

  vertex[temp].add(temp2);
  i++;
}

Create an Linked List array of linked lists with a loop java

Here you go:

private static LinkedList[] bucket = new LinkedList[19];

static {
    for (int i = 0; i < bucket.length; ++i) {
        bucket[i] = new LinkedList();
    }
}

Why can't I create a singly linked list from an array?

Let's look at this

head=null;     // you are setting head to null
for(i=0;i<setSize;i++){
    head.next = head;  // see two lines up, head is null, it can not have next

Your constructor has some problems. Try using this version:

public SLLSet(int[] sortedArray){ //second constructor
    head = null;
    if (sortedArray == null || sortedArray.length == 0) {
        setSize = 0;
    }
    setSize = sortedArray.length;
    head = new SLLNode(sortedArray[0], null);
    SLLNode curr = head;

    for (int i=1; i < setSize; ++i) {
        curr.next = new SLLNode(sortedArray[i], null);
        curr = curr.next;
    }
}

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