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

Question

So I need to take in input of edges of a bipartite graph like this:

The first number is the number of edges. After that edges are listed. See how for example vertex 1 has multiple different edges and I want to keep track of what 1 is connected to, I was thinking that each vertex of the graph would have some sort of list of vertices it is connected to which leads me to try to create an array of linked lists but I'm not sure how I would do that. I tried

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

But I get a nullpointerexception at the add line.

You haven't initialized the elements in the array, only the array itself. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Nov 25, 2013 at 20:43
Did you think of creating classes like - Vertex, Edge? And have a List<Edge> in another class called Graph? — Rohit Jain
– Rohit Jain, Commented Nov 25, 2013 at 20:44
Also, arrays use 0-based indices which means that an array of size 5 has indices 0...4. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Nov 25, 2013 at 20:44

Stefan Haustein · Accepted Answer · 2013-11-25 20:58:09Z

32

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++;
}

answered Nov 25, 2013 at 20:58

Stefan Haustein

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3 Comments

jaamit Over a year ago

LinkedList<Integer>[] vertex = new LinkedList[5]; Why do I get the following warning when I do the above? How to mitigate this? Type safety: The expression of type LinkedList[] needs unchecked conversion to conform to LinkedList<Integer>[]

Shubham Mittal Over a year ago

@jaamit @SuppressWarnings("unchecked") LinkedList<Integer>[] vertex = new LinkedList[5]; worked for me.

jaamit Over a year ago

@ShubhamMittal @SuppressWarnings("unchecked") this will suppress the warning. I am more interested in knowing why the warning came in the first place.

run run chicken · Accepted Answer · 2016-08-10 21:23:04Z

8

//initialize array
LinkedList<Integer>[] vertex = new LinkedList[5];
//initialize array elements(objects of LinkedList)
for (int j=0; j<5; j++)
    vertex[i]=new LinkedList<Integer>();

int i = 0, m = 6;
while(i!=m){
    int temp = sc.nextInt();
    int temp2 = sc.nextInt();
    vertex[temp].add(temp2);
    i++;
}

Normally Arrays are not encouraged in Java. Alternatively you can use this:

//initialize array
List<LinkedList<Integer>> vertex = new ArrayList<LinkedList<Integer>>();
//initialize arraylist elements(objects of LinkedList)
for (int j=0; j<5; j++)
    vertex.add(new LinkedList<Integer>());

answered Aug 10, 2016 at 21:23

run run chicken

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1 Comment

Saheel Sapovadia Over a year ago

vertex[i]=new LinkedList<Integer>(); it should be--> vertex[j]=new LinkedList<Integer>();

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