Question

how can I make array of ArrayList or Pair Class which I made myself at the code below.

ex1)

import java.util.*;

class Pair{
  static int first;
  static int second;
}

public class Main{
  public static void main(String[] args){
    Vector<Pair>[] v = new Vector<Pair>[100](); //this gives me an error
  }
}

1.why the code above gives me an error?
2.my goal is to make an array of vector so that each index of vector holds one or more Pair classes. How can I make it?

another example) : array of ArrayList

import java.util.*;

public class Main{
  public static void main(String[] args){
    ArrayList<Integer> arr = ArrayList<Integer>(); //I know this line doesn't give error
    ArrayList<Integer>[] arr = ArrayList<integer>[500]; // this gives me an error
  }
}

3.why does the code above give me an error?
4.my goal is to make an array of ArrayList so that each index of Array has ArrayList/Queue/Vector/Deque whatever. How can I make it?

Answer 1

The syntax you have used is not what Java uses. If you want to have an array of ArrayLists then do:

ArrayList[] arr = new ArrayList[100];

for(int i=0; i<arr.length; i++)
{
    arr[i] = new ArrayList<Pair>(); // add ArrayLists to array
}

Here the type argument <Pair> specifies that the ArrayList should contain items of type Pair. But you can specify any type you wish to use. The same goes for ArrayList, you could replace ArrayList with Vector in the example.

It would be best to use an ArrayList instead of an array in the example. Its much easier to maintain without worrying about the changing length and indexes.

Hope this helps.