Is it possible to dynamically instantiate Arraylist in java?

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For example, if I need to create arraylists for each person q register in my system:

ArrayList pessoa1 ArrayList();
ArrayList pessoa2 ArrayList();
ArrayList pessoa3 ArrayList();

The problem is I don’t know how many people are going to register so it would have to be created dynamically, like:

for(int i = 0; i < numeroPessoasCadastradas; i++){
    pessoa[i] = new ArrayList();
}

It’s possible to do something like that?

java
  • 2

    Yes, perfectly possible. Arraylist has no set size. It even has a size but expands as it is filled.

  • Hello, I tried to create this for but java gives me the following error: Exception in thread "main" java.lang.Nullpointerexception

  • 1

    So it would be interesting to better explain the problem, because it was not very clear the purpose of this.

  • Probably the Arraylist pessoa was not created.

  • Try to start pessoa before. You are using a vector of ArrayList, which is strange to me; anyway, you could, before the for, do pessoa = new ArrayList[numeroPessoasCadastradas], but it’s very strange to do this. What is the context of you needing to do this? Why the ArrayLists are not typed?

  • @Thiago, I’d like to add an addendum on Arraylist: here. I believe I can help in understanding your question. In your solution you use Arraylist.

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It’s possible, only, the way you wrote, pessoa[] it’s just one Array, and Arrays do not grow dynamically as you would like:

import java.util.ArrayList;

public class MeuPequenoArray {

    public static void main(String[] args) {

        int numeroPessoasCadastradas = 12345; // tamanho fixo
        ArrayList[] pessoa = new ArrayList[numeroPessoasCadastradas];

        for (int i = 0; i < numeroPessoasCadastradas; i++) {
            pessoa[i] = new ArrayList();
        }
    }
}

A possible solution is to create a List of ArrayLists - or a ArrayList of ArrayLists:

import java.util.ArrayList;

public class MeuEnormeArrayList {

    public static void main(String[] args) {

        int numeroPessoasCadastradas = 12; // esse número não importa mais
        ArrayList<ArrayList> pessoa = new ArrayList<ArrayList>();

        for(int i = 0; i < numeroPessoasCadastradas; i++){
            pessoa.add(new ArrayList<>());
        }
    }   
}

  • would like to add an Addendum on Arraylist: here. I believe it can help in understanding your answer. In your solution you use Arraylist.

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