How to fill an Arraylist with values of another Arraylist caught randomly?

Question

I have the class Vetor which will have a method to start an array with random numbers:

public class Vetor{
    private int tamanho;
    private int[] vetor;

    public Vetor(int tamanho) {
       this.tamanho = tamanho;
       this.vetor = vetor;
    }

    public void iniciaVetor() {
        for (int i = 0; i < tamanho; i++) {
           this.vetor[i] = (int) Math.random();
        }
    }
}

Now I got the class Arraylist which will have a method to hold a array vector:

public class Arraylist{
    private int tamanho;
    private ArrayList<Vetor> arraylist;

    public Arraylist(int tamanho) {
        this.tamanho = tamanho;
        this.arraylist = new ArrayList<>();
    }

    public void iniciaArray(int tamanho_vetor) {
        Vetor v;
        for (int i = 0; i < tamanho_vetor; i++) {
            v = new Vetor(tamanho_vetor);
            v.iniciaVetor();
            arraylist.add(v);
        }
    }   
}

Finally, I have the class selects which has a method that will randomly select an index (an array from the list of vectors) and will populate that vector in the list temp. It will end by the temp be the same size as arraylist.

public class Selecao{
    public ArrayList<Vetor> seleciona(ArrayList<Vetor> arraylist, tamanho_vetor) {
        ArrayList temp = new ArrayList<>();
        while (arraylist.size() != temp.size()) {
            int numero_aleatorio = (int) Math.random();
            //cromo adicionar o vetor da posição "numero_aleatorio" no arraylist temp
        }
    }
}

Answer 1

This code has several red flags, but I will focus only on what you need for the question.

What you want is to shuffle the array. Java has it ready. Use the method shuffle(). You don’t even have to create a method for this, but if you want to create an abstraction you can do it:

import java.util.*;
import java.util.Collections;

class HelloWorld {
    public static void main (String[] args) {
         ArrayList<Integer> lista = new ArrayList<>();
         for (int i = 0; i < 50; i++) lista.add(i);
         seleciona(lista);
         for (int i = 0; i < 50; i++) System.out.println(lista.get(i));
    }
    public static void seleciona(ArrayList<Integer> arraylist) {
        Collections.shuffle(arraylist);
    }
}

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.

I put a Integer in the list, but it could be anything else. I only did so to facilitate since the question did not have an easy code to test.

With the comment below it became clearer and I made this solution:

import java.util.*;
import java.lang.Math;

class HelloWorld {
    public static void main (String[] args) {
         ArrayList<Integer> lista = new ArrayList<>();
         for (int i = 0; i < 50; i++) lista.add(i);
         lista = seleciona(lista);
         for (int i = 0; i < 50; i++) System.out.println(lista.get(i));
    }
    public static ArrayList<Integer> seleciona(ArrayList<Integer> arraylist) {
        Random rnd = new Random();
        ArrayList<Integer> temp = new ArrayList<>(arraylist.size());
        for (int i = 0; i < arraylist.size(); i++) {
            temp.add(arraylist.get(rnd.nextInt(arraylist.size())));
        }
        return temp;
    }

}

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.