How can I make generated numbers in Java be all different using Random?

Question

Basically, I need to generate 5 random numbers that are unique every time they are generated (so they don't repeat) and then output them to a TextView.

I managed to generate the 5 numbers, however, they are not unique all the time, meaning sometimes the same number shows up twice or more when the button is pressed to generate those numbers.

Here's what I have gotten so far:

import androidx.annotation.IntegerRes;
import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import java.util.ArrayList;
import java.util.Random;

public class MainActivity extends AppCompatActivity {

    private TextView numbersGenerated;
    private Button btnGenerate;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        numbersGenerated = findViewById(R.id.numbers);
        btnGenerate = findViewById(R.id.btnGenerate);

        btnGenerate.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ArrayList<Integer> arrayRandom = new ArrayList<>();

                Random rand = new Random();
                for (int i = 0; i < 5; i++)
                {
                    Integer r = rand.nextInt(50) + 1;
                    arrayRandom.add(r);
                }
                numbersGenerated.setText(arrayRandom.toString());
            }
        });
    }
}

I am not sure what type of validation I can do since this is my first ever contact with Java, so any help is appreciated.

score 3 · Accepted Answer · answered Oct 05 '19 at 12:04

3

instead of

ArrayList<Integer> arrayRandom = new ArrayList<>();

use a Set

Set<Integer> set = new HashSet<>();

and then do a

while (set.size() < 5) {
    set.add(rand.nextInt(50) + 1);
}

answered Oct 05 '19 at 12:04

Horatiu Jeflea

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Could you explain what's the difference between ArrayList and Set? As I mentioned, I'm new to Java so pardon me for not understanding what it does. Thank you. – Pedro Dias Oct 05 '19 at 12:08
More mathematics than Java :-) A set can contain only a single instance of an element. So, if you 'add' the number 99 to a set already containing 99, there is no change. – Oct 05 '19 at 12:10
Also, a set has no defined order, ie the order you add numbers is not necessarily the order you get if you loop over its content. – Joachim Isaksson Oct 05 '19 at 12:15
order can be solved with a LinkedHashMap – Horatiu Jeflea Oct 05 '19 at 12:16
1

*LinkedHashSet I meant – Horatiu Jeflea Oct 05 '19 at 12:29

How can I make generated numbers in Java be all different using Random?

1 Answers1