Joining PART of 2 lists in Java

Question

I have two lists with items (List 1 and List 2). I'll create a third one, here called list_mix, with 10 items, and it will receive:

The 6 first positions or 60% of list1 and
The 4 first positions or 40% of list2.

That new list (list_mix) should receive the items from the lists 1 and 2 randomly.

Here is an example: public class JoinLists {

    public static void main(String[] args) {

        List<String> list1 = new ArrayList<String>();
        list1.add("dog","cat","rabbit","bat","zebra","bear","turtle","mouse","fly","bird");

        List<String> list2 = new ArrayList<String>();
        list2.add("bee","horse","mule","cow","fish","shark","lion","tiger","elephant","panther");

    List<String> list_mix = new ArrayList<String>();
        list_mix.addAll(list1);
        list_mix.addAll(list2);

    System.out.println("list1 : " + list1);
    System.out.println("list2 : " + list2);
    System.out.println("list_mix : " + list_mix);

}

What I want is the following result in list_mix: Output

list1 : [dog,cat,rabbit,bat,zebra,bear,turtle,mouse,fly,bird]
list2 : [bee,horse,mule,cow,fish,shark,lion,tiger,elephant,panther]
list_mix : [dog,bee,cat,horse,rabbit,mule,bat,cow,zebra,bear] //A list with 10 items, using 60% of list 1 and 40% of list 2

If I use ".addAll" I'll get all the items, but I want only the first ones of the two lists. How can I get this?

Jean-François Savard · Accepted Answer · 2015-03-03 17:48:55Z

1

You could use the subList method :

list_mix.addAll(list1.subList(0, 6));
list_mix.addAll(list2.subList(0, 4));

Then to randomize the order of the list use Collections.shuffle(list_mix);

answered Mar 3, 2015 at 17:48

Jean-François Savard

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Comments

Sachin Thapa · Accepted Answer · 2015-03-03 18:12:56Z

You can create a method that will helps to extract part of a list for a given percentage.

See Java sublist docs

Once you have your sublist you should use Collections.shuffle to randomize results.

Also I updated the way you initialized array strings in your code, here is updated code.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class RandomList {

private static List<String> getSubList(List<String> list, int percentage) {
    return list.subList(0, (percentage / 10));
}

public static void main(String[] args) {

    String[] array1 = { "dog", "cat", "rabbit", "bat", "zebra", "bear",
            "turtle", "mouse", "fly", "bird" };

    List<String> list1 = new ArrayList<String>();
    list1.addAll(Arrays.asList(array1));

    String[] array2 = { "bee", "horse", "mule", "cow", "fish", "shark",
            "lion", "tiger", "elephant", "panther" };

    List<String> list2 = new ArrayList<String>();
    list2.addAll(Arrays.asList(array2));

    List<String> list_mix = new ArrayList<String>();
    list_mix.addAll(getSubList(list1, 60));
    list_mix.addAll(getSubList(list2, 40));

    Collections.shuffle(list_mix);

    System.out.println("list1 : " + list1);


    System.out.println("list2 : " + list2);
        System.out.println("list_mix : " + list_mix);

    }
}

Output

list1 : [dog, cat, rabbit, bat, zebra, bear, turtle, mouse, fly, bird] 
list2 : [bee, horse, mule, cow, fish, shark, lion, tiger, elephant, panther] 
list_mix : [cat, cow, zebra, rabbit, mule, dog, bear, bat, horse, bee]

Hope this helps.

Cheers !!

Thanks Sacnhin, of course it helped :)I really appreciate your care with the code, it was the best answer

Rogue · Accepted Answer · 2015-03-03 17:54:20Z

Here's a method that could clear things up for you:

public List<String> getSubList(List<String> in, int magnitude) {
    List<String> back;
    if (in.size() > 10) { // if > 10, (magnitude*10)% can be more than magnitude
        //use percentage
        back = new ArrayList<>(in); //prevent modifying outside contents
        Collections.shuffle(back); //randomize, use ThreadLocalRandom instead if working with big collections
        magnitude = (in.size() * (magnitude * 10)) / 100; //set our new magnitude
    } else {
        back = in; //just use inserted elements if not using percentage
    }
    return back.subList(0, magnitude); //return our sublist
}

Then it's as simple as:

List<String> mixed = new ArrayList<>();
mixed.addAll(getSubList(list1, 6));
mixed.addAll(getSubList(list2, 4));

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