How to shuffle the contents of an array

Question

so my program is supposed to access a text document then do all that jazz that currently works. The only problem that I can't figure out is how to shuffle the contents of an array without having them end up on top of each other. Both internets and multiple tries with random and for loops have been unfruitful. here is my code:

import java.io.*;
import java.util.*;
public class lab_6 {
public static void main(String[] args)throws FileNotFoundException {
    Scanner input = new Scanner(System.in); //reads from keyboard
    System.out.println("What is the name of your file. ");
    String name = input.nextLine();
    Scanner reader = new Scanner(new File(name));// Open text file
    System.out.println("how many names are in your array");
    int num = input.nextInt();
    String[] names = new String[num];    
    for (int index = 0; index< names.length; index++)
    {
        names[index] = reader.nextLine();// Gets a line while there is one 
    } 
    System.out.println("\nOriginal List");
    printList(names);
    System.out.println("\nShuffled List");
    shuffle(names);
    printList(names);
    System.out.println("\nSorted List");
    Arrays.sort(names);  // this is a built in method
    printList(names);
    System.out.println("What name are you looking for");
    Scanner input1 = new Scanner(System.in); //reads from keyboard
    String find = input1.nextLine();
    int index = search(names,find);
    if(index == -1)
        System.out.println("The name was not there");
    else
        System.out.println(find+" was found at position "+index);
    System.out.println("The average length of all the names is "+averageLength(names));
}
public static void printList(String[] array) // print the list of names numbered
{
    for (int i=0; i<array.length; i++)
    {
        System.out.println((i+1)+") "+ array[i]);
    }

}
public static void shuffle (String[] array) // mix-up the array
{


}
public static int search(String[] array, String find) 
{   
    for(int i=0; i<array.length; i++) {

        if (array[i].equals(find) ) return i;

    } 
    return -1;
}
public static double averageLength(String[] array) //return the average length of the names 
{       
    int sum=0;
    for (int i=0; i<array.length; i++)
    {
        int l= array[i].length();
        sum +=l;
    }
    int average = sum/(array.length);
    return average; 

}

}

Answer 1

String[] names = ...;
Collections.shuffle(Arrays.asList(names));
// done

Note that Arrays.asList() returns a modifiable (but fixed-length) list, backed by the array, and not a copy of the array. So the array will be shuffled.

Answer 2

Just use the Fisher-Yates shuffle (Knuth algorithm P):

private Random rand = new Random();

public static void shuffle(String[] array) { // mix-up the array
    for (int i = array.length - 1; i > 0; --i) {
        int j = rand.nextInt(i + 1);
        String temp = array[i];
        array[i] = array[j];
        array[j] = temp;
    }
}

See:

1. Knuth, D. 1969, 1998: Seminumerical Algorithms 1st & 3rd Eds. The Art of Computer Programming Series, Volume 2, p. 125.
2. Fisher, Ronald A.; Yates, Frank (1948) [1938]. Statistical tables for biological, agricultural and medical research (3rd ed.). London: Oliver & Boyd. pp. 26–27.
3. http://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle
4. https://stackoverflow.com/a/1520212/636009

Answer 3

You can use the Collections class, with the shuffle method. The documentation is available here.

Example

int[] values = {1,2,3,4,5};

List<Integer> valuesList = Arrays.asList(values);

Collections.shuffle(valuesList);

// valuesList is shuffled.