CODEWITHSUNDEEP
javapermutationgeneric-programming
RuneScape
RuneScape

Reputation: 75

permutations of a list java

I am trying to generate all possible permutations of a given list in a generic function. I think I have the function code correct. However, during testing time I keep getting compiler errors from my main program. Please help!

import java.util.*;

class perms <E>{

  public List<List<E>> generatePerm(List<E> original) {
      if (original.size() == 0) { 
          List<List<E>> result = new ArrayList<List<E>>();
          result.add(new ArrayList<E>());
          return result;
      }

      E firstElement = original.remove(0);
      List<List<E>> returnValue = new ArrayList<List<E>>();
      List<List<E>> permutations = generatePerm(original);

      for (List<E> smallerPermutated : permutations) {
          for (int index=0; index <= smallerPermutated.size(); index++) {
              List<E> temp = new ArrayList<E>(smallerPermutated);
              temp.add(index, firstElement);
              returnValue.add(temp);
          }
      }

      return returnValue;
  }

  public static void main(String[] params){
      List<Integer> p = new ArrayList<Integer> ();
      p.add(1);
      p.add(2);

      System.out.print(p.toString());

      List<List<Integer>> resultantList=generatePerm(p);

      System.out.print(resultantList.toString());
  }
}

I keep generating the following error in my testing portion:

perms.java:33: generatePerm(java.util.List<E>) in perms<E> cannot be applied to (java.util.List<java.lang.Integer>)
List<List<Integer>> resultantList=generatePerm(p);

Upvotes: 0

Views: 1337

Answers (3)

Sage
Sage

Reputation: 15418

  1. Your generatePerm function is not static: as main is static you won't call a non-static function from a static context.
  2. You should put an <E> after the access modifier of generatePerm declaration to specify that this function is generic.

So working version of generatePerm function would be:

public static <E>List<List<E>> generatePerm(List<E> original) { 
  //your code block

}

Upvotes: 1

Petr &#218;jezdsk&#253;
Petr &#218;jezdsk&#253;

Reputation: 1269

You forgot 2 things:

  • make generatePerm() static
  • then you have to make generatePerm() generic adding <E>
  • (java convention) class names begin with capital letter - rename class to Perms

Result:

public class Perms
...
public static <E> List<List<E>> generatePerm(List<E> original)
...

Btw you don't have to declare <E> in class definition because here you use it only in static context.

Upvotes: 0

Joni
Joni

Reputation: 111259

The error message is not helpful at all; the real problem is that you are calling a method that belongs to an object, but don't provide an object. If it wasn't for generics you would get the famous "cannot call non-static method from static context" error.

One way to fix it is creating an object:

  perms<Integer> computation = new perms<Integer>();
  List<List<Integer>> resultantList = computation.generatePerm(p);

Upvotes: 1

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