Generates permutations from a list of collections. - Java java.util

Java examples for java.util:Collection Element

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Description

Generates permutations from a list of collections.

Demo Code


//package com.book2s;
import java.util.Collection;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;

public class Main {
    public static void main(String[] argv) {
        List collections = java.util.Arrays.asList("asdf", "book2s.com");
        System.out.println(generatePermutations(collections));
    }/*  w  ww  . java2  s  . co m*/

    /**
     * Generates permutations from a list of collections. Each permutation contains an element
     * from each of the collections. 
     * 
     * @param <T>         the generic type for object in the collections
     * @param collections   the list of collections
     * @return            the collection of permutations
     */
    public static <T> Collection<List<T>> generatePermutations(
            List<Collection<T>> collections) {
        if (collections == null || collections.isEmpty()) {
            return Collections.emptyList();
        } else {
            Collection<List<T>> res = new LinkedList<List<T>>();
            permutationsImpl(collections, res, 0, new LinkedList<T>());
            return res;
        }
    }

    private static <T> void permutationsImpl(List<Collection<T>> ori,
            Collection<List<T>> res, int d, List<T> current) {
        if (d == ori.size()) {
            res.add(current);
            return;
        }
        // iterate from current collection and copy 'current' element N times, one for each element
        Collection<T> currentCollection = ori.get(d);
        for (T element : currentCollection) {
            List<T> copy = new LinkedList<T>();
            copy.addAll(current);
            copy.add(element);
            permutationsImpl(ori, res, d + 1, copy);
        }
    }
}

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