I have a list

l = ["a","z","c","c","a","e","e"]

I need to generate a set of list as below in java

l1=["a"]
l2=["z"]
l3=["c","c"]
l4=["a"]
l5=["e","e"]

below is my code so far

package test

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class test {

    public static void main(String[] args) {
        List<String> l =new ArrayList<>();
        List<List> ll =new ArrayList<>();
        List<String> nl;
        l.add("a");
        l.add("z");
        l.add("c");
        l.add("c");
        l.add("a");
        l.add("a");
        l.add("e");
        String prev="";
        nl = new ArrayList<>();
        for (String x : l){
            if(prev.equals(x))
            {
                nl.add(x);
            }
            else {
                if(!nl.isEmpty()) {
                    ll.add(nl);
                }
                else{
                List<String> nl1 = new ArrayList<>();
                nl1.add(x);
                ll.add(nl1);
                }
            }
            prev=x;
        }

        System.out.println(ll);
    }
}

output

[[a], [z], [c], [c, a], [c, a]]

expected

[[a], [z], [c,c], [a,a], [e]]

CodePudding user response：

With some little modifications to your code the below snippet should give you the disired result:

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

public class Example {

    public static void main(String[] args) {
        List<String> values = Arrays.asList("a","z","c","c","a","e","e");
        List<List<String>> result = new ArrayList<>();

        String previous = null;
        List<String> subList = new ArrayList<>();

        for (String value : values) {
            if (previous == null || value.equals(previous)) {
                subList.add(value);
            } else {
                result.add(subList);
                subList = new ArrayList<>();
                subList.add(value);
            }
            previous = value;
        }
        if (!subList.isEmpty()) {
            result.add(subList);
        }
        System.out.println(result);
    }
}

Output:

[[a], [z], [c, c], [a], [e, e]]

Another approach using streams and AtomicReference:

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;
import java.util.stream.Collectors;

public class Example {

    public static void main(String[] args) {
        List<String> input = Arrays.asList("a", "z", "c", "c", "a", "e", "e");

        AtomicReference<String> ar = new AtomicReference<>(input.get(0));
        AtomicInteger ai = new AtomicInteger();

        List<List<String>> result = new ArrayList<>(
                input.stream().collect(
                        Collectors.groupingBy(str -> str.equals(ar.getAndSet(str)) ? ai.get() : ai.incrementAndGet()))
                .values());
        System.out.println(result);
    }
}