SearchInList.java

import com.google.common.collect.Lists;

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

public class SearchInList {

    public static final ArrayList<List<String>> LIST = Lists.newArrayList(
        Lists.newArrayList("a", "b"),
        Lists.newArrayList("c", "d"),
        Lists.newArrayList("e", "f", "g"),
        Lists.newArrayList("h", "i", "j", "k"),
        Lists.newArrayList("l", "m", "n", "o")
    );

    public static final boolean containsIn2D (List<List<String>> list, String str) {
        return list.stream().flatMap(l -> l.parallelStream()).anyMatch(v -> v.equals(str));
    }
    public static void main(String[] args) {
        System.out.println(containsIn2D(LIST, "a"));
    }
}