public class Main {
    public static void main(String[] args) {
        String s1 = "Hello";
        String s2 = s1;
        s1 = s1.concat(" World");
        System.out.println(s2);
    }
}

import java.util.HashMap;

 public class Main {
    public static void main(String[] args) {
        HashMap<String, Integer> map = new HashMap<>();
        map.put(null, 1);
        map.put("a", 2);
        map.put(null, 3);
        System.out.println(map.get(null) + map.get("a"));
    }
}

List<String> list = Arrays.asList("one", "two", "three", "four");
Map<Integer, String> size2Str = list.stream()
        .collect(Collectors.toMap(String::length, Function.identity()));
System.out.println(size2Str.size()); 

.collect(Collectors.toMap(String::length, Function.identity(), (s1, s2) -> s1))
// Этот вариант оставит первое значение. 

List<String> list = Arrays.asList("Java", "Python", "C++");
Collections.sort(list, new Comparator<String>() {
    @Override
    public int compare(String s1, String s2) {
        return s1.length() - s2.length();
    }
});
System.out.println(list); // [Java, C++, Python]