Objects and Collections - General Questions

What is the output of the following program?

import java.util.*;
public class Main {
    public static void main(String[] args) {
        Map<String, Integer> m = new HashMap<>();
        m.put("a", 1);
        m.put("a", 2);
        System.out.println(m.get("a") + " " + m.size());
    }
}

What is the output of the following program?

public class Main {
    public static void main(String[] args) {
        java.util.Set<String> s = new java.util.HashSet<>();
        s.add("a"); s.add("a"); s.add("b");
        System.out.println(s.size());
    }
}

Which collection keeps insertion order and allows duplicates?

What is the output of the following program?

import java.util.*;
public class Main {
    public static void main(String[] args) {
        Map<Character, Integer> freq = new TreeMap<>();
        for (char c : "banana".toCharArray()) freq.merge(c, 1, Integer::sum);
        System.out.println(freq);
    }
}

What is the output of the following program?

public class Main {
    public static void main(String[] args) {
        java.util.Deque<Integer> st = new java.util.ArrayDeque<>();
        st.push(1); st.push(2); st.push(3);
        System.out.println(st.pop() + " " + st.peek());
    }
}