What's the difference between java.util.Map.putAll(Map<>) and java.util.Map.put(Integer, Object) in a loop

Question

In the code below, I found that using the putAll method can cause problems if we pass the map in parameter

public class Main {

    public static void main(String...strings ) {
        Etudiant e1=new  Etudiant(5, "A");
        Etudiant e2=new  Etudiant(6, "B");

        Map<Integer, Etudiant> map= new HashMap<>();
        map.put(1, e1);
        map.put(2, e2);

        Map<Integer, Etudiant> map2= new HashMap<>();
        map2.put(1,map.get(1));
        map2.put(1,map.get(2));

        changeMe(map2);
        System.out.println(map.get(1));

        Map<Integer, Etudiant> map3= new HashMap<>();
        map3.putAll(map);
        changeMe(map3);
        System.out.println(map.get(1));
    }

    private static void changeMe(Map<Integer, Etudiant> etudiants) {
        etudiants.get(1).name="K";
    }
}
}

Here is the ouput result:

Etudiant [age=5, name=A]
Etudiant [age=5, name=K]

Could you explain the difference ?

Why after the use of putAll the object changes ?

Answer 1

Your code explained in detail

Etudiant e1=new  Etudiant(5, "A");
Etudiant e2=new  Etudiant(6, "B");

Map<Integer, Etudiant> map= new HashMap<>();
map.put(1, e1);
map.put(2, e2);

map now contains {1=Etudiant(5, "A"), 2=Etudiant(6, "B")}

Map<Integer, Etudiant> map2= new HashMap<>();
map2.put(1,map.get(1));
map2.put(1,map.get(2));

map2 now contains {1=Etudiant(6, "B")}

changeMe(map2);
System.out.println(map.get(1));

Etudiant(6, "B") has been renamed Etudiant(6, "K"), so:
map now contains {1=Etudiant(5, "A"), 2=Etudiant(6, "K")}
map2 now contains {1=Etudiant(6, "K")}
and it printed:

Etudiant(5, "A")

Map<Integer, Etudiant> map3= new HashMap<>();
map3.putAll(map);

map3 content is a copy of map content, so:
map3 now contains {1=Etudiant(5, "A"), 2=Etudiant(6, "K")}

changeMe(map3);
System.out.println(map.get(1));

Etudiant(5, "A") has been renamed Etudiant(5, "K"), so:
map now contains {1=Etudiant(5, "K"), 2=Etudiant(6, "K")}
map2 now contains {1=Etudiant(6, "K")}
map3 now contains {1=Etudiant(5, "K"), 2=Etudiant(6, "K")}
and it printed:

Etudiant(5, "K")

The code works exactly the way you coded it to do.

---

All the above can easily be seen by adding a bunch of print statements, which is one way to debug your code.

public class Test {
    public static void main(String[] args) {
        Etudiant e1=new  Etudiant(5, "A");
        Etudiant e2=new  Etudiant(6, "B");

        Map<Integer, Etudiant> map= new HashMap<>();
        map.put(1, e1);
        map.put(2, e2);
        System.out.println("map:  " + map);

        Map<Integer, Etudiant> map2= new HashMap<>();
        map2.put(1,map.get(1));
        map2.put(1,map.get(2));
        System.out.println("map2: " + map2);

        changeMe(map2);
        System.out.println("map:  " + map);
        System.out.println("map2: " + map2);
        System.out.println(map.get(1));

        Map<Integer, Etudiant> map3= new HashMap<>();
        map3.putAll(map);
        System.out.println("map3: " + map3);

        changeMe(map3);
        System.out.println("map:  " + map);
        System.out.println("map2: " + map2);
        System.out.println("map3: " + map3);
        System.out.println(map.get(1));
    }
    private static void changeMe(Map<Integer, Etudiant> etudiants) {
        System.out.print("Renamed " + etudiants.get(1));
        etudiants.get(1).name="K";
        System.out.println(" to " + etudiants.get(1));
    }
}
class Etudiant {
    int id;
    String name;
    Etudiant(int id, String name) {
        this.id = id;
        this.name = name;
    }
    @Override
    public String toString() {
        return "Etudiant(" + this.id + ", \"" + this.name + "\")";
    }
}

Output

map:  {1=Etudiant(5, "A"), 2=Etudiant(6, "B")}
map2: {1=Etudiant(6, "B")}
Renamed Etudiant(6, "B") to Etudiant(6, "K")
map:  {1=Etudiant(5, "A"), 2=Etudiant(6, "K")}
map2: {1=Etudiant(6, "K")}
Etudiant(5, "A")
map3: {1=Etudiant(5, "A"), 2=Etudiant(6, "K")}
Renamed Etudiant(5, "A") to Etudiant(5, "K")
map:  {1=Etudiant(5, "K"), 2=Etudiant(6, "K")}
map2: {1=Etudiant(6, "K")}
map3: {1=Etudiant(5, "K"), 2=Etudiant(6, "K")}
Etudiant(5, "K")