TestSingleton.java which is imperfect in multi-threading environment

TestSingleton.java

public class Communication {
    static Communication instance;

    private Communication() {
    }

    public static Communication getInstance() {
        if(instance == null)
            instance = new Communication();

        return instance;
    }


    public void send(String message) {
        // sends message
    }
}


public class Main {
    public static void main(String[] args) {
        //


        Communication instance = Communication.getInstance();
        Communication communication = Communication.getInstance();

        instance.send("hi");
        communication.send("hello car!");
        Communication.getInstance().send("hello");
        Communication.getInstance().send("world");
        // handle two responses

        //
        MainSub1 mainSub1 = new MainSub1(Communication.getInstance());
        mainSub1.setCommunication(communication);
        mainSub1.does();
        mainSub1.does1();
    }
}


public class MainSub1 {
    Communication communication;


    public MainSub1(Communication communication) {
        this.communication = communication;
    }

    public void setCommunication(Communication communication) {
        this.communication = communication;
    }

    void does() {
        communication.send("hi");
    }

    void does1() {
        Communication.getInstance().send("hi");
    }

    public static void main(String[] args) {

    }
}


public class MainSub2 {
}