public class Transponder {
    public String ID;
    public String someProperty;
}

public class Channel {
    public String ID;
    public Transponder transponder;
}

public class Main {
    private Map<String, Transponder> transponders;
    private Map<String, Channel> channels;

    public Main(){
        transponders = new HashMap<String, Transponder>();
        channels = new HashMap<String, Channel>();

        Transponder t1 = new Transponder();
        t1.ID = "12345";
        t1.someProperty = "something";

        transponders.put(t1.ID, t1);

        Channel c1 = new Channel();
        c1.ID = "42";
        c1.transponder = t1;

        channels.put(c1.ID, c1);
    }
}