Reflection , java , iterate on collection

I'm writing a function that will iterate on some collection , I don't know what collection I'm getting , so I wrote this till now

void PrintFieldsOfClass(Object obj){
    if(obj ==null){
        return;
    }
    Class<?> mainClass = obj.getClass(); // create instance of the class
    Class<?>[] interfaces =null;
    Object collection = null;
    int isCollection = 0; //1 - for List , 2 -  For Set , 3 - Map
    if(mainClass !=null ){
        interfaces = mainClass.getInterfaces();
    }
    try {
        if(interfaces!=null ){
            for(Class<?> interface1 : interfaces ){



                if(interface1.getName().toString().equals("java.util.List"))                     {
                    isCollection = 1;
                    collection = new ArrayList<Object>();
                }
                else            if(interface1.getName().toString().equals("java.util.Set")){
                    isCollection=2;
                    collection = new HashSet<Object>();
                }
                else if(interface1.getName().toString().equals("java.util.Map")){
                    isCollection=3;
                    collection = new HashMap<Object,Object>();
                }
            }
        }

`

After I know what collection I got , how do I cast the object to that collection ? How do I iterate over it ? Thanks in advance !