How to destroy instance - navigation

Question

how to destroy instance ?
eg
class screen1
runprgm() {
screen2 s2 = new screen2()
class screen2
frame.removeall();
//do all database work
//load all components
frame.add (components);
button_click() {
screen3 s3 = new screen3();
now screen2 is loaded..
class screen3
frame.removeall();
//do all database work
//load all components
frame.add (components)
now on the button_click on screen2 to screen3 i want to remove the memory occupied by screen1. ?
one more question
when there is a back button, what is the normal way of programming (create an instance again) or any other caching mechanism is there ?

Answer 1

to free the memory you got to kill the reference to the object
screen1 s1  = new screen1();   //memory is allocated to the object s1
s1 = null;  //the reference to the object s1 is killedfrom time to time the garbage collector will show in and release all the memory that stores object without any references.
Of course you can call the garbage collector yourself if you like that better.
For Your question with the 'back button'. I would say: it depends...
If you got enough free memory just keep the objects you want to have in your history. If you don't have enough memory... well it seems like you have to think about something else to store your data: database, filesystem, (have a look at the Object.toString() method), compress your data,...
hope this helps