ArrayList is reinitialising when a method is called

From a quick review of that block :

while(a!=0){
... 
else if(a==2)
{d=new User_Interface2();
d.showData();}
else if(a==3)
{d=new User_Interface2();
d.main();}
}

You are creating a new instance on each iteration.
Create d outside the loop and reuse that instance.

d=new User_Interface2();
while(a!=0){
    ...
    else if(a==2) {
        d.showData();
    } else if(a==3) {
        d.main();
    }
}