View All Java Casting Operations

sir, i got alot confusion in casting operations please provide me some notes and also here is one prog in which how should we do casting
public class main{
public static void main(String[] args){
Test t = new Test();
String s1 = new String();
String s2 = new String();

s1 =t.a;
s2 =t.b;

if (s1==s2){
System.out.println(“equal memory”);
}
else{
System.out.println(“diffent memory”);
}
}

}

in another java file ****************
public class Test{
int a= 20;
int b=20;
}