i have a problem . I'm trying to add and them retrieve objects of the same class to ArrayList , but when i try to retrieve them i get incompatible types erroe
import java.util.*;
public class Test{
Test(){
main();
}
List test = new ArrayList();
public void main(){
test.add(new Square(10));
Iterator i = test.iterator();
while(i.hasNext()){
Square temp = i.next();
}
}
}
Yes - you're not using generics anywhere, so i.next(); is just returning Object as far as the compiler is concerned.
Options:
Cast to Square:
Square temp = (Square) i.next();
Personally I'd favour using generics - it's much nicer to have type safety where you can :)
// Not clear why this is an instance variable - and please make fields private :)
List<Square> test = new ArrayList<Square>();
public void main() {
test.add(new Square(10));
Iterator<Square> i = test.iterator();
while(i.hasNext()) {
Square temp = i.next();
// Use square here
}
}
Two asides:
The enhanced for loop can replace your while loop:
for (Square square : test)
It's odd to have an instance method called main with no parameters; it's more common to have public static void main(String[] args). There's nothing illegal about what you've done here, just odd.
You should use the generic type List<Square>, ArrayList<Square> and Iterator<Square> to avoid this.
If you don't the compiler cannot "know" what is the actual types of elements in test, and it "thinks" them as of type Object.
List<Square> test = new ArrayList<Square>();
public void main(){
test.add(new Square(10));
Iterator<Square> i = test.iterator();
while(i.hasNext()){
Square temp = i.next();
}
}
You have to cast to Square:
while(i.hasNext()){
Square temp = (Square) i.next();
}
The iterator will return Object type elements only.
With Java 1.5+ and generics, we can do it without casting:
// a list, that will only accept Square instances
List<Square> test = new ArrayList<Square>();
public void main(){
test.add(new Square(10));
Iterator<Square> i = test.iterator();
while(i.hasNext()){
Square temp = i.next();
}
}
Object type objects only and that we need to cast to solve incompatibel type issues.