java - difference between 2 types of downcasting -

public class {     main() {         // different implications of these 2 down casts          superclass trefvar = new subclass();          // down cast example 1         subclass avar = (subclass) trefvar;            // down cast example 2         ((subclass) trefvar).somemethodinsubclass();       } } 

implications, wise, differences between example 1 cast, , example 2 cast?

there virtually no difference. first example creates new local variable, , second example doesn't. that's it.

to peek under hood bit , verify that, let's consider simple class, 2 methods you're doing in example:

public class downcasts {   public int stringlength1(object o) {     string s = (string) o;     return s.length();   }    public int stringlength2(object o) {     return ((string) o).length();   } } 

the bytecode these methods (which can see javap -c downcasts) are:

public int stringlength1(java.lang.object);   code:      0: aload_1      1: checkcast     #2                  // class java/lang/string      4: astore_2      5: aload_2      6: invokevirtual #3                  // method java/lang/string.length:()i      9: ireturn  public int stringlength2(java.lang.object);   code:      0: aload_1      1: checkcast     #2                  // class java/lang/string      4: invokevirtual #3                  // method java/lang/string.length:()i      7: ireturn 

the first method these things:

• string s = (string) o:
  • loads o stack
  • checks it's string
  • stores register 2
• return s.length():
  • loads register 2 stack (that's 1 saved)
  • invokes string::length (a virtual function) on it
  • returns result

the second method does:

• return ((string) o).length():
  • loads o stack
  • checks it's string
  • invokes string::length (a virtual function) on it
  • returns result