Casting variables in Java

Question

I wonder if anyone could tell me how casting works  I understand when I should do it  but not really how it works  On primitive data types I understand partially but when it comes to casting objects I don t understand how it works   How can a object with the type Object just suddenly be cast to  let s say  MyType  just an example  and then get all the methods

User · Answer

Casting a reference will only work if it's an instanceof that type. You can't cast random references. Also, you need to read more on Casting Objects.

e.g.

String string = "String";

Object object = string; // Perfectly fine since String is an Object

String newString = (String)object; // This only works because the `reference` object is pointing to a valid String object.

User · Answer

Casting in Java isn t magic  it s you telling the compiler that an Object of type A is actually of more specific type B  and thus gaining access to all the methods on B that you wouldn t have had otherwise  You re not performing any kind of magic or conversion when performing casting  you re essentially telling the compiler  trust me  I know what I m doing and I can guarantee you that this Object at this line is actually an  lt Insert cast type here gt    For example   Object o    str   String str    String o    The above is fine  not magic and all well  The object being stored in o is actually a string  and therefore we can cast to a string without any problems   There s two ways this could go wrong  Firstly  if you re casting between two types in completely different inheritance hierarchies then the compiler will know you re being silly and stop you   String o    str   Integer str    Integer o    Compilation fails here   Secondly  if they re in the same hierarchy but still an invalid cast then a ClassCastException will be thrown at runtime   Number o   new Integer 5   Double n    Double o    ClassCastException thrown here   This essentially means that you ve violated the compiler s trust  You ve told it you can guarantee the object is of a particular type  and it s not   Why do you need casting  Well  to start with you only need it when going from a more general type to a more specific type  For instance  Integer inherits from Number  so if you want to store an Integer as a Number then that s ok  since all Integers are Numbers   However  if you want to go the other way round you need a cast - not all Numbers are Integers  as well as Integer we have Double  Float  Byte  Long  etc   And even if there s just one subclass in your project or the JDK  someone could easily create another and distribute that  so you ve no guarantee even if you think it s a single  obvious choice   Regarding use for casting  you still see the need for it in some libraries  Pre Java-5 it was used heavily in collections and various other classes  since all collections worked on adding objects and then casting the result that you got back out the collection  However  with the advent of generics much of the use for casting has gone away - it has been replaced by generics which provide a much safer alternative  without the potential for ClassCastExceptions  in fact if you use generics cleanly and it compiles with no warnings  you have a guarantee that you ll never get a ClassCastException

User · Answer

Suppose you wanted to cast a String to a File  yes it does not make any sense   you cannot cast it directly because the File class is not a child and not a parent of the String class  and the compiler complains    But you could cast your String to Object  because a String is an Object  Object is parent   Then you could cast this object to a File  because a File is an Object   So all you operations are  legal  from a typing point of view at compile time  but it does not mean that it will work at runtime    File f    File  Object   Stupid cast     The compiler will allow this even if it does not make sense  but it will crash at runtime with this exception   Exception in thread  main  java lang ClassCastException      java lang String cannot be cast to java io File

User · Answer

The right way is this   Integer i   Integer class cast obj     The method cast   is a much safer alternative to compile-time casting

User · Answer

Actually  casting doesn t always work  If the object is not an instanceof the class you re casting it to you will get a ClassCastException at runtime

[java] Casting variables in Java

Examples related to java

Examples related to casting