Java rules for casting

Question

When can a certain object be cast into another object? Does the casted object have to be a subtype of the other object? I\'m trying to figure out the rules...

Answer 1

Suppose we want to cast d object to A,

A a = (C)d;

So internally 3 rules have been checked by Compiler and JVM. The compiler is checking first 2 rules at Compile time and JVM will check last one rule at Runtime.

Rule 1 (Compile time checking):

Type of 'd' and C must have some relation (child to parent or parent to child or same time).If there is no relationship then we will get a compile error(inconvertible types).

Rule 2 (Compile time checking):

'C' must be either same type or derived type(subclass) of 'A' otherwise we will get a compile error(incompatible types).

Rule 3 (Runtime Exception):

Runtime object type of 'd' must be same or derived a type of 'C' otherwise we will get a runtime exception (ClassCastException Exception).

Find following examples to get more idea,

String s = new String("hello"); StringBuffer sb = (StringBuffer)s;  // Compile error : Invertible types because there is no relationship between.

Object o = new String("hello"); StringBuffer sb = (String)o;       // Compile error : Incompatible types because String is not child class of StringBuffer.

Object o = new String("hello"); StringBuffer sb = (StringBuffer)o; // Runtime Exception : ClassCastException because 'o' is string type and trying to cast into StingBuffer and there is no relationship between String and StringBuffer.