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Inheritance II - Casting ​

Inheritance I - Interface and Implementation Inheritance

Compile-Time Type Checking and Expressions ​

Based on compile-time types:

  • Method calls
  • Assignments

Compile-Time Assignments ​

An expression using the new keyword has the specified compile-time type. Example:

java
SLList<Integer> sl = new VengefulSLList<Integer>();
  • Compile-time type of right hand side (RHS) expression is VengefulSLList.
  • A VengefulSLList is-an SLList, so assignment is allowed.
java
VengefulSLList<Integer> vsl = new SLList<Integer>();
  • Compile-time type of RHS expression is SLList.
  • An SLList is not necessarily a VengefulSLList, so compilation error results.

Compile-Time Method Calls ​

Method calls have compile-time type equal to their declared type.

java
public static Dog maxDog(Dog d1, Dog d2) { … }
  • Any call to maxDog will have compile-time type Dog!
java
Poodle frank  = new Poodle("Frank", 5);
Poodle frankJr = new Poodle("Frank Jr.", 15);

Poodle largerPoodle = maxDog(frank, frankJr);
  • Compilation error! RHS has compile-time type Dog

(Static) Casting ​

Java has a special syntax for specifying the compile-time type of any expression.

  • Put desired type in parenthesis before the expression.
  • Tells compiler to pretend it sees a particular type.
java
Poodle largerPoodle = (Poodle) maxDog(frank, frankJr);
// compiles! Right hand side has compile-time type Poodle after casting

Casting is a powerful but dangerous tool.

  • Tells Java to treat an expression as having a different compile-time type.
  • Ignore type error at compile time but might cause runtime error.
  • Does not actually change anything: sunglasses don’t make the world dark.
java
Poodle frank = new Poodle("Frank", 5);
Malamute frankSr = new Malamute("Frank Sr.", 100);

Poodle largerPoodle = (Poodle) maxDog(frank, frankSr); // runtime exception!
// This return s a Malamute
// Can't be casted into Poodle