Monday, June 16, 2014

Restrictions on Generics (The Java(tm) Tutorials



Cannot Instantiate Generic Types with Primitive Types

Generics in Java are an entirely compile-time construct - the compiler turns all generic uses into casts to the right type. This is to maintain backwards compatibility with previous JVM runtimes.
This:
List<ClassA> list = new ArrayList<ClassA>();
list.add(new ClassA());
ClassA a = list.get(0);
gets turned into (roughly):
List list = new ArrayList();
list.add(new ClassA());
ClassA a = (ClassA)list.get(0);
So, anything that is used as generics has to be convertable to Object (in this example get(0) returns an Object), and the primitive types aren't. So they can't be used in generics.
C# is a separate matter - generics are implemented directly as part of the runtime, so primitive types can be used - the CLR generates new versions of generic classes for primitives and structs as they are used. The only disadvantage is (until .NET 4) no generic covariance or contravariance was allowed, unlike Java (see the superand extends keywords in generic definitions)

Cannot Create Instances of Type Parameters

Cannot Declare Static Fields Whose Types are Type Parameters

A class's static field is a class-level variable shared by all non-static objects of the class. Hence, static fields of type parameters are not allowed.

Cannot Use Casts or instanceof with Parameterized Types

Because the Java compiler erases all type parameters in generic code, you cannot verify which parameterized type for a generic type is being used at runtime:

Cannot Create Arrays of Parameterized Types


As mark said, the types are not reifiable, which is a problem in the following case:
try {
   doSomeStuff();
} catch (SomeException<Integer> e) {
   // ignore that
} catch (SomeException<String> e) {
   crashAndBurn()
}
Both SomeException<Integer> and SomeException<String> are erased to the same type, there is no way for the JVM to distinguish the exception instances, and therefore no way to tell which catch block should be executed.

If arrays of parameterized lists were allowed, the previous code would fail to throw the desired ArrayStoreException.

Cannot Overload a Method Where the Formal Parameter Types of Each Overload Erase to the Same Raw Type

A class cannot have two overloaded methods that will have the same signature after type erasure.
The overloads would all share the same classfile representation and will generate a compile-time error.

Labels

Review (572) System Design (334) System Design - Review (198) Java (189) Coding (75) Interview-System Design (65) Interview (63) Book Notes (59) Coding - Review (59) to-do (45) Linux (43) Knowledge (39) Interview-Java (35) Knowledge - Review (32) Database (31) Design Patterns (31) Big Data (29) Product Architecture (28) MultiThread (27) Soft Skills (27) Concurrency (26) Cracking Code Interview (26) Miscs (25) Distributed (24) OOD Design (24) Google (23) Career (22) Interview - Review (21) Java - Code (21) Operating System (21) Interview Q&A (20) System Design - Practice (20) Tips (19) Algorithm (17) Company - Facebook (17) Security (17) How to Ace Interview (16) Brain Teaser (14) Linux - Shell (14) Redis (14) Testing (14) Tools (14) Code Quality (13) Search (13) Spark (13) Spring (13) Company - LinkedIn (12) How to (12) Interview-Database (12) Interview-Operating System (12) Solr (12) Architecture Principles (11) Resource (10) Amazon (9) Cache (9) Git (9) Interview - MultiThread (9) Scalability (9) Trouble Shooting (9) Web Dev (9) Architecture Model (8) Better Programmer (8) Cassandra (8) Company - Uber (8) Java67 (8) Math (8) OO Design principles (8) SOLID (8) Design (7) Interview Corner (7) JVM (7) Java Basics (7) Kafka (7) Mac (7) Machine Learning (7) NoSQL (7) C++ (6) Chrome (6) File System (6) Highscalability (6) How to Better (6) Network (6) Restful (6) CareerCup (5) Code Review (5) Hash (5) How to Interview (5) JDK Source Code (5) JavaScript (5) Leetcode (5) Must Known (5) Python (5)

Popular Posts