Understanding Dynamic and Static Typing in Object-Oriented Programming

1. Object Oriented Programming 236703 Spring 2007 Types

2. Values 5 Color.RED "abc" new String[20] new Scanner("c:/temp/in.txt")‏ new Properties()‏

3. Supported Operations int x = 5; Color.RED == null "abc".equalsIgnoreCase("ABC")‏ (new String[20])[0] new Scanner("c:/temp/in.txt").next()‏ new Properites(0).getProperty("a")‏

4. Unsupported Operations int x = 5.9; Color.RED.equals(5,5)‏ "abc".remove(0)‏ (new String[20])[0][0] = "xyz"; new Scanner("c:/temp/in.txt") = null Integer.parseInt(Properites(0))‏

5. Dynamic Typing • The dynamic typing approach: • Compile-time: Allow all operations • Run-time: Before carrying out an operation, check that it is supported by the value • Requirement: • A run-time data structure that determines the “support” question • Consequences: • Assignments to variables never fail • No need to define types (of variables, return types, etc.)‏ • Flexibility • Run-time errors if we are not careful

6. Static Typing • The static typing approach: • Compile-time: Check that operations are correct • Run-time: Just carry out the operation • The idea: The compiler “simulates” the evaluation of expressions • Compile-time computation of the type of the expression • Corresponds to the run-time computation of the result of the expression • If a type cannot be computed => Compilation error • The computed type of subexpressions is used in computing the type of bigger expressions

7. Static Typing (cont.)‏ • Requirements • Variables have types • Methods have return types • Assignments are checked by the compiler • Message sends are checked by the compiler • Consequences • More type errors are detected at compile-time • Programmer must type more text • Not as flexible: “You can't always do what you want” • Invocation of an action is faster • But this factor becomes less and less important...

8. Supported? Yes/No?DT = Dynamic Typing; ST = Static Typing interface List { void add(Object o); } class ArrayList implements List { void add(Object o) { ... } } class LinkedList implements List { void addFirst(Object o) { ... } void add(Object o) { ... } } LinkedList w = new LinkedList(); w.addFirst("1"); // DT ST List x = new LinkedList(); x.addFirst("1"); // DT ST List y = new ArrayList(); y.addFirst("1"); // DT ST LinkedList z = new ArrayList(); // ST z.addFirst("1"); // DT

9. What is a Type? • Theoretically a type is defined as a set of values • A more practical definition: A function that maps a value to either “yes”/”no” • The type is the set of all sources that are mapped to “yes” • The compiler usually represents types as a directed graph • Two types of nodes: Messages, Types • Edges are labeled (ordinal numbers)‏ • A type node has outgoing edges to all messages it can receive • A message node has outgoing edges to its return type, types of its parameters

10. Type Checking: Statically Typed Language • Occurs when the compiler encounters an assignment • (Parameter passing is actually a sequence of assignments) void f(F f) { R r = f; // <- Type checking ...} • The idea: Compare the set of messages of F, R • In Java: • A subtype shares messages with its supertypes • The sets are compared for inclusion (not equality)‏

11. Subtyping Schemes • Nominal subtyping: A type Y shares messages with X if Y declares X as a super type • A fixed set of super types • Structural conformance: A type Y shares messages with X if the messages have the same signature • One node for all messages of with the same signature • Regardless of where the messages are defined • An unlimited set of super types • This is actually a recursive definition

12. Nominal vs. StructuralN = Nominal; S = Structural public interface R1 { public void a(); } public interface R2 { public void a(); } public interface F extends R1 { public void b(); } void g(F f) { R1 r1 = f; // N: S: R2 r2 = f; // N: S: ... }

13. Typing Holes • Recall: A type system should prevent us from applying an operation to a value that does not support it • Is it really so? public class A { void f() { ... } } .. void g(A a) { a.f(); } • What values (of the variable a) will not support the operation a.f() ?

14. Typing Strength • Strong typing: • An unbreakable association between a value and its type • Weak typing: • One can change the value-type association • Modern languages: Strong typing • With either static or dynamic type checking