Types in programming languages What are types, and why do we need them? Types in programming languages1

Definition: Type Type: A collection of values bool: true, false int: -4, 3, 6, 2, etc. Data type: Type + operations to manipulate the type Bool + {&&, ||, !} Int + {+, -, *, etc. } Types in programming languages2

Definition: Data item Data belong to a type 34 belongs to int False belongs to bool Simple data item No subparts Examples: 34 and false Aggregate data item Has sub-parts Classes aggregate other types Types in programming languages3

Abstract Data Type (ADT) Abstract data type Defines a type in terms of type + operations Focus on what you can do with data items, not how it is done Can be programmed using interfaces Data type Implementation of the ADT Focus on how Programmed using a class Example: Collections ADT: IList Data type: List and LinkedList Types in programming languages4

Why do we need types? A variable is just a name / alias of a memory location (one or more bytes in memory) Using types we can have rules saying which operations can legally be applied to variables (memory locations) And which operations to disallow Example: String str1, str2; String str = str1 + str2; String s = str1 – str2; // illegal Types in programming languages5

Strong vs. weak typing Strong typing Variables, expressions, etc. have types Types must “match” Languages: C#, Java, and many other programming languages Weak typing No types Variables are just aliases for memory locations Languages: Assembly, BASIC, and many other languages When strong typing was introduced (late 1960’es) programmers used to say “Strong typing is for programmers with weak minds” Meaning: A programmer should be able to remember the “types” of variables himself. Hungarian notation: iVar is an integer, sName is a string, etc. Types in programming languages6

Static vs. dynamic type checking Static type checking Types of variables, expressions etc. are checked at compile-time C#, Java etc. used static type checking Dynamic checking Type of variables, expressions etc. are checked at runtime. C# when you use type casts, checking is deferred to run-time: You might get a InvalidCastException Check as mush as possible at compile-time An error message from the compiler to the programmer is much better than an error message form the program to the end-user Types in programming languages7

Types in object-oriented programming In object-oriented programming the programmer creates his own types, called classes. From theses classes we make variables, called objects Types are organized in an inheritance hierarchy C#, Java: A single class hierarch rooted in the class Object C++: More hierarchies, not single root Types in programming languages8

Subtypes an substitution Whenever you have an object of a super type you should be able to substitute that object with an object of a subtype Example IList MyList; MyList can be List, LinkedList etc. MyList.Add(“Anders”); Works no matter whether it is List or LinkedList Types in programming languages9

Subtypes and substitution (2) Class S { virtual B method(A a) { … } } Class T : S { override Y method(X x) { … } } Requirements Parameters A must be a subtype of X Return types Y must be a subtype of B Called co-variant return types These requirements are not handled well in many programming languages, like C# Types in programming languages10

Method overriding vs. method overloading Overridden methods same signature in super-type as in subtype Overloaded methods Same name but different parameters Some C# keywords Virtual, used on base-class methods The method can be overridden in sub-classes Override, used on sub-class methods The method overrides a method from the base-class New, used on sub-class methods The method is not overridden. Instead the sub-class has is own (new) version of the method. Sealed, used on sub-class methods The method was virtual in a base-class, but cannot be overridden in sub-sub-classes. Example: MethodOverriding Types in programming languages11