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

Types in programming languages2 Type definitions Type: A collection of values –boolean: true, false –int: -4, 3, 14593, and much more Data type: Type + operations to manipulate the type –boolean + {&&, ||, !} –int + {+, -, *, etc.}

Types in programming languages3 Definition: Date item Data items –Data, belongs to a type –34, belongs to int –false belongs boolean Simple data item –No subparts Aggregate data item –Has subparts –Classes aggregate other types

Types in programming languages4 Abstract data type (ADT) ADT: Defines a data type in terms of type + operations –Focus on what, not how. –Java: Interface, or public parts of a class Data structure –Physical implementation of an ADT –Focus on how. Example: Java collections –ADT: List –Data structures: ArrayList, LinkedList, etc.

Types in programming languages5 Why do we need types? A data item is just a name/alias/mnemonic of a memory location, i.e. one or more bytes in memory. Using types we can have rules saying which operations can legally be applied to variables (i.e. memory locations). –And which operations to disallow –Example: If 2 variables (memory locations) is declared String it may not make sense to perform the operation ‘-’ (minus).

Types in programming languages6 Strong vs. weak typing Strong typing –Variables, expressions, etc. have types. –Types must “match”. –Languages: Java, C#, and many other languages Weak typing –No types. –Variables are just aliases for memory locations (i.e. bytes) –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 the variables / memory locations himself.

Types in programming languages7 Static vs. dynamic type checking Static type checking –Types of variables, expressions, etc. checked at compile-time. –Java: Uses static type checking Dynamic checking –Types of variables, expressions, etc. checked at run-time. –Java: When you use type casts, checking is deferred to run-time. If something is wrong you get a ClassCastException. Goal: Check as much as possible at compile-time –An error message from the compiler to the programmer is much better than an error message from the program to the end-user. –Java: The main reason for introducing generics in Java 5.0

Types in programming languages8 Types in object-oriented programming In object-oriented programming the programmer creates his own types, called classes. From these classes we make variables, called objects. Types are organized in inheritance hierarchies. –Java (and C#): One single class hierarchy rooted in class Object. –Other language (like C++): More hierarchies, no single root.

Types in programming languages9 Subtypes and substitution Whenever you have an object of a super type you should be able to substitute that object with an object of a subtype. Java example –List mylist; // mylist can be an object of ArrayList, LinkedList –mylist.add(“Anders”) Works no matter is myList is an ArrayList or LinkedList

Types in programming languages10 Subtypes and substitution (2) class S { –B method(A a) { … } } class T extends S { –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 –Java Requires that A is the same type as X –Otherwise Java assumes we want overloading, not overriding. Allows Y to be a subtype of B. –New in Java 5.0

Types in programming languages11 Method overriding in Java Overridden methods must have same signature in subtype as in super type. –Same parameters in same order –Return same type or more specific (subtype). –Throw same or more specific (subtype) exceptions. Exceptions can be considered a kind of “return types” Example: MethodOverriding.java Idea for future Java version: –Parameters of same of more general type. Not considered an overriding in present day Java. Reason: Method overloading (methods with same name, but different parameter types (or number of parameters).

Types in programming languages12 Primitive types in object-oriented programming languages Most programming languages have primitive types like int, double, char, boolean, etc. These types are considered primitive because they are not classes. Java: Primitive types and wrapper classes, like int ~ Integer, boolean ~ Boolean, etc. Java 5.0: Automatic conversion from primitive to wrapper and vice versa (called “auto boxing”)