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Types in programming languages

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

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Types in programming languages

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

  2. 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 languages

  3. 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 languages

  4. 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 languages

  5. Why do we need types? • A variable 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 languages

  6. 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 languages

  7. 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 languages

  8. 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 languages

  9. 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 languages

  10. 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 Subtypes and substitution (2) Types in programming languages

  11. 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 languages

  12. 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”) Types in programming languages

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