Early programming languages ca. 1960

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1 Session 5: Intro. to Java collections History Collection / container concept Shortcoming of original version Parameterized collections Example: ArrayList Comp 271, Spring, 2012 Mr. Weisert Early programming languages ca Fortran, Algol, & others supported arrays of 1,2, or 3 dimensions, used mostly for vector and matrix manipulation in scientific programs. Cobol supported similar arrays, but called them tables, used mostly for maintaining relationships in business data processing. Lisp supported linked list structures from which one could build sophisticated data structures, used mostly for artificial intelligence applications. Drawbacks of the early array support The program could address elements by subscript, but not the whole array. To add two vectors you had to write out a loop. To multiply two matrices you needed a nested loop. APL, PL/I, et al (ca. 1966) introduced operations on same-dimensioned arrays themselves (corresponding elements). That capability is not supported by the C family of languages, but you can build it in C++, C#, or (awkwardly) Java. Arrays in C To keep the language small, C's designers built arrays on top of the notion of pointers. You could do arithmetic on pointers, so that if P is a pointer to the origin of an array: P[k] is just a more readable way of specifying the element fetched by *(P+k) Ugly, but some C programmers liked it! (Like going back to machine language.) COMP 271.Spring, copyright 2010 Conrad Weisert

2 Serious shortcomings of C arrays No bounds checking! Program can fetch and store into any part of the program's memory space. Avoidable in C++ or C# through well-designed classes.) Java enforces bounds checking. No way for a function to discover the size of its array argument! Usual bypass was to pass a separate parameter giving the array size. Error prone! Features of Java's built-in arrays Out of bounds subscripts are always checked: ArrayIndexOutOfBounds exception is thrown. Two ways to create new operator with explicit dimension, or initializer list Dimension can be retrieved as arrayname.length So, no need to pass size to a function Don't confuse with String length(). Array restrictions in C, C++, and Java Always have 0-origin Only one-dimensional 2-dimensional (matrix) can be created (awkwardly) as an array of arrays. similarly for higher dimensionality. But C++ and Java let us create array-like classes that are more flexible. Generalizing the concept Once we have the ability to define a class, we can provide a wider range of container / collection facilities that: Support a variety of data structures, some dynamic. Provide consistent interfaces to user programs. Can be tuned for efficiency and safety. C++ launched its standard template library In Java you can define your own container class, or use one from Java's standard collections library COMP 271.Spring, copyright 2010 Conrad Weisert

3 Early Java container classes Java got off to a weak start with typeless collections. User programs had to cast every retrieved element to its proper class! You could store only references to Object! After you retrieved an item, you had to cast it to the type you knew it was! Example: member = (Person) roster.get(k); where roster is collection, such as an ArrayList Tedious and error-prone. Java eventually gets it right! Parameterized collections New with Java 1.5 Eliminates the need to cast retrieved elements. The collection knows what type they are. How does it know? When the program declares and creates a collection, it specifies the type of elements that will be stored in it: ArrayList<String> titles = new ArrayList<String>(200) Important restriction The type of element stored in a parameterized collection may not be a Java primitive type, such as int. What is actually stored is always a reference to an object. Let's start by examining ArrayList A Java library collection class providing similar functionality to Java's built-in arrays additional flexibility What's wrong with using Java's built-in arrays Nothing! In fact: They can store not only references to objects, but also Java primitive data. They're usually more efficient than ArrayList, provided that: 1. The array will stay the same size once created. 2. The programmer will never want to switch to a different kind of container, such as a linked list. Java's built-in arrays are safer and easier to use than the equivalent C++ facility. But ArrayList is more flexible. COMP 271.Spring, copyright 2010 Conrad Weisert

4 ArrayList derivation Class hierarchy Interfaces Object AbstractCollection Abstract List ArrayList Collection List Why do we need to know this? ArrayList methods inherited from List Let T be the type of item to be stored in the list. x be an item of type T k be the position in the list of the kth item (like an array subscript) Then: get(k) set(k,x) add(x) add(k,x) returns the kth item stores x as the kth item appends x to the end of the list inserts x after the kth item remove(k) deletes the element at position k and closes the gap More! See a Java manual. Some notes on those methods..a.get(k).. is the equivalent of ordinary array subscripting in an expression..a[k].. a.set(k,x) is the equivalent of subscripting in an assignment a[k] = x; add was a poor choice of name. They should have called it append or insert. Why? ArrayList constructors 1. ArrayList<T>(int n) Allocates space with initial capacity to hold n references to objects of type T 2. ArrayList<T>(ArrayList<T> x) Allocates space to hold the content of x and initializes the new content to be a copy of those references. Note: the actual data referred to are not copied; this is a shallow copy. 3. ArrayList<T>() Allocates default size (currently 10) (Avoid this one unless you're sure the list will be tiny.) COMP 271.Spring, copyright 2010 Conrad Weisert

5 Managing ArrayList size An existing ArrayList is reallocated whenever: user-program invokes add(x) when the capacity is already full. user-program invokes ensurecapacity(n) when n > current capacity. Why would we do that? If the program knows that no more elements will be added and there's already unused capacity, it can get rid of the wasted space by invoking: trimtosize() Why would we do that? A smart index/subscript operator: the iterator No, they don't iterate (that's what for and while loops do) An iterator in C ++ or Java Provides access to an element of a collection (like a C++ subscript or a Java get) Simplifies ordered traversals of a collection The explanation in our textbook ( ) is a little confusing. Read it carefully, more than once. Hint: Concentrate on the bottom half of p. 144 Ask about anything you don't understand Creating an iterator object for an existing collection ArrayList<String> tbl = new ArrayList<String>(100);.. Iterator<String> tblx = tbl.iterator(); The new ArrayList iterator object Function in the ArrayList class that returns an iterator for that class. Navigating through a collection using an iterator while (tblx.hasnext()) {String item = tblx.next(); or whatever the contained type is <do something with item> } returns the item and advances the position COMP 271.Spring, copyright 2010 Conrad Weisert

6 Avoiding iterators The recently added form of the for statement defines an iteration loop over all the elements of any kind of collection: for (T x : a) {<code to do something with each x> } Read "for each x in a" a is a collection of items of type T x is a bound loop variable Example of ArrayList use Our textbook shows a reasonable example in section (pp. 244 ff) However well need to simplify, clarify, and otherwise tidy up the code. How? Why? What's wrong with it? The other example (section 6.3) is less reasonable and rather poorly motivated. Read it and understand it, but don't try to duplicate or extend it, unless it interests you. Lab exercise (your choice) Easy exercise: Do the Book exercise presented Tuesday Harder (longer) exercise: Revise the ArrayListExample program (Collins pp ) to be simpler, more flexible, easier to use, and more reliable. We shall discuss both February 14 COMP 271.Spring, copyright 2010 Conrad Weisert

Week 2: The Clojure Language. Background Basic structure A few of the most useful facilities. A modernized Lisp. An insider's opinion

Week 2: The Clojure Language. Background Basic structure A few of the most useful facilities. A modernized Lisp. An insider's opinion Week 2: The Clojure Language Background Basic structure A few of the most useful facilities A modernized Lisp Review of Lisp's origins and development Why did Lisp need to be modernized? Relationship to