Sedgewick Specialties

Transcription

1 Sedgewick Specialties Our textbook is apparently a follow-up on another CS1083-ish book that uses a lot of simplified libraries, to avoid overwhelming newbies with the standard Java libraries. This book continues using these simplified libraries. You need to be able to read the textbook's code, though I encourage you to write your code with the standard Java libraries Next few slides mentions some of the textbook's library classes.

2 StdRandom (page 30) StdRandom is their alternative to Math.random() and the Java library class Random. Static methods to generate random numbers, uniformly or by other probability distributions.

3 StdStats (p 30) Convenience class with static methods to compute various statistical functions on an array of doubles. max, mean, median, stddev etc.

4 StdOut (p 37) Their alternative to System.out StdOut has static methods for printing to the screen. methods print() and println() are what you expect. method printf() does formatted output. (There is also a System.out.printf() method, but you may not be familiar with it. This approach is borrowed from C; authors appear to like C...)

5 printf() method - p37 Printf takes at least one parameter, a format string. The format string has embedded 0 or more conversion codes that begin with %. %s substitute a string here %d substitute a decimal integer here %10d substitute integer; pad its width to 10 spaces %5.2f pad a float's width to 5 places, with 2 after the decimal point printf() takes extra parameters after the format string to represent the actual values, in same order as format string. StdOut.printf( hi %s, your hourly rate is %5.2f, owen,6.1);

6 StdIn (p 39, 41) StdIn has static methods to get various types from the keyboard. char foo = StdIn.readChar(); Also has methods to tell whether the input has run out (isempty()) And a method to read whole next line (you then parse it) And a readall() method to grab multiple lines (that you probably then split apart) And methods like readalldoubles() to get an array

7 Don't use StdIn Simple input from Java is inconvenient when you use the normal libraries. But do it anyway; you need to get used to the pain sometime... See my web page (or Google) for examples that use the normal Java libraries for input

8 In (p83) Textbook's In class : constructor takes a file name or a Web URL name. Opens the file, after which methods named after StdIn's methods can be used to read from the file. In foo = new In( ); while (! foo.isempty()) StdOut.printf( value is %d\n,foo.readint());

9 Out (p 83) Textbook's Out class has (nonstatic) methods like the static methods in StdOut. Most notable is printf(). Constructor for Out object takes the name of a file into which output will go. Programmer's responsibility to call close() when done

10 Don't use In and Out They're specific to this textbook, not standard things in the normal Java libraries. Instead, see Google or my course webpage for examples of the normal way to do file or URL input and output. They're less complicated because efficiency and error handling have been compromised. Still, I wish they were normal Java...

11 StdDraw (p42) There is a class StdDraw with static methods to do simple graphics drawings methods point(), circle(), square(), filledcircle(), setpencolor(), clear()... These things are otherwise a pain to do in Java Go ahead and use StdDraw rather than the normal Java routines, if you'd like.

12 StdAudio (not in book) There's also a StdAudio class for easily working with sounds. While not described in the book, it is documented on the book's website

13 Whitelisting The concept of whitelisting requires you to have a long white list of items. You're then presented with a another list of items. Print all of them that are not in the whitelist. Book example: whitelist of known valid credit card numbers. Second list is of people presenting credit card numbers (some of which might be bad). Recurring application in textbook.

14 More book ADTs (70, 76-79) Point2D, Interval1D, Interval2D: geometric. Counter: you give it a name, pass it around, call its increment() method. tally() gives the current count, and tostring() gives both its name & count. Accumulator and VisualAccumulator edu.princeton.cs.algs4.date, as opposed to the usual java.util.date

15 Using Packages: Compilation Recall from 1083? Either give the compiler the full name of a class, or use an import statement. Either way, the compiler needs to be able to find out about the class, so it can check your use of methods, etc. Uses a classpath to find out about the used class (either as a standalone source-code file or as a member of a JAR file, a pre-packaged and compressed library) The Maven software tool will work with you to take care of classpath things, in the CS2383 labs and programming assignments.

16 Using Packages: Runtime Once your program is compiled to byte code, when executed, the Java class loader needs to find the compiled byte code of any other packages you used. class loader also uses the class path setting. Again, Maven can help out.

17 Making Packages The classes you write belong to a package. You specify which one with the package keyword. Omit this, and your class belongs to the default package Protection is looser between things in the same package. In CS2383, I'll want you to use a specific package (ca.unbsj.cs2383) for all the code you create in this course. This may not be good software engineering.

18 Assertions Good software engineering to use assertions, checkable statements about the state of the program at a given line. documents something important can help you debug: the world is not really as you supposed. Java keyword is assert. assert j > k; // or assert j > k : foo bar baz ; When Java interpreter runs with -ea argument, if the condition is not met, program will throw an exception java -ea MyClass versus java Myclass

19 Java Execution: the JIT Compiler Our Java system (and others) has a Just-In-Time compiler built in. Execution defaults to slowly interpreting the compiled byte code of a class. But if a portion of code is hot (executed a lot), the system detects it and translates this part's byte codes into the CPU's native instructions. Translation (done once) is slow, but then whenever the hot code runs, it's much faster than before. This messes up our attempts to time algorithm implementations. More later.

20 ADTs, Clients, Implementations (p64) An Abstract Data Type is a data type whose implementation is hidden. The only way to manipulate the ADT is via its public interface. An API for a data type includes constructors and instance methods (including at least a brief informal description of their parameters and effects). Formal methods goal would be to have a precise mathematical description of the effects, remains mostly a dream. Clients use the ADT via its public API. Someone, of course, must implement the API.

21 Wrapper Class (1073/1083 review) Recall that Java has basic types like float, char. It also has reference types like Object and every other class (String, MyNiceClass...) Sometimes you want to do something that only works for objects...with a basic type. Wrapper classes like Float, Char to the rescue! Java auto-boxing feature can silently convert between basic types and wrappers (potentially at a big speed penalty.)

22 Bags and other Collections One fundamental kind of ADT represents a collection of items. Usually you can know how many items you have. What else you can do with the items varies: Bag: put items into it, then loop over them. Stack: Bag + remove the newest item in the bag Queue: Bag + remove the oldest item in the bag Deque: Add or remove from either end. pushleft + popleft or pushright + popright Stack pushleft + popright or pushright + popleft Queue Deques are part of the standard Collections for Java

23 Generics and Collections (Review, maybe??) How to use someone else's generic class, so you don't have to have a separate class to handle lists of Strings, lists of Integers, etc. List<String> v1 = new ArrayList<>(); List<Integer> v2 = new ArrayList<>(); Before Java 7, we didn't have diamond operator <> Before Java 5, we didn't have generics at all, and you'd have to make v1 & v2 ArrayLists (implicitly of Object) and typecast everything that came out of the list. Easy to get it wrong and have ClassCast exceptions at runtime.

24 DIY Generic Class (not review?) In CS2383, we need to make our own generic classes, for others to use. Can also use generics just in individual methods. You write code using a type parameter. Library writers tend to use single capital letters for this (T is really popular), but your textbook likes short capitalized names. Annoying fact: while you usually can treat T like a real class's name, it's illegal to create an array of T. Instead, you're forced to make an array of objects and typecast it.

25 Generic Class Example public class Ex<T extends Object> { // extends Object is optional. But could change to Person... private T onet; private T [] arrayofts; private ArrayList<T> morets; public Ex(T arg) { onet = arg; arrayofts = (T []) new Object[10]; // gets a warning, annotation if this bugs you morets = new ArrayList<>(); } } public static void main( String[] argv) { Ex<String> foo = new Ex<>("hello"); // 36 would make a compile error Ex<Integer> bar = new Ex<>(36); } // autoboxed, as if you wrote new Integer(36)