CompSci 100E 3.1 1 Functions (Static Methods) ! Java function. " Takes zero or more input arguments. " Returns one output value. ! Applications. " Scientists use mathematical functions to calculate formulas. " Programmers use functions to build modular programs. " You use functions for both. ! Examples. " Built-in functions: Math.random(), Math.abs(), Integer.parseInt(). " Our I/O libraries StdDraw.show(), StdAudio.play(). " User-defined functions: main(). CompSci 100E 3.2 2 Anatomy of a Java Function ! Java functions. Easy to write your own. f(x) = !x!input 2.0 1.414213… output CompSci 100E 3.3 3 Flow of Control ! Flow of control. Functions provide a new way to control the flow of execution of a program. "pass-by-value" CompSci 100E 3.4 4 Libraries ! Library. A module whose methods are primarily intended for use by many other programs. ! Client. Program that calls a library. ! API. Contract between client and implementation. ! Implementation. Program that implements the methods in an API.CompSci 100E 3.5 5 Modular Programming ! Modular programming. " Divide program into self-contained pieces. " Test each piece individually. " Combine pieces to make program. ! Ex. Flip N coins. How many heads? " Read arguments from user. " Flip one fair coin. " Flip N fair coins and count number of heads. " Repeat simulation, counting number of times each outcome occurs. " Plot histogram of empirical results. " Compare with theoretical predictions. CompSci 100E 3.6 Flashback: Data processing ! Scan a large (~ 107 bytes) file ! Print the words together with counts of how often they occur ! Need more specification? ! How do you do it? ! What is we only wanted the top k (say 20) words? CompSci 100E 3.7 What can you put into an ArrayList? ! Any Object ! Use a wrapper class (see java.lang.*) " int, double, char, boolean, … " Integer, Double, Character, Boolean, ! Can have your cake and eat it too ArrayList<Integer> list = new ArrayList<Integer>(); for (int k = 0; k < 10; k++){ list.add(k*k); } for (Integer jj : list){ System.out.println(jj); } ! All made practical by Version 5 of Java CompSci 100E 3.8 Exploring ArrayLists ! Look at the Java 6 API ! Note interfaces implemented " Serializable, Cloneable, Iterable " Collection, List, RandomAccess ! Note other descriptive text " Regarding performance " Constructors " Methods " Don’t forget methods in parent classesCompSci 100E 3.9 Exploring ArrayLists ! Some Commonly Used Methods " boolean add(E o) // append " void add(int index, E element) // insert " void clear() " boolean contains(Object elem) " E get(int index) " int indexOf(Object elem) " boolean remove(Object o) " E remove(int index) " E set(int index, E elem) // replace " int size() CompSci 100E 3.10 Exploring ArrayLists ! Performance " Constant Time # size, isEmpty, get, set, iterator, listIterator operations # add (amortized) " Linear Time # All of the other operations run in linear time " What does all of this mean? " Why do we care? " Exercise: Implement on an array the equivalent of # void add(int index, E element) # E remove(int index) " Remember: Memory is an array (well sort of) CompSci 100E 3.11 What is a char? ! Differences between unicode and ASCII " Why is unicode used? Why should we care? What should we know? How many of the details are important? ! A char value can be treated like an int value " Add integer to it, cast back to char " Subtract character from it, get int back counters[s.charAt(k)- ’A’]++; " Anatomy of the statement above?? CompSci 100E 3.12 Inheritance and Interfaces ! Inheritance models an "is-a" relationship " A dog is a mammal, an ArrayList is a List, a square is a shape, … ! Write general programs to understand the abstraction, advantages? void execute(Pixmap target) { // do something } ! But a dog is also a quadruped, how can we deal with this?CompSci 100E 3.13 Single inheritance in Java ! A class can extend only one class in Java " All classes extend Object --- it's the root of the inheritance hierarchy tree " Can extend something else (which extends Object), why? ! Why do we use inheritance in designing programs/systems? " Facilitate code-reuse (what does that mean?) " Ability to specialize and change behavior o If I could change how method foo() works, bar() is ok " Design methods to call ours, even before we implement o Hollywood principle: don't call us, … CompSci 100E 3.14 Comparable and Comparator ! Both are interfaces, there is no default implementation " Contrast with .equals(), default implementation? " Contrast with .toString(), default? ! Where do we define a Comparator? " In its own .java file, nothing wrong with that " Private, used for implementation and not public behavior o Use a nested class, then decide on static or non-static o Non-static is part of an object, access inner fields ! How do we use the Comparator? " Sort, Sets, Maps (in the future) ! Does hashing (future topic) have similar problems? CompSci 100E 3.15 Sets ! Set is an unordered list of items " Items are unique! Only one copy of each item in set! ! We will use two different implementations of sets ! TreeSet " A TreeSet is backed up by a tree structure (future topic) " Keeps items sorted (+) " Slower than HashSets ?? (-) ! HashSet " A HashSet is backed up by a hashing scheme (future topic) " Items not sorted – should seem to be in random order (-) " Faster than TreeSets ?? (+) CompSci 100E 3.16 Using Both ArrayList and Sets ! You may want to use a set to get rid of duplicates, then put the items in an ArrayList and sort them! ! Problem: " Often data comes in the form of an array " How do we go from array to ArrayList or TreeSet? ! Problem: " Often we are required to return an array " How do we go from a Collection such as an ArrayList or TreeSet to an array? ! Can do it the “hard” way with loops or iterators: " one item at a time !
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