Lecture 6 More Java Advanced Programming Techniques API Best reference is online http docs oracle com javase 7 docs api Generated by Javadoc Garbage Collection All memory allocated on the heap that is no longer referenced is freed up by the Garbage Collector There is no telling when that will happen There is a way to force garbage collection if needed System gc Java procedures are mainly call by reference This means that procedures can easily alter objects passed to them as parameters Building your own exceptions Class ArgumentNegativeException extends Exception exceptions class Factorial Public static int compute int k throws ArgumentNegativeException other exceptions if k 0 throw new ArgumentNegativeException else return 1 and later in a program try System out println answer is Factorial compute n catch ArgumentNegativeException e System err println e oops continue super used to call the parent class s constructor with arguments if desired using the syntax super arg0 arg1 must be the very first line in every constructor unless omitted in which case it will be automatically inserted without arguments super xxx to call methods of the parent only goes one level up no super super Java lacks some C features No overloading of operators although function overloading is okay No i o operators No general multiple inheritance Jave does have something cooler though the Interface Multiple inheritance is messy No environment variables in Java No such thing as environment variables as there are with C C and Unix because this is not OS independent There s a system property setting mechanism that accesses properties declared in the command line invocation called System getProperty read about it in the Java documentation Generics templates New with Java 5 Mechanism is a little different than the template Allows containers to hold Objects and first class types instanceOf can upcast and downcast but wrong cast can cause exceptions during runtime To check whether a certain object is of a particular class use instanceOf keyword this To invoke one method from another in the same class an object must send a message to itself this methodName does the job To refer to a field use this fieldName When no ambiguity is possible this is optional private String text public Greeting String text this text text Object equals Object e finalize hashCode toString Java Arrays Objects but not built with new Array C like syntax Size must be known at instantiation Homogeneous all elements of same type Can contain primitive types Array Syntax Idioms declare initialize allocate from heap int a new int 3 stylish Java int b new int 6 like C less good note scope of i in for loop like C for int i 0 i a length i System out println a i a i b a a b reference same array old b can be garbage collected String colors red blue green Command line arguments class CommandLineArgsDemo java public static void main String args for int i 0 i args length i System out println args i java CommandLineArgsDemo foo bar b q foo bar b q args 0 isn t program name Java 2D Arrays char board new char 8 8 for int y 0 y board length y for int x 0 x board y length x System out print board x y System out println Collections in Java Manipulate grouped data as a single object Java provides List Set Map add contains remove size loop over sort Insulate programmer from implementation array linked list hash table balanced binary tree Like C Standard Template Library STL Can grow as necessary Contain only Objects reference types List Like an array elements have positions indexed 0 size 1 duplicate entries possible Unlike an array can grow as needed can contain only Objects heterogeneous easy to add delete at any position API independent of implementation ArrayList LinkedList Set Like a List can grow as needed can contain only Objects heterogeneous easy to add delete API independent of implementation HashSet TreeSet Unlike a List elements have no positions duplicate entries not allowed Collection API methods some int size boolean add Object obj returns true if Collection changes as a result of the add it always will for List may not for Set boolean contains Object obj to see the rest study the API warning look at List not List a GUI class Iterator Interface suppose Collection of Programmer objects Iterator iter engineers iterator while iter hasNext Programmer p Programmer iter next p feed pizza Note cast Programmer since Collection and Iterator manage anonymous objects When collection has a natural ordering Iterator will respect it Supports safe remove of most recent next StringTokenizer Like an Iterator for a String Elements are white space delimited words StringTokenizer st new StringTokenizer now is the time while st hasMoreTokens String word st nextToken Should implement Iterator interface hasNext next Map Table lookup abstraction void put Object key Object value Object get Object key can grow as needed any Objects for key and value keys tested for equality with equals of course API syntax independent of implementation HashMap TreeMap Iterators for keys values key value pairs Conversions String Object String s I am obj invokes obj toString s String valueOf Object obj ditto and overloaded for primitive types char boolean char int long float double int Integer valueOf String s Integer is wrapper class for int Integer Integer parseInt String s Equality reminder for objects tests pointer not contents Object class implements public boolean equals Object obj return this obj public int hashCode probable unique id String class overrides if g equals goodbye not if goodbye equals g ok too Strings hash as they should for deep copy override Object clone field by field copy String matches and searches boolean equalsIgnoreCase String anotherString int compareTo String anotherString 0 boolean startsWith String prefix boolean endsWith String suffix boolean regionMatches int toffset String other int ooffset int len int int int int indexOf int ch indexOf String str indexOf int fromIndex lastIndexOf Methods Returning Constructing New Strings concat String str Can also use replace char old char new Not in place substring int beginIndex New string substring int beginIndex int endIndex toLowerCase toLowerCase Locale locale toUpperCase toUpperCase Locale locale trim New string Class StringBuffer mutable size and content StringBuffer str new StringBuffer World str insert 0 Jello str append str setCharAt 0 H now Hello World str reverse now dlroW olleH String s str toString public