Duke CPS 10811.1From C++ to Java● Java history: Oak, toaster-ovens, internet language, panacea● What it is➤ O-O language, not a hybrid (like C++)➤ compiled to byte-code, executed on JVM➤ byte-code is “highly-portable”, write once run “anywhere”simple, object-oriented, portable, interpreted, robust, secure, architecture-neutral, distributed, dynamic, multithreaded, high performance● create both applets and applications➤ current version 1.3b, we’re using 1.2, which is Java 2➤ browsers support 1.1.x, plug-in needed for 1.2/SwingDuke CPS 10811.2Java Fundamentals● primitive types: int, double, char, boolean (and some others that differ in size, e.g., float, long)➤ size of int is guaranteed to be 32 bits (unlike, say, C++)➤ these primitive types are different than all other types, do NOT belong to inheritance hierarchy, are NOT allocated using new, can be compared using ==, are copied, ...● All other types descend from base type Object, must be allocated using new➤ no delete, garbage collection is automatic➤ all parameters are passed by value, no reference params➤ everything is a pointer (really a reference) --- can change value of parameter via a method call, not via assignment➤ no address operator, “safe pointers”Duke CPS 10811.3C++ and Java confusion● == only works for primitive typesString s = “hello”; String t = “hello”; if (s == t) ...➤ equal() to check for semantic equality, == for pointers● assignment is shallow copy, no new values definedFoo f = new Foo(123); Foo g = f; g.change(); ➤ What happens to f?➤ use clone() function, from interface cloneable()● no semi-colons after class declarations● repeat public/private each time needed, default is packageDuke CPS 10811.4Java Classes: Strings and Arrays● String➤ immutable, once set cannot be changed (but make a new one), see also StringBuffer➤ concatenation using +, this is the only non-arithmetic use of +, in Java no user-defined overloaded operators (+= also works, what about immutable?) any class can be concatenated if it implements toString()● array and Vector➤ array is typed, non-growable, random-access collection• See ArrayList and the new Collections hierarchy➤ Vector is non-typed, growable, random-access collection• casting is required, but is checked at runtime, therefore safev.set(1,“hello”); String s = (String) v.get(1);Duke CPS 10811.5Compiling/Executing Java Programs● class Foo must be stored in Foo.java➤ file name corresponds to class name➤ directory hierarchy corresponds to package hierarchy• java.lang.String is in package java.lang, must be stored in path /xxxxx/*/*/java/lang➤ package is set of related classes• CLASSPATH specifies where packages are looked for● compile, run, document➤ javac, compiler: javac -deprecation Foo.java➤ java, runtime: java Foo➤ javadoc: javadoc -author *.java● import java.lang.*➤ different from #include in C++, namespace deviceDuke CPS 10811.6Inheritance, Interfaces● All classes in Java extend the class Object➤ explicit extension/inheritance is possible, but only single inheritance➤ possible to implement multiple interfaces● An interface is like an abstract base class, all methods/member functions must be implemented ➤ example: Iterator is new version of Enumeration, same pattern, different namesboolean hasMoreElements() boolean hasNext()Object nextElement() Object next()● Possible to declare an object of type Iteration, but cannot use new Iteration (but see anonymous class exception)➤ class Foo extends Widget implements IterationDuke CPS 10811.7Public, private, protected, package● similar to use in C++➤ public methods/classes callable from client code➤ private methods/instance variables not accessible NOFRIENDs➤ protected limits access to subclasses➤ no designation is package access (this approximates friend in C++, but it’s both better and worse)● Package is a module of related classes➤ package classes can access all package functions/data➤ can be used like friend functions --- belong to a package➤ directory hierarchy mimics package designation, CLASSPATH must be set properlyDuke CPS 10811.8Java I/O and other non-pretty stuff● I/O is not pretty➤ Reader base class: Reader supports reading characters only, no formatted input➤ use a BufferedReader constructed from another Reader➤ formatted I/O: use Integer, Double, etc., see ConsoleInput● Integer: an int wrapped in a class➤ static Integer valueOf(String) -- returns an Integer➤ int intValue() -- corresponding int➤ static int parseInt(String) -- returns an int● Double:➤ parseDouble(), other similar
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