Java Programming Review: OOP, Interfaces, Inheritance, and I/O Streams in CMSC433, Spring , Study Guides, Projects, Research of Programming Languages

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CMSC433, Spring 2004 Programming Language Technology and Paradigms Java Review Jeff Foster January 29, 2004 2 Administrivia • Project 1 will be posted later today – Due February 11 • Reading: (today) Liskov ch. 1, 2, (Tue) ch. 4 – Supplemental: Eckel ch. 1, 7, 8, 9 3 Java • Descended from Simula67, SmallTalk, others – Superficially similar to C, C++ • Fully specified, compiles to virtual machine – machine-independent • Secure – bytecode verification (“type-safe”) – security manager 4 Object Orientation • Combining data and behavior – objects, not developers, decide how to carry out operations • Sharing via abstraction and inheritance – similar operations and structures are implemented once • Emphasis on object-structure rather than procedure structure – behavior more stable than implementation – … but procedure structure still useful 5 Example public class Complex { private double r, i; public Complex(double r, double i) { this.r = r; this.i = i; } public String toString() { return “(“ + r + “, “ + i + “)”; } public Complex plus(Complex that) { return new Complex(r + that.r, i + that.i); } } 6 Using Complex public static void main(String[] args) { Complex a = new Complex(5.5,9.2); Complex b = new Complex(2.3,-5.1); Complex c; c = a.plus(b); System.out.println(“a = “ + a); System.out.println(“b = “ + b); System.out.println(“c = “ + c); } 7 The Class Hierarchy • Classes by themselves are a powerful tool – Support abstraction and encapsulation • Java also provides two other abilities – Interfaces allow different classes to be treated the same – Inheritance allows code reuse • Note: When you inherit from a class, you also “implement” the class’s “interface” 8 Project 1: Interfaces public interface MiniServlet extends Runnable { void setArg(String arg); void setOutputStream(OutputStream out); } class HelloWorld implements MiniServlet { ... } class Print implements MiniServlet { ... } MiniServlet s = new HelloWorld(); if (...) s = new Print(); s.setArg(...); 9 Interfaces • An interface lists supported (public) methods – No constructors or implementations allowed – Can have final static variables • A class can implement (be a subtype of) zero or more interfaces • Given some interface I, declaring I x = ... means – x must refer to an instance of a class that implements I, or else null 10 Interface Inheritance • Interfaces can extend other interfaces – Sometimes convenient form of reuse (see project 1) • Given interfaces I1 and I2 where I2 extends I1 – If C implements I2, then C implements I1 • Since a class can implement multiple interfaces, interface extensions are often not needed 11 Inheritance • Each Java class extends or inherits code from exactly one superclass • Permits reusing classes to define new objects – Can define the behavior of the new object in terms of the old one 12 Example class Point { int getX() { ... } int getY() { ... } } class ColorPoint extends Point { int getColor() { ... } } • ColorPoint reuses getX() and getY() from Point • ColorPoint “implements” the Point “interface” – They can be used anywhere a Point can be 25 Example Client/Server ServerSocket s = new ServerSocket(5001); Socket conn = s.accept(); InputStream in = conn.getInputStream(); OutputStream out = conn.getOutputStream(); Socket conn = new Socket (“www.cs.umd.edu”, 5001); InputStream in = conn.getInputStream(); OutputStream out = conn.getOutputStream(); Server code server client Client code 26 Example Client/Server ServerSocket s = new ServerSocket(5001); Socket conn = s.accept(); InputStream in = conn.getInputStream(); OutputStream out = conn.getOutputStream(); Socket conn = new Socket (“www.cs.umd.edu”, 5001); InputStream in = conn.getInputStream(); OutputStream out = conn.getOutputStream(); Server code server client ? Client code 27 Example Client/Server ServerSocket s = new ServerSocket(5001); Socket conn = s.accept(); InputStream in = conn.getInputStream(); OutputStream out = conn.getOutputStream(); Socket conn = new Socket (“www.cs.umd.edu”, 5001); InputStream in = conn.getInputStream(); OutputStream out = conn.getOutputStream(); Server code server client Note: The server can still accept other connection requests on port 5001 Client code 28 Example Client/Server ServerSocket s = new ServerSocket(5001); Socket conn = s.accept(); InputStream in = conn.getInputStream(); OutputStream out = conn.getOutputStream(); Socket conn = new Socket (“www.cs.umd.edu”, 5001); InputStream in = conn.getInputStream(); OutputStream out = conn.getOutputStream(); Server code server client in out Client code 29 Example Client/Server ServerSocket s = new ServerSocket(5001); Socket conn = s.accept(); InputStream in = conn.getInputStream(); OutputStream out = conn.getOutputStream(); Socket conn = new Socket (“www.cs.umd.edu”, 5001); InputStream in = conn.getInputStream(); OutputStream out = conn.getOutputStream(); Server code server client in out in out Client code 30 Possible Failures • Server-side – ServerSocket port already in use – Client dies on accept • Client-side – Server dead – No one listening on port • In all cases IOException thrown – Must use appropriate throw/try/catch constructs 31 Class Objects • For each class, there is an object of type Class • Describes the class as a whole – used extensively in Reflection package • Class.forName(“MyClass”) – returns class object for MyClass – will load MyClass if needed • Class.forName(“MyClass”).newInstance() – creates a new instance of MyClass • MyClass.class gives the Class object for MyClass