Short Introduction to the Concepts of Programming in Java Overview over the most important constructs

Transcription

1 Introduction to Java Short Introduction to the Concepts of Programming in Java Overview over the most important constructs OOIS 1998/99 Ulrike Steffens Software Systems Institute harburg.de harburg.de Data Types Two different kinds of data types primitive types reference types Primitive Types boolean char byte short int long float double OOIS StWe-Java 2 1

2 Reference Types Predefined or user-defined Classes String Array The values of reference types are not manipulated directly. Instead handles (references) to the actual values are used. OOIS StWe-Java 3 Reference Type Variables A variable of a reference type contains a reference to an anonymous object in storage. a Point a,b; An uninitialized variable contains a null-reference. Objects are anonymous. They have an immutable identity. Objects are created at run-time. new Point(0,0) b 0 0 OOIS StWe-Java 4 2

3 Binding Reference Type Variables Assigning an object to a reference type variable, the variable gets a reference to the object as new value. The value of the object does not change a = new Point(0,0); b = new Point(3,4); Assigning values of other b variables means assigning the references! a = b May be a pitfall, as the object may change OOIS StWe-Java 5 a a b Comparing Objects Comparing two reference type variables means comparing the references they contain and, thus, comparing the identity of the referenced objects. a b 3 4 a == b c 3 4 a!= c OOIS StWe-Java 6 3

4 Comparing the Contents of Objects The contents of objects belonging to pre-defined classes can be compared using the method equals. If you define a class of your own, equals by default only compares references, as == does. => You have to override equals for new defined classes. OOIS StWe-Java 7 Creating New Data Types: class Class keyword defines new data type class ATypeName { /* class body goes here */ ATypeName a = new ATypeName(); Fields class DataOnly { int i; float f; boolean b; Each instance of DataOnly gets its own copy of the fields In a class, primitives get default values. OOIS StWe-Java 8 4

5 Methods, Arguments and Return Values Methods: how you get things done in an object Traditionally called functions Can only be defined inside classes returntype methodname (/*argument list*/){ /* method body */ Example method call: int x = a.f(); //For object a OOIS StWe-Java 9 The Argument List Types of the objects to pass in to the method Name (identifier) to use for each one Whenever you seem to be passing objects in Java, you re actually passing handles int size(string s){ return s.length(); OOIS StWe-Java 10 5

6 Control Flow Statements if (condition) {... else {... (condition)? value1: value2 while (condition) {... do {... while (condition) for(int i = 0; i < 10; i++) {... switch(value){ case (value1):... ; case (value2):... ;... default:...; OOIS StWe-Java 11 The static Keyword Normally each object gets its own data What if you want only one piece of data shared between all objects of a class? ( class data ) class StaticTest { static int i = 47; What if you want a method that can be called for the class, without an object? ( class method ) class StaticFun { static void incr() { StaticTest.i++; StaticFun.incr(); OOIS StWe-Java 12 6

7 Guaranteed Initialization with the Constructor class Rock { Rock() { // This is the constructor System.out.println("Creating Rock"); public class SimpleConstructor { public static void main(string args[]) { for(int i = 0; i < 10; i++) new Rock(); OOIS StWe-Java 13 Nuance We can deduce meaning from context Wash the shirt Wash the car Wash the dog Not shirtwash the shirt carwash the car dogwash the dog OOIS StWe-Java 14 7

8 Method Overloading void wash(shirt s) { //... void wash(car c) { //... void wash(dog d) { //... Unique argument combinations distinguish overloaded methods OOIS StWe-Java 15 Method Overloading One word, many meanings: overloaded class Tree { int height; Tree() { System.out.println("A seedling"); height = 0; Tree(int i) { System.out.println("A new Tree, " + i + " feet tall"); height = i; OOIS StWe-Java 16 8

9 void info() { System.out.println("Tree is " + height + " feet tall"); void info(string s) { System.out.println(s + ": Tree is + height + " feet tall"); OOIS StWe-Java 17 Default Constructor: Takes no Arguments Compiler creates one for you if you write no constructors class Bird { int i; public class DefaultConstructor { public static void main(string args[]) { Bird nc = new Bird(); // default! OOIS StWe-Java 18 9

10 this: Handle to Current Object public class Leaf { private int i = 0; Leaf increment() { i++; return this; void print() { System.out.println("i = " + i); public static void main(string args[]) { Leaf x = new Leaf(); x.increment().increment().increment().print(); OOIS StWe-Java 19 this in Constructors A very common kind to use this is in constructors to initialize data fields with the constructor's arguments public class Animal { private int numberoflegs; Animal(int numberoflegs) { this.numberoflegs = numberoflegs; OOIS StWe-Java 20 10

11 Overloading on Return Values Why not also use return values in method overloading? void f() { //... int f() { //... Then what would this mean? f(); OOIS StWe-Java 21 Member Initialization void f() { int i; // No initialization i++; Produces compile-time error Inside class, primitives are given default values if you don t specify values class Data { int i = 999; long l; // defaults to zero //... OOIS StWe-Java 22 11

12 Array Initialization int a1[]; // this... int[] a1; // is the same as this! Creates a handle, not the array. Can t size it. To create an array of primitives: int a1[] = { 1, 2, 3, 4, 5 ; An array of class objects: Animal a[] = new Animal[20]; // 20 x null System.out.println(a.length + " animals"); for(int i = 0; i < a.length; i++) { a[i] = new Animal((i % 2 + 1) * 2); Bounds checked, length produces size OOIS StWe-Java 23 Array Initialization Can also use bracketed list (The size is then fixed at compile-time) Integer[] a = { new Integer(1), new Integer(2), new Integer(3), ; If you do anything wrong either the compiler will catch it or an exception will be thrown OOIS StWe-Java 24 12

13 Multi-dimensional Arrays It is possible to define multi-dimensional arrays in the same way as described. int[][] a; The brackets even may be distributed between type and identifier. int[] a[]; Initialization can be done directly int[][] a = { { 1, 2, 3, { 5, 6 ; It can also be done by nested iterations over the array and its components. arrays may not be rectangular OOIS StWe-Java 25 Strings Strings are immutable objects of the class String. String literals are zero, one or more characters included within double quotes. When a binding to a string literal is executed for the first time, a new String object is created. If any other bindings to this literal appear, the respective variables will hold reference to the same object. String a = "abc"; String b = "abc"; a b "abc" OOIS StWe-Java 26 13

14 String Constructors String objects can also be created by calling a constructor. String constructors create new objects whenever they are called. String c = new String(" abc"); a b c "abc" "abc" There are several constructors defined for strings. OOIS StWe-Java 27 String Concatenation Strings can be concatenated by using +. String c = "A " + "concatenation"; The concatenation also creates a new String object. Values of other types can be concatenated to strings, too. They are implicitly converted to String. String n = "Number " + 49; OOIS StWe-Java 28 14

15 Package: the Library Unit import java.util.*; import java.util.vector; Managing name spaces Class members are already hidden inside class Class names could clash Need completely unique name Compilation units (.java files) Name of.java file == name of single public class Other non-public classes are not visible Each class in file gets its own.class file Program is a bunch of.class files (no.obj or.lib) OOIS StWe-Java 29 Creating a Library of Classes package mypackage; public class is under the umbrella mypackage Client programmer must import mypackage.*; Creating unique package names Location on disk encoded into package name Convention: first part of package name is Internet domain name of class creator (reversed) Java interpreter uses CLASSPATH environment variable as starting point for search OOIS StWe-Java 30 15

16 Library Location C:\DOC\Java\SomeDirectory\util CLASSPATH takes care of first part: CLASSPATH=.;D:\JAVA\LIB;C:\DOC\ JavaT Programs can be in any directory import SomeDirectory.util.*; public class LibTest { public static void main(string args[]) { Vector v = new Vector(); List l = new List(); Compiler starts search at CLASSPATH OOIS StWe-Java 31 Java Access Control public Interface Access private Only Accessible Within the class protected Sort of private OOIS StWe-Java 32 16

17 Friendly Default access, has no keyword public to other members of the same package, private to anyone outside the package. Easy interaction for related classes (that you place in the same package) Also referred to as package access OOIS StWe-Java 33 public: Interface Access package c05.dessert; public class Cookie { public Cookie() { System.out.println("Cookie constructor "); void foo() { System.out.println("foo"); // Separate file: import c05.dessert.*; public class Dinner { public Dinner() { System.out.println("Dinner constructor"); public static void main(string args[]) { Cookie x = new Cookie(); //! x.foo(); // can't access OOIS StWe-Java 34 17

18 private: Can t Touch That! class Sundae { private Sundae() { static Sundae makeasundae () { return new Sundae(); public class IceCream { public static void main(string args[]) { //! Sundae x = new Sundae(); Sundae x = Sundae.makeASundae(); OOIS StWe-Java 35 protected Finer grained, deals with inheritance Generally: when you need it, you ll know OOIS StWe-Java 36 18

19 An Object has an Interface Type name Interface Light on() off() brighten() dim() Object : Characteristics and Behavior Object Creation: Light lt = new Light(); Message: lt.on(); That s programming with objects! OOIS StWe-Java 37 Inheritance Inheritance: Automatically duplicates the interface Shape draw() erase() Circle draw() erase() Square draw() erase() Line draw() erase() Reusing interface (and implementation) Objects related by inheritance all have the same type (interface) OOIS StWe-Java 38 19

20 public class Shape { public void draw() { /* code drawing a shape */ public void erase() { /* code erasing a shape */ public class Circle extends Shape{ public void erase() { /* code erasing a circle */ OOIS StWe-Java 39 Polymorphism One piece of code works with all this objects draw() erase() Shape Circle Square Line OOIS StWe-Java 40 20

21 An Amazing Trick void dostuff(shape s) { s.erase();... s.draw(); Circle c = new Circle(); Triangle t = new Triangle(); Line l = new Line(); dostuff(c); // dynamic binding dostuff(t); dostuff(l); OOIS StWe-Java 41 Composition vs. Inheritance Car Wheel[] Window[] Door[] Engine Pure composition: Code reuse Gray area Useful void f() void g() MoreUseful void f() void g() void u() void v() void w() Circle draw() erase() Inherit to extend or change interface Shape draw() erase() Sqare draw() erase() Line draw() erase() Pure inheritance: interface duplication for interchangeable objects OOIS StWe-Java 42 21

22 An abstract Instrument Abstract classes leave some part of the implementation open Some methods and data may only be defined abstract class Instrument4 { int i; // storage allocated for each public abstract void play(); public String what() { return "Instrument4"; public abstract void adjust(); Rest of the code is the same Abstract classes cannot be instantiated by new OOIS StWe-Java 43 An Instrument interface No concrete elements in interface You don t extend, you implement interface Instrument { void play(); // Automatically public String what(); void adjust(); class Guitar implements Instrument { public void play() { System. out.println("guitar.play()"); public String what() { return "Guitar"; public void adjust() { OOIS StWe-Java 44 22

23 How to use Java on STS-Sun-Workstations? Set the paths PATH and CLASSPATH correctly by typing: def_jdk12 Create a file with your source code ending on.java Compile your code: javac filename.java Run your application java filename OOIS StWe-Java 45 23