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

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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

ul. steffens @tu- harburg .de http:// www.sts.tu- harburg .de

Data Types

• Two different kinds of data types – primitive types – reference types

Primitive Types

• boolean • char • int • long

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OOIS-11-98-StWe-Java 3

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.

a b

0 0

Reference Type Variables

• A variable of a reference type contains a reference to an anonymous object in storage.

Point a,b;

• An uninitialized variable contains a null-reference. • Objects are anonymous.

• They have an immutable identity. • Objects are created at run-time.

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OOIS-11-98-StWe-Java 5 a b 0 0 3 4 a b 0 0 3 4

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

variables means assigning the references! a = b

• May be a pitfall, as the object may change

a b 34 3 c a == b a != c

Comparing Objects

• Comparing two reference type variables means comparing the references they contain and, thus, comparing the identity of the referenced objects.

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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.

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

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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

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 (); }

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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: ...; }

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++; } }

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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();

} }

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”

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Method Overloading

void wash (shirt s) { // ... void wash (car c) { // ... void wash (dog d) { // ...

Unique argument combinations distinguish overloaded methods

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; }

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OOIS-11-98-StWe-Java 17 void info () {

System. out.println ("Tree is " + height + " feet tall ");

}

void info (String s) {

System. out.println (s + ": Tree is “ + height + " feet tall "); }

}

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 !

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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 (); }

}

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 ; }

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Overloading on Return Values

• Why not also use return values in method overloading?

void f() { // ... int f() { // ...

• Then what would this mean? f();

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;

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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

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

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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

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.

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OOIS-11-98-StWe-Java 27 "abc" a b c "abc"

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");

• There are several constructors defined for strings.

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;

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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)

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)

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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

Java Access Control

Interface Access

Only Accessible Within the class

“Sort of private“

public

private

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“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”

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() {

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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 (); }

}

protected

• Finer grained, deals with inheritance • Generally: when you need it, you’ll know

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An Object has an Interface

• Object : Characteristics and Behavior

• Object Creation: Light lt = new Light ();

• Message: lt.on();

That‘s programming with objects! Light on() off() brighten() dim() Interface Type name

Inheritance

• Inheritance: Automatically duplicates the interface Line draw() erase() Shape draw() erase() Circle draw() erase() Square draw() erase()

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OOIS-11-98-StWe-Java 39 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 */} }

Line Shape

Circle Square ... works with all this objects One piece of code ...

Polymorphism

draw() erase ()

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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);

Composition vs. Inheritance

Car Wheel [] Window[] Door [] Engine Pure composition: Code reuse Shape draw() erase () Circle draw() erase () Sqare draw() erase () Line draw() erase () Pure inheritance: interface duplication Useful

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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

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 () {}

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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