java Features

java Features
Version April 19, 2013
by
Thorsten Kracht
Contents
1
Introduction
1.1 Hello World . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
2
2
Variables, Types
3
3
Input/Output
3.1 Standard I/O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Scanner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
4
4
4
Operators
4.1 Boolean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Increment/Decrement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
5
5
5
Control Structures
5.1 If . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
6
6
6
Standard Classes
6.1 Import . . .
6.2 Math . . . .
6.3 Random . .
6.4 String . . .
6.5 toString . .
6.6 Wrapper . .
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7
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9
Classes
7.1 General . . . . . . . . . . . . . .
7.2 Definition . . . . . . . . . . . . .
7.3 Classes in separate files . . . . . .
7.4 Overloading . . . . . . . . . . . .
7.5 Referenced objects can be changed
7.6 Encapsulation . . . . . . . . . . .
7.7 Inheritance . . . . . . . . . . . .
7.8 Abstract classes, polymorphism .
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1
Chapter 1
Introduction
This note is intended to help people getting started with java. It contains some explanations and covers the topics by
examples.
1.1 Hello World
//
// file Hello.java
//
class Hello
{
public static void main ( String[] args )
{
System.out.println("Hello World!");
}
}
This program can be compiled and executed with:
javac Hello.java
creates Hello.class
java Hello
executes Hello.class
2
Chapter 2
Variables, Types
These are the primitive data types:
byte
8 bit, -128
short 16 bit, -32768
int
32 bit, -2E9
long
64 bit, -10e18
float 32 bit, -3.4E38
double 64 bit, -1.7e308
char
16 bit (!)
boolean true, false)
-
+127
+32767
+2E9
+10e18
(long i = 3L;)
+3.4e38 (float x = 1.23F;)
+1.7e308 (double x = 123.0D;)
The type names can be used as cast operators:
int i = (int) 1.5;
Primitive variables cannot be changes by a method.
Reference variables can have the value null.
String a = null;
if( a != null) {
...
}
3
Chapter 3
Input/Output
3.1 Standard I/O
System.in
System.out
System,err
3.2 Scanner
import java.util.Scanner;
// loads java.io implicitly
class echo
{
public static void main (String[] args)
{
String line;
Scanner inp = new Scanner( System.in );
System.out.println("Enter:");
line = inp.nextLine();
System.out.println("You typed:" + line );
}
}
4
Chapter 4
Operators
4.1 Boolean
A
A
A
A
A
A
== B
< B
<= B
> B
>= B
!= B
&&
||
!
4.2 Increment/Decrement
i++;
i--;
5
Chapter 5
Control Structures
5.1 If
class ifTest
{
public static void main (String[] args)
{
double x = 1.0;
if( x < 0) {
System.out.println(" x < 0");
}
else if (x == 0.) {
System.out.println(" x == 0");
}
else {
System.out.println(" x > 0");
}
}
}
5.2 Loops
class ifTest
{
public static void main (String[] args)
{
double x = 1.0;
if( x < 0) {
System.out.println(" x < 0");
}
else if (x == 0.) {
System.out.println(" x == 0");
}
else {
System.out.println(" x > 0");
}
}
}
6
Chapter 6
Standard Classes
The examples in this section can be compiled and executed by:
$ javac mathTest.java && java mathTest
cos 60.0 = 0.5000000000000001
The string ’mathTest’ has to be replaces by the actual class name.
6.1 Import
// import a single class
import java.util.Random;
// import all classes of a package
import java.util.*;
6.2 Math
class mathTest
{
public static void main (String[] args)
{
double degree = 60.;
System.out.println(" cos " + degree + " = " + Math.cos( Math.toRadians( degree)));
}
}
$ javac mathTest.java && java mathTest
cos 60.0 = 0.5000000000000001
6.3 Random
import java.util.Random;
public class randTest
{
public static void main ( String[] args )
{
Random rand = new Random();
int i = 10;
while ( i > 0 )
{
// other methods: nextFloat(), nextDouble(), nextGaussian()
7
System.out.println("random " + rand.nextInt(1000) ); // numbers from 0 to 999
i = i - 1;
}
}
}
6.4 String
The string class (package java.lang.String) contains these functions (and others):
public char charAt( int index )
public String concat( String str )
public boolean endsWith( String suffix )
public boolean equals( Object anObject )
str1.equals( str2)
public boolean equalsIgnoreCase( String anotherString )
public int indexOf( int ch )
public int indexOf( String str )
public int length()
public boolean startsWith( String prefix )
public String substring( int beginIndex )
public String substring( int beginIndex, int endIndex )
public String toLowerCase()
public String toUpperCase()
public String trim()
Example:
//
class Beispiel
{
public static void main ( String[] args )
{
java.lang.String str;
str = new String( "ein test string");
String str1 = new String( "noch ein string");;
System.out.println( "string = " + str + " length " + str.length());
System.out.println( "upperCase-str1 = " + str1.toUpperCase());
}
}
6.5 toString
Produces a string representation of an object.
import java.awt.*;
class toStringTest
{
public static void main (String[] args)
{
Point a = new Point( 3, 4);
System.out.println(" toString " + a.toString());
}
}
Result:
$ javac toStringTest.java && java toStringTest
toString java.awt.Point[x=3,y=4]
6.6 Wrapper
The wrapper classes, package java.lang, create an object.
primitive type wrapper type
byte
Byte
short
Short
int
Integer
long
Long
float
Float
double
Double
char
Character
boolean
Boolean
Example:
Integer i = new Integer(123);
Chapter 7
Classes
7.1 General
• static members belong a class not the opject.
• private visible from within the object.
• public visible from outside the object.
• If a member is neither declared private or public, it is public to the package.
7.2 Definition
class Hello
{
void say()
{
System.out.println( "this is Hello.say");
}
}
class classTest
{
public static void main (String[] args)
{
Hello a = new Hello();
a.say();
}
}
7.3 Classes in separate files
Here is a main programm that uses code which is stored in a different file:
// file classTest.java
class classTest
{
public static void main (String[] args)
{
classHello a = new classHello( "moin");
a.say();
}
}
The class classHello:
// file classHello.java
class classHello
10
{
String msg;
classHello( String a)
{
this.msg = a;
System.out.println( "this is the Constructor, classHello.Hello");
}
void say()
{
System.out.println( "this is classHello.say: " + this.msg);
}
}
This command compiles both files and executes main().
$ javac classTest.java classHello.java && java classTest
7.4 Overloading
class classHello
{
private String msg;
classHello( String a)
{
this.msg = a;
}
public void say()
{
System.out.println( "this is classHello.say:
}
// overloading a function
public void say( String str)
{
System.out.println( "this is classHello.say:
}
}
class classTest
{
public static void main (String[] args)
{
classHello a = new classHello( "Moin");
a.say();
String b = new String( "Guten Tag");
a.say( b);
}
}
7.5 Referenced objects can be changed
class Value
{
public int i = 1;
}
class Klasse
{
void func( Value obj) {
obj.i = 11;
return;
" + this.msg);
" + str);
}
}
class classTest
{
public static void main (String[] args)
{
Value b = new Value();
Klasse a = new Klasse();
System.out.println( "Vorher " + b.i);
a.func( b);
System.out.println( "Nachher " + b.i);
}
}
7.6 Encapsulation
class Value
{
private int i = 1;
public void setValue( int argin){
i = argin;
}
public int getValue( ){
return i;
}
}
class classTest
{
public static void main (String[] args)
{
Value b = new Value();
System.out.println( "Vorher " + b.getValue());
b.setValue( 2);
System.out.println( "Nachher " + b.getValue());
}
}
7.7 Inheritance
class Human
{
int age;
String name;
public Human( String name, int age){
this.name = name;
this.age = age;
}
}
class male extends Human {
String gender;
public male( String name, int age) {
super( name, age);
this.gender = "male";
}
}
class classTest
{
public static void main (String[] args)
{
male a = new male( "Fritz", 30);
System.out.println( a.name + " " + a.age + " " + a.gender);
}
}
7.8 Abstract classes, polymorphism
An abstract class is never instanciated. It servers the purpose of being a superclass for other classes.
abstract class Animal
{
String name;
public Animal( String name){
this.name = name;
}
// abstract methods have to be implemented by subclasses
public abstract String sound();
}
class dog extends Animal {
public dog( String name) {
super( name);
}
public String sound() {
return "wau";
}
}
class cat extends Animal {
public cat( String name) {
super( name);
}
public String sound() {
return "miau";
}
}
class classTest
{
public static void main (String[] args)
{
Animal a = new dog( "Rex");
Animal b = new cat( "Miezzi");
Animal c;
c = a;
System.out.println( c.name + " says " + c.sound());
c = b;
System.out.println( c.name + " says " + c.sound());
}
}
Polymorphism: depending which type of animal is referenced by c, the sound() method produces a different result.
Index
abstract classes, 13
encapsulation, 12
inheritance, 12
overloading, 11
polymorphism, 13
toString, 8
14