Aleksandra Tešanovi ć (alete@ida.liu.se)
Calin Curescu (calcu@ida.liu.se)
Real-Time Systems Laboratory
Department of Computer and Information Science
Linköping University
October 2005
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Gives an introduction to Java language
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Provides an overview over basic knowledge required by unskilled Java programmers to program the RoboCup lab assignments with success
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Focuses on specific issues required for programming the RoboCup lab assignments
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Tailored for students that have some experience with object-oriented programming (e.g., C++)
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Java overview and features
Control Statements
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Variables
Classes and inheritance
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Type conversion
Operator summary
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Compiling and running programs
Resources
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Java Programming myScheduler.java
compiler myScheduler.class
Java Platform program bytecode
Java Virtual
Machines
(JVM) interpreter my scheduler interpreter my scheduler myScheduler.java
Java API
(+RoboCup API)
_______________
JVM
Hardware Platform
Java (+RoboCup)
Platform
Windows Solaris
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Object oriented language
Interpreted at runtime
Garbage collection
Java runtime environment deletes objects that are no longer being used different than in C++!
automatic deletion of objects=> no deconstructors
Strong type checking
Exception handling
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Loops
while, do…while , for similar to C and C++!
Branching
if…else, switch…case
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Exceptions
try…catch…finally, throw, throws
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Aborts
break, continue, label:, return
Scope of these is block dependent
public class Scheduler{ public string PolicyName; public Scheduler() {this(true);}
Variable
Constructor
} public void fireThread(RealtimeThread s){ s.run();
}
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Method constructor does not have to be specified
system uses default constructor default constructor does nothing classes have modifiers
public, protected, private a class is an implementation of an interface
interface is Java’s response to multiple inheritance public static void main(String[] args)
method used for executing a program similar to C++!
CLASS
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„ declaration: type name
RLThread newThread
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„ can be declared as static
class-wide, same value for all instance of the class have modifiers
access modifiers: public, protected, private
static, final, transient, volatile are valid within a scope similar to C++!
Class member variable scope class Scheduler{ public string PolicyName;
Method Parameter public void fireThread
(RealtimeThread s)
{
...
if (..)
{ int i=7;
}
Scope
Local block variable scope
}
System.out.println("The value of i = "+i);
} Error
(”i” does not exist out of scope)
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Primitive type
byte,short, int, long
float double
char, boolean double missedDeadlines= 0.0; manipulation of the value!
similar to C++!
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Reference type
reference is a memory address (pointer in C++) different than in C++!
RTLThread newThread; manipulation of the address!
arrays are objects and are of reference type
all objects are manipulated by Java using their references!
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„ are created from a class (as you know)
AbsoluteTime abs= new AbsoluteTime(); class object name allocates space for an object constructor
„ used as in C++ objectReference.variableName
manipulate or inspect its variables
call its methods
„ remember that objects are of reference type
„ represent the behavior of a class class Scheduler{ public string PolicyName;
} public void fireThread(RealtimeThread s){ s.run();
} modifier output input
„ have parameters
several input
one output (return)
„ and modifiers
public, protected, private
static, final, synchronized, native
OBS! Value of the pointer is passed if an input parameter is an object !
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public void tricky(Point arg1, Point arg2) {
Point temp = arg1; arg1 = arg2; arg2 = temp; arg2.x = 200;
}
What is the value of p1.x, p2.x after the following sequence of statements?
p1.x = 50; p2.x = 100; tricky(p1,p2);
public class EarliestDeadlineFirstScheduler extends Scheduler{…}
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Subclass
(EarliestDeadlineFirstScheduler) inherits all members of superclass
(Scheduler)
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Exceptions to inheritance are when
members are not accessible (i.e., private)
members are explicitly overridden (i.e., redefined) in subclass
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Constructors are not inherited
since they are not members of a class
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Object casting!
Upcast (widening conversion)
from subclass to ancestor
usually automatic; always safe
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Downcast (narrowing conversion)
from ancestor to subclass
checked at runtime for validity
if not an instance – an Exception is thrown
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”instanceof” operator can be used to check if an object belongs to a class
Priority Operators
[ ]
1
()
.
2
3
4
5
6
++
--
Operation array index method call member access pre- or postfix increment pre- or postfix decrement unary plus, minus + -
~
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bitwise NOT boolean (logical) NOT type cast
(type)
New
+ -
* / % object creation multiplication, division, remainder addition, substraction string concatenation
+
<<
>> signed bit shift left signed bit shift right
>>>
< <=
> >=
Instanceof unsigned bit shift right less than, less than or equal to greater than, greater than or equal to reference test
Associativity left right left left left left
Priority
7
8
9
10
11
12
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Operators
==
Operation equal to
!= not equal to
&&
||
|
|
&
&
^
^ bitwise AND boolean (logical) AND bitwise XOR boolean (logical) XOR bitwise OR boolean (logical) OR boolean (logical) AND boolean (logical) OR
? :
= conditional assignment
*= /= += -= %= <<=
>>= >>>= &= ^= |= combinated assignment
(operation and assignment)
Associativity left left left left left left right right
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Class
HighResolution
Time
Relevant Members setTime(HighResolutionTi me new)
Subclass
AbsoluteTime
Relevant members
....
ReleaseParamet ers
.....
PeriodicParameters
...
...
AbsoluteTime getStart()
RealtimeThread
ReleaseParameters getReleaseParameters()
....
RLThread ....
ThreadNode ....
RMThreadNode
AbsoluteTime deadline
AbsoluteTime nextRelease
Assume:
RLThread[] threadList=(RLThread[]) getThreads(); threadNode= new RMThreadNode(threadList[i]) ;
How can we set the next release time of the threadNode (using setTime ). The value of the (old) release time of the thread can be obtained using getRealeseParameters.
What should follow?
This statement: threadNode.nextRelease.setTime(((PeriodicParameters)threa dList[i].getReleaseParameters()).getStart());
...is equivalent with:
AbsoluteTime nextRel = threadNode.nextRelease; RLThread thr
= threadList[i];
ReleaseParameters ppo = thr.getReleaseParameters();
PeriodicParameters pp = (PeriodicParameters)ppo;
AbsoluteTime sTime = pp.getStart(); nextRel.setTime(sTime);
When not sure use ”()” to enforce presedence.
Expression between ”()” is always evaluated first!
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Subclass Class
PlannerControl
Relevant Members restart()
ThreadNode ....
RMThreadNode
Additional members
AbsoluteTime deadline
AbsoluteTime nextRelease
Assume:
RLThread[] threadList=(RLThread[]) getThreads(); threadNode= new RMThreadNode(threadList[i]) ;
How can we restart the planner of the threadNode in the list ?
What should follow?
What is the result of the following statements?
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„ if(!t.isGreater(a.deadline)&&b<a+c) {...}
System.out.println(”temperature is ”+3+2);
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System.out.println(”temperature is
”+(3+2));
(PlannerControl)threadList[1].restart();
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((PlannerControl)threadList[1]).restart();
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Each class is in its own .java file
ClassName has to be the same as FileName in CyclicScheduler.java: public class CyclicScheduler extends BasicScheduler
exception is that inner classes contained wihin the class do not have its own .java file
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Related classes are grouped in packages”
in CyclicScheduler.java: package rt; similar to namespaces in C++ packages are structured hierarchically each .java file should start with the declaration of the package import soccorob.rt.*; import soccorob.ai.agent.*; public class CyclicScheduler extends BaseCyclicScheduler {
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Import allows you to use short names for classes
„ correct compiling depends on setting the CLASSPATH environment variable
points to the directories under which the compiler can find classes on which new compiled class depends
In Windows: set CLASSPATH=%CLASSPATH %;c:\ programs\Robolab;z:\robolab
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Class files must respect the package hierarchy
„ class rt.CyclicScheduler should be in CLASSDIR\rt\, where
CLASSDIR is contained in CLASSPATH
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Compiling command example javac [–classpath C:\programs\Robolab] CyclicScheduler.java
”-classpath ...” option can be used instead of the CLASSPATH variable
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Result of the compilation is a .class file
sorted in the same directory as the source file
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Can be changed with ”–d DestinationDirectory”
„ command for running the program
java myPackage.ClassName
this
Can be used to access methods or fields
Reference to “this ” object can be in a return statement super
Only for accessing the parent’s methods or fields public class HelloWorld extends Hello {
String msg; public HelloWorld(String message) { super(message); msg = message;
} public HelloWorld() { this("hello world");
} public void changeMsg(msg) { this.msg = msg;
} public HelloWorld returnMe() { return this;
}
}
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Standard I/O: stdin, stdout, stderr
defined in java.lang.System
object as public static final InputStream in; public static final PrintStream out;
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Example
String mes1 = ”Hello World\nSecond line:”;
String mes2 = ”\tValue of ion =”
String mes3 = mes1 + mes2; int ion = 3;
System.out.println(mes3);
System.out.println(mes2 + ion);
Escape codes for unprintable characters:
\b - Backspace
\t - Tab
\n - Newline
\f - Form feed
\r - Carriage return
\‘ - Single quote
\” - Double quote
\\ - A backslash
\nnn - The character with octal value nnn
('\012' == '\n')
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On-line tutorials
Help you to learning the Java language
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Provide explantions for essential Java classes
Available at http://java.sun.com/docs/books/tutorial/
Other available material at IDA
Introduction to Java
Available at http://www.ida.liu.se/~TDDI48/intro.pdf
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Introduces RoboCup Real-Time
Scheduling Lab
Environment: software and hardware
RoboCup Lab assignments
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Provides really good help for successful completions of the lab assignments
Scheduled for November 4th 15-17
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