How can I use abstract class methods

[Vas Miriyala]

Hi firends
I have a abstract class and I want to use someof methods from abstract class. I mean I want to to use abstract class methods in other class

[Janeice DelVecchio]

If you want to use the abstract methods in another class, you might consider making it into an interface.

[Vas Miriyala]

well Thanks for your reply and Here class is only abstract and methods are not but defined methods in abstract class are returning something and I want to use that

[Suhas Bilaye]

You simply need to create a subclass which extends from the abstract class. Then using the instance of this subclass you can use the defined methods of the abstract class.

[Rob Spoor]

Suhas is right. You can call abstract methods of an abstract class without problems. This is because you can only instantiate concrete classes, and concrete sub classes of abstract classes have implemented the abstract methods.

Consider:

abstract class A
{
    abstract void print();
}

class B extends A
{
    void print() { System.out.println("I am B"); }
}

public class Test
{
    public static void main(String[] args)
    {
        A a;
        a = new B();
        a.print(); // actually calls B.print
    }
}

[Vas Miriyala]

thats quiet hepfull, and bit confused also and actually I have a file names Hello.java which is a abstract class and I have a one method init and I have onemore class called use.java and both are in same packeage and I want to use method in Hello.java which is a abstract class in Use.java How can I do that?

[Christian Dillinger]

Did you understand Rob's sample? If not, read it again. If you did, your problem should be solved. But to help us to help you, you should post your code here...

[Vas Miriyala]

oh.forgive my madness and Here is the code

 

package traffic.object;
import java.io.*;
import java.util.*;
import traffic.*;
import rescuecore.OutputBuffer;
import rescuecore.RescueConstants;

public abstract class MovingObject extends RealObject implements Obstruction {
    public MovingObject(int id) {
        super(id);
    }

    private int m_position;
    private double m_positionExtra;
    // private int m_direction;
        private Route m_routePlan;


    public void setRoutePlan(Route route) {
        m_routePlan = route;
    }

    public Route routePlan() {
		System.out.println("m_routePlan = " +m_routePlan);
        return m_routePlan;
    }

   

I want to use routePlan method in below class named mySimulator.java

package traffic;
import java.net.*;
import java.util.*;
import traffic.object.*;
import traffic.object.MovingObject;

public class mySimulator  implements Constants    {
    private final IO io;
    private final MovingObject mv;
    private int id;
        public mySimulator(InetAddress kernelAddress, int kernelPort) {
	io = new TCPIO(kernelAddress,kernelPort);
	io.sendConnect();
	id = io.receiveConnectOk();
	System.out.println("Connected. Simulator ID is " + id);
	io.sendAcknowledge();
    }



    public void simulate() {
	System.out.println("start");
	while(true) {
	    io.receiveCommands();
	    System.out.println("time " + WORLD.time());
	    move();
	    io.sendUpdate();
	    io.receiveUpdate();
	}
    }
public Route route;

	private void move() {
	    System.out.println("under construction");
	    System.out.println(" route = " + route);
	    mv.calculateAgentsMovingLength();

	}
}

[Christian Dillinger]

Get a reference to an Object that extends MovingObject and call the method. Do you have a concrete class that can be instantiated?

[Vas Miriyala]

sorry,I didnt get you can you give example using my code

[Christian Dillinger]

public class MyMovingObject extends MovingObject{
...
}

...
MovingObject mo = new MyMovingObject();
mo.routePlan();
...

That's what Rob's sample was about...

[Vas Miriyala]

But got the error which says

   
Use.java:6: traffic.Use is not abstract and does not override abstract method type() in traffic.object.RescueObject
public class Use extends MovingObject {
       ^
1 error

 

[Christian Dillinger]

There is a method type() in RescueObject you have to implement in order to get a concrete class - as long as you don't you'll never ever get a class that not abstract.

[Vas Miriyala]

what I have to implement...No idea about that

  
package traffic.object;

import traffic.*;

/** The base class of all RoboCupRescue's objects */
public abstract class RescueObject implements Constants {
  public final int id;

  public RescueObject(int id) { this.id = id; }
  public abstract int type();

  /** sets a property
   *  @param property an integer indicating a type of the property
   *  @param value    value of the property
   */
  public void input(int property, int[] value) { /* nothing to do */ }

  public int hashCode() { return id; }

  public String toString() { return new StringBuffer().append(super.toString()).append(" (ID:").append(id).append(")").toString(); }
}

  

[Christian Dillinger]

There is an abstract method in RescurObject: public abstract int type();

You have to implement this method in your subclass.

public int type(){
return 12345;
}

And that's only an example! May be should should read some tutorials about basic java concepts.

[Vas Miriyala]

okay thanks but got this error

   
Use.java:6: cannot find symbol
symbol  : constructor MovingObject()
location: class traffic.object.MovingObject
class Use extends MovingObject {
^
1 error
 

Use.java

package traffic;
import traffic.object.*;
import traffic.*;
import java.util.*;
import traffic.mySimulator;
class Use extends MovingObject {
    void Use() {}
    //int i;
    //    ArrayList arr = new ArrayList();
    MovingObject mo =new Use();
    //    mo.routePlan();
   public int type() {
	return 1;
    }
}

[David Newton]

Are you compiling using the correct classpath?

[Vas Miriyala]

yes I am compiling with cirrect classpath only this is is classpath

 export PATH=$PATH:/opt/jdk1.6.0_15/bin/

[David Newton]

That's a path, not a classpath.

[Vas Miriyala]

Here it is my Classpath correct me if this is wrong

root@VAIO:/home/miriyala/Desktop/test/rescue-1.0-prealpha/oldsims# echo $CLASSPATH
:/opt/jdk1.6.0_15/jre/lib/:/home/miriyala/Desktop/test/rescue-1.0-prealpha/jars/:/home/miriyala/Desktop/test/rescue-1.0-prealpha/lib/junit-4.5.jar

[David Newton]

Yes; you have no reference to the classes you've already compiled.

Note that most here will encourage you *not* to set the classpath environment variable, as it can cause a lot of confusion. It's often easier to supply the classpath directly to the Java compiler.

[Vas Miriyala]

sorry for late reply I have been quite busy with some other work ,as you said about supplying classpath to the java compiler, here there is a makefile to compile all java programs at once and I am confused how to supply the classpath at compilation time

here is makefile and makefile.in

Makefile

 
DEBUG=-g -Wall
OPTIMISE=

export CC=gcc
export CXX=g++
export CFLAGS=$(DEBUG) $(OPTIMISE) -pipe
export CPPFLAGS=$(DEBUG) $(OPTIMISE) -pipe
export CXXFLAGS=$(DEBUG) $(OPTIMISE) -pipe
export LDFLAGS=
export JAVAC=javac
export CLASSPATH=$(shell pwd)
export JFLAGS=-g
export TOP_DIR=$(shell pwd)

SUBDIRS = rescuecore librescue blockadessimulator collapsesimulator firesimulator gis miscsimulator civilian traffic viewer ticker yab
.PHONY: $(SUBDIRS)

all:
	@@for dir in $(SUBDIRS) ; do\
		(echo $$dir ; $(MAKE) -C $$dir all)\
	done

clean:
	rm -f *~ 
	for dir in $(SUBDIRS) ; do\
		($(MAKE) -kC $$dir clean)\
	done

$(SUBDIRS):%:
	$(MAKE) -C $@ all

$(patsubst %,%-clean,$(SUBDIRS)):%-clean:
	$(MAKE) -C $* clean

   

Makefile.in

CLASSES=$(FILES:.java=.class)

.PHONY: clean classes subdirs

all: classes subdirs

classes: $(CLASSES)
	@if [ -e to_compile ]; then $(JAVAC) $(JFLAGS) -classpath '$(CLASSPATH)' @to_compile; rm -f to_compile; fi

%.class: %.java
	@echo $< >> to_compile

clean:
	rm -f *.class *~
	@for DIR in $(SUB_DIRS); do $(MAKE) -C $$DIR clean; done

subdirs:
	@for DIR in $(SUB_DIRS); do $(MAKE) -C $$DIR; done

[David Newton]

A make file?!

Um... hmm. Interesting. Baffling, but interesting.

In any case, see where you pass CLASSPATH to javac? Put your classes in it.

[Vas Miriyala]

I compiled the program successfulyy but I was unable to use the values returned by the method which is in abstract class. I dont no whats wrong with this code ..

MovingObject.java

   
package traffic.object;
import java.io.*;
import java.util.*;
import traffic.*;
import rescuecore.OutputBuffer;
import rescuecore.RescueConstants;

public abstract class MovingObject extends RealObject implements Obstruction {
    public MovingObject(int id) {
        super(id);
    }

    private int m_position;
    private double m_positionExtra;
    // private int m_direction;
    // private int[] m_positionHistory;
    private Route m_routePlan;


    public void setRoutePlan(Route route) {
        m_routePlan = route;
    }
[b]// I just want to use values returned by this method[/b]
    public Route routePlan() {
	//	System.out.println("m_routePlan = " +m_routePlan);
        return m_routePlan;
    }

 

So I created one class which extends MovingObject.java and named as myMovingObject.java
But Its not calling any methods in superclass!!!
myMovingObject.java

package traffic;
import traffic.object.*;
import traffic.Route;
import traffic.object.MovingObject;

public class myMovingObject extends MovingObject {
    private Route m_routePlan;    
    
    public myMovingObject() {
	this(0);    
    }
    
    public myMovingObject(final int id) {
	super(id);
    }
    
[b]    // called this method in another program named mySimulator.java(see below ) Here I am calling the method which is in super class but its returning Null [/b]
    public void Hello() {
	MovingObject mv = new myMovingObject();
	m_routePlan = mv.routePlan();
	System.out.println(m_routePlan);
	
    }
    
    
    
    public int type() {
	return 12;
    }
    
}

mySimulator.java

package traffic;
import java.net.*;
import java.util.*;
import traffic.object.*;
import java.io.*;
import traffic.Route;
import traffic.object.MovingObject;
import rescuecore.InputBuffer;
import traffic.myMovingObject; 

public class mySimulator extends myMovingObject   implements Constants    {
  
    private final IO io;
    private int id;
        public mySimulator(InetAddress kernelAddress, int kernelPort) {
	io = new TCPIO(kernelAddress,kernelPort);
	io.sendConnect();
	id = io.receiveConnectOk();
	System.out.println("Connected. Simulator ID is " + id);
	io.sendAcknowledge();
	}


    
      public void simulate() {
	  System.out.println("start");

	  while(true) {
	      io.receiveCommands();
	      System.out.println("time " + WORLD.time());
	      move();
	      io.sendUpdate();
	      io.receiveUpdate();
	  }
      }

    public void move() {
	myMovingObject mv = new myMovingObject();
	mv.Hello();
	System.out.println(" working on that");
    }


}
    

Any Idea whats wrong with this???

[Rob Spoor]

srinivas miriyala wrote:Any Idea whats wrong with this???

1) Classes myMovingObject and mySimulator should start with a capital M.

2) mySimulator uses the constant interface anti-pattern.

3) your only assignment to m_routePlan occurs on line 21: "m_routePlan = mv.routePlan();". mv's own copy of m_routePlan has never been assigned though, so it still returns null. mv gets created on line 20. Its constructor does not set its m_routePlan field so it stays null. You then ask for it on line 21, and since it's null your object's m_routePlan field is set to null as well.

[Vas Miriyala]

thanks for your reply and what am I suppose to do to overcome this problem?

[Campbell Ritchie]

You will have to find out about the constant interface anti-pattern and alter your interfaces accordingly.
You will have to alter the constructor to make sure the route plan is instantiated.

[Rob Spoor]

The latter being the most important advice. The anti-pattern doesn't prevent your program from working, it's just bad style. And yes, Sun has used this bad style as well (in Swing among others. SwingConstants, sight...).

[Vas Miriyala]

Can somebody tell me the solution for my problem
Thanks in advance

[David Newton]

Campbell wrote:You will have to alter the constructor to make sure the route plan is instantiated.

[Vas Miriyala]

thanks for you reply and I didn't get it can you explain with example

[David Newton]

Instantiate (create, make a new one) the route plan in the constructor.

Rather than us just giving you the code, try something and see what you come up with--if you run into an issue, we can help with specific issues.

[Vas Miriyala]

You mean I need to crate one constructor then I need to instantiate the routeplan in this constructor right

[Rahul Kurup]

When you create a new object as

MovingObject mo = new Use();

constructor for Use() will be called which will in turn invoke the parent class constructor(MovingObject). Now MovingObject does not have a zero argument constructor. Also
Use does not have a constructor that calls super().

Could this be the problem? Experts please confirm.

[Vas Miriyala]

I have been working on this past two weeks but no progress and I really appriciate if somebody could sort the problem..experts I really Need your valuable suggestions thanks in advance

[David Newton]

I would *strongly* suggest starting a new thread, posting your complete code, and asking some very specific questions--this thread has gotten very difficult to follow, and I'm not even sure what problem you're having at this point.

Normally I wouldn't, but I'm going to lock this thread to make you start over. Before posting the new thread, I'd make sure your code is as clean as possible, formatted nicely, and so on. It'll also give you an opportunity to look over your existing code and figure out exactly what's wrong and what you need to ask. You may be able to answer your own question, too--sometimes taking a step back is very helpful.

Thanks!