scope of an Object + Vectors

[mikemcleanuk]

Hey have created a simple example of some object and vector code using 1 superclass and 1 sub-class.

The problem i have is the scope in which an object i assign has. When i replace the comment marks the code works fine but i get the compile error when i try to do the object calls outside of the IF statement.

For this example it wouldnt matter but i need a way to be able to use the objects methods from anywhere within the code after i have done the "instance of" assignements.

Thanks in Advance
Mike


import java.util.*;
import javax.swing.*;

public class Vectors
{
public static void main(String[] args)
{
Vector <Appliance> myAppliances = new Vector <Appliance>();

myAppliances.add(new Television(true, 1,1));

if (myAppliances.elementAt(0) instanceof Television)
{
Television tv1 = (Television)myAppliances.elementAt(0);
/*
System.out.println(tv1);
tv1.setPower(false);
System.out.println(tv1);
tv1.setPower(true);
System.out.println(tv1);
tv1.setChannel(2);
System.out.println(tv1);
tv1.setVolume(2);
System.out.println(tv1);
*/
}

System.out.println(tv1);
tv1.setPower(false);
System.out.println(tv1);
tv1.setPower(true);
System.out.println(tv1);
tv1.setChannel(2);
System.out.println(tv1);
tv1.setVolume(2);
System.out.println(tv1);
}
}



C:\JPR\ICA3\ICA3\Java\Demos>javac Vectors.java
Vectors.java:28: cannot find symbol
symbol : variable tv1
location: class Vectors
System.out.println(tv1);
^
Vectors.java:29: cannot find symbol
symbol : variable tv1
location: class Vectors
tv1.setPower(false);
^
Vectors.java:30: cannot find symbol
symbol : variable tv1
location: class Vectors
System.out.println(tv1);
^
Vectors.java:31: cannot find symbol
symbol : variable tv1
location: class Vectors
tv1.setPower(true);
^
Vectors.java:32: cannot find symbol
symbol : variable tv1
location: class Vectors
System.out.println(tv1);
^
Vectors.java:33: cannot find symbol
symbol : variable tv1
location: class Vectors
tv1.setChannel(2);
^
Vectors.java:34: cannot find symbol
symbol : variable tv1
location: class Vectors
System.out.println(tv1);
^
Vectors.java:35: cannot find symbol
symbol : variable tv1
location: class Vectors
tv1.setVolume(2);
^
Vectors.java:36: cannot find symbol
symbol : variable tv1
location: class Vectors
System.out.println(tv1);
^
9 errors

superclass:


import javax.swing.*;

public abstract class Appliance
{
public boolean power;
public ImageIcon picture;

public Appliance(ImageIcon ii)
{
picture = ii;
}

boolean getPower()
{
return power;
}

void setPower(boolean p)
{
power = p;
}

public abstract ImageIcon getPicture();

public String toString()
{
return "Appliance " + power;
}
}

sub-class:


import javax.swing.*;

public class Television extends Appliance
{
private int channel;
private int volume;

public Television()
{
super(new ImageIcon("../images/television.jpg"));
power = false;
channel = 1;
volume = 5;
}

public Television(boolean p, int c, int v)
{
super(new ImageIcon("../images/television.jpg"));
power = p;
channel = c;
volume = v;
}

public void setChannel(int C)
{
channel = C;
}

public int getChannel()
{
return channel;
}

public void setVolume(int V)
{
volume = V;
}

public int getVolume()
{
return volume;
}

public ImageIcon getPicture()
{
return picture;
}

public String toString()
{
return "Television " + power + " " + picture + " " + channel + " " + volume + "\n";
}
}

[dwhitney67]

The bottom line is that the variable tv1 is not accessible out of the if-block. It's lifetime expires as soon as the if-block concludes.

You can define an Appliance handle outside the if-block, then recast as needed within the if-block. Once the if-block is finished, use the Appliance handle for your set/get method calls.

Wrt printing a Television (or Applicance) object, you probably want to call the toString() method.