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This page contains an archived post to the Java Answers Forum made prior to February 25, 2002.
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Message:
Storing Objects in a Vector
Posted by Matt Gerrans on October 26, 2001 at 3:14 AM
Same message, better formatting (I edited my response in my editor, then pasted it in, not realizing the newlines would all get translated to <br> (html newlines), resulting in the less than aesthetically pleasing results you were just subjected to)
This is not just a simple matter of changing the Name member variable to not be static.Christian, the problem is, when you make a static variable, there is only one instance of that variable for the whole program, not one for each object. So the Name variable in the v1 variable is really the exact same Name as in v2. When you create v2 you are just changing this "global" value of VectorItem.Name. By making it not be static, there could be a different Name for every VectorItem object you create. A few other tips, by the way:
- The standard convention in Java for member variable names is to not capitalize the first character of the name, but do use capitalization to separate words in variables (for example "name" not "Name" and "itemName" not "ItemName."
- Use static sparingly, when you really do need a single variable to be shared by all instances of a class. It is more commonly used in combination with final, which results in a class constant.
- You don't have to use the this reference to access an object's variable or method, unless there is a name clash. An example of a name clash would be if your member variable was "name" and your constructor also took a parameter called "name." In that case "this.name = name;" would be necessary, since "name = name;" would be silly.
- You might want to use a List (with either ArrayList or LinkedList Collection object) instead of a Vector, unless you are using an old JDK or you need the thread safety of Vector. By the way, List has less verbose methods (add() instead of addElement() and so on) and Vector has been retrofitted to the List interface, so it can use those, too.
- The best way to go through all the items in a List (or Vector), is with an Iterator (see example, below); it is easy to use and will probably have much better performance than your own for-loop that accesses each item.
You can download Bruce Eckel's book Thinking in Java for free. Also, Sun has nice online tutorials. Based on the fact that you had everything as static, it looks like you may have more experience with a procedural language (C, or BASIC perhaps?) and you may want to get more acquainted with the object-oriented approach. Thinking in Java kind of takes the approach that the reader knows C and introduces Java from that perspective. Finally, here is what your code might look like if you applied some of the above suggestions (for simplicity, I put both classes in one file, which required that the ListItem not be public. However, your practice of having a separate java file for each class is actually best, especially as a project grows and the classes become more complex):
import java.util.*;class ListItem
{
private String name;
public ListItem(String itemName)
{
name = itemName;
}
public String getName()
{
return name;
}
} public class ListTest
{
static Random random = new Random(); // Initialize the list here or in the constructor,
// but not in main()!
private List itemList = new ArrayList();
public void show()
{
Iterator i = itemList.iterator(); while( i.hasNext() )
System.out.print(((ListItem)i.next()).getName() + " ");
} public void addSomeCharacters()
{
} public void addSomeShadyCharacters( int howMany )
{
for( int i = 0; i < howMany; i++ )
{
// This is broken up into many lines for clarity (it is
// possible to lump it all in one statement, but much
// harder to read).
char randomChar = (char)( 'a' + random.nextInt(26) );
Character shadyCharacter = new Character( randomChar );
String itemName = shadyCharacter.toString();
ListItem listItem = new ListItem( itemName );
itemList.add( listItem );
}
} public static void main( String [] args )
{
ListTest test = new ListTest(); int howMany = 10;
try
{
howMany = Integer.parseInt( args[0] );
}
catch( Exception e )
{
System.out.println( "Failed to read 'howMany' option " +
"from command line, defaulting to " +
howMany + "." );
} test.addSomeShadyCharacters( howMany );
test.show();
}
}
- mfg
>
> > > Hi there,> > > I tried to use a Vector to store some simple Objects. The problem is, that I don't get the correct global variables of the Objects back. (see source code)
> > > Instead of returning A and B, I just get B two times. I don't know the reason. I always get the name of the last added Object x the Vector size.
> > > I can't figure out the problem - but maybe you can. > > > Thanks for all answers, > > > Chris.
>
> > > public class VectorItem {
> > >
> > > private static String Name;
> > >
> > > public VectorItem(String ItemName) {
> > >
> > > this.Name = ItemName;
> > > }
> > >
> > > public String getName() {
> > >
> > > return this.Name;
> > > }
> > > }> > > --------------------------------------------------- > > > import java.util.Vector; > > > public class Test {
> > >
> > > private static Vector ObjectVector;
> > >
> > > public static void show() {
> > >
> > > for (int i = 0; i < ObjectVector.size(); i++) {
> > >
> > > System.out.println( ((VectorItem) ObjectVector.elementAt(i)).getName());
> > > }
> > > }
> > >
> > > public static void main(String [] argv) {
> > >
> > > VectorItem v1 = new VectorItem("A");
> > > VectorItem v2 = new VectorItem("B");
> > >
> > > ObjectVector = new Vector();
> > >
> > > ObjectVector.addElement(v1);
> > > ObjectVector.addElement(v2);
> > >
> > > show();
> > > }
> > > }
>
>
>
> > Change Name in the class VectorItem to be non-static i.e. instance variable.
Replies:
- hmmmm Chin Loong October 26, 2001 at 4:27 AM
(6)
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