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passing objects to constructors

Posts: 6
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I have a question about passing objects to constructors with different type object parameters.

I am using three classes:
The main class StringEditorGUI class allows me to enter two string objects which are passed to constructors (a default constructor and one with specified parameters) in a StringEditor class. These string objects then I beleive need to be passed to a linked list (MyList) class (that contains two constructors and together create two linked objects. So the problem I am having is that the MyList constructor with specified parameter is of the MyList type and the objects being sent are of the String type. Here is the abbreviate code. I need to understand how to pass the String objects from
the StringEditor (String left, String right) constructor to the
MyList(MyList lst) constructor.


public class StringEditorGUI extends JFrame implements ActionListener {

// create empty StringEditor
private StringEditor ed = new StringEditor();

static public void main(String [] args) {
new StringEditorGUI().setVisible(true);

public void actionPerformed(ActionEvent ae) {
Object objSource = ae.getSource();

if (objSource == buttonSet) {
String strSet = JOptionPane.showInputDialog("Please enter text to set to editor:");
if (strSet != null)
ed = new StringEditor(strSet, "");

} else if (objSource == buttonInsert) {
String strInsert = JOptionPane.showInputDialog("Please enter text to insert into " +
"editor at cursor position:");
for (int i = 0; strInsert != null && i < strInsert.length(); i++)

public class StringEditor {

/** Creates a new instance of StringEditor */
// Effect: StringEditor object is created
// Postcondition: no characters are in the string
public StringEditor() {

// Effect: StringEditor object is created
// Precondition: parameters strings are non-null
// Postcondition: characters from strings left and right are represented
// in the editor
public StringEditor(String left, String right) {


public class MyList {

private int numItems;
private Object item;
private MyList next;

/** Creates a new instance of MyList */
// Effect: new MyList object created
// Precondition: None
// Postcondition: an empty MyList object exists
public MyList() {
next = null;

// Effect: new MyList object created that is copy of lst
// Precondition: lst is not null
// Postcondition: an MyList object that is identical to lst exists
public MyList(MyList lst) {
next = lst;

Ranch Hand
Posts: 1282
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Aside from the fact that you need to move your code inside the code tags,   MyList(MyList lst)   suggests to me that you are stuck on getting going. Giving you a direct answer will probably further drill you in. Let me ask you some questions:

  • What does the list contain ? Strings ?
  • Is there a base class for MyList that would serve as a list container ?

  • To pass an item / an object, from one constructor to the next suggests a class hirearchy, which is done by immediately calling super() as the first line in the constructor. You are providing contract statements of what the constructor Preconditions / Postconditions are, which sounds to me like you being instructed by someone who has had to disentangle this stuff many times. I suggest you focus on basics for a little while until you can fulfil conditions.

    For one thing, there is a rich toolkit in java Collections, try looking in there for something. If it is a class and not an interface, you can make it a base class for MyList and simplifiy some of your work. By keeping a reference to next in your MyList class, you may be able to construct a list. One of the OO ranchers will have to help you with that.

    I suggest not having a default constructor if the intent of the code is to pass two string objects ... but to answer your question:

    I have not tried to do a linked list in any programming language, I always go to a map or hashtable, but since you are trying to do a list, try something like LinkedList<E> as your base class for MyList. That is gonna be awful steep for a beginner, but no worse than trying to provide contract pre/post conditions without knowing how to get them fulfilled.
    You showed up just in time for the waffles! And this tiny ad:
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