java中的数组问题

Question

Hello I'm making a program that tracks the number of users that are logged on by entering names through textfield0. I hit a road block being new to java I still don't understand some things I was wondering how one would add names to the array User in real time and have the array be count it show it in textfield1.

Thank you in advance.

public class UserTracking {


    public static void main(String[] args){
        final Kong f = new Kong();
        f.pack();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setSize(200,110);
        f.setVisible(true);

    }


}
class Kong extends JFrame implements  ActionListener
{
    JTextField textfield0 = new JTextField();
    JTextField textfield1 = new JTextField();
    JLabel  label0 = new JLabel("User");
    JLabel  label1 = new JLabel("Number of Users:");
    JButton btnon = new JButton("Log on");
    JButton btnoff = new JButton("Log off");
    String User[] ={};


    public Kong()
    {
        super("Shazam");

    JPanel panel1 = new JPanel(new BorderLayout());
        panel1.add(textfield0,BorderLayout.CENTER);
        panel1.add(label0,BorderLayout.WEST);
    JPanel panel2 = new JPanel(new BorderLayout(10,10));
        panel2.add(btnon,BorderLayout.WEST);
        panel2.add(btnoff,BorderLayout.EAST);
    JPanel panel3 = new JPanel(new BorderLayout());
        panel3.add(textfield1,BorderLayout.CENTER);
        panel3.add(label1,BorderLayout.WEST);



    JPanel frame =  new JPanel(new BorderLayout(5,5));
        frame.add(panel1,BorderLayout.NORTH);
        frame.add(panel2,BorderLayout.EAST);
        frame.add(panel3,BorderLayout.SOUTH);
        setContentPane(frame);
    }

    public void actionPerformed(ActionEvent e)
    {

    }
}

Answer 1

If you've had experience with C++, think of Java arrays as C++ arrays in that once their size is defined, it stays that way. If you wish to increase the capacity, you need to recreate the array and add the contents of the old array inside. Essentially this is what vector did.

In Java if you want an expandable array, you essentially want a List object. I would strongly encourage you to use an ArrayList which is essentially the equivalent of vector in Java, allowing you to add as many objects as you want without worrying about its capacity.

If you ever need an array, you can convert it using the toArray() method (though in my experience, ArrayList does everything you'd require).

Answer 2

The size of an array in java can't be changed after the first initialization.

If you want a dynamic sized data container, you can use any implementation of the List interface. For example ArrayList :

ArrayList<String> user = new ArrayList<String>();

And to add each user to the ArrayList :

user.add(username);

Answer 3

使用ArrayList 。

Answer 4

你需要的不是一个数组，而是一个更灵活的数据类型（能够“增长”大小），就像一些List实现一样：ArrayList实际上是由一个数组支持的，但是你不需要自己管理（重新）分配。

Answer 5

You are also missing linking buttons to ActionListener. Once that is done, you could use ArrayList to add users to the list as they login and remove users from list as they logoff.

private ArrayList<String> loggedOnUsers = new ArrayList<String>();

and

void actionPerformed(event)
{
    if(it's logOn button action)
    {
        add user to loggedOnUsers list
    }
    else
    {
       remove user from loggedOnUsers list
    }
}

Answer 6

Instead of taking String array you can take ArrayList it would be easier. Something like this

ArrayList<String> user = new ArrayList<String>();

On some action like button or text change event write this

user.add(textField1.getText());

Answer 7

The actionPerformed method should

• add a string into the User array (which should be named users )
• ask the array its new length
• change the value in the count textfield ( textfield1 , which really has a bad name)

The problem is that Java arrays have a fixed length. They can't grow. That'w why java.util.ArrayList exists : it's a class that behaves like a dynamic-length array. Read its javadoc to know how to use it.