Array that Holds JButton Objects

问题:

Ok, so I'm attempting to do an exercise from a book I'm using to learn Java. Here is the code that I have so far:

import javax.swing.*;
import java.awt.GridLayout;
import java.awt.BorderLayout;
public class Calculator {
    //Declaration of all calculator's components.
    JPanel windowContent;
    JTextField displayField;
    JButton button0;
    JButton button1;
    JButton button2;
    JButton button3;
    JButton button4;
    JButton button5;
    JButton button6;
    JButton button7;
    JButton button8;
    JButton button9;
    JButton buttonPoint;
    JButton buttonAdd;
    JButton buttonEqual;
    JPanel pl;

    //Constructor creates the components in memory and adds the to the frame using combination of Borderlayout.
    Calculator() {
        windowContent= new JPanel();

    // Set the layout manager for this panel
        BorderLayout bl = new BorderLayout();
        windowContent.setLayout(bl);

    //Create the display field and place it in the North area of the window
        displayField = new JTextField(30);
        windowContent.add("North",displayField);

    //Create button field and place it in the North area of the window
        button0=new JButton("0");
        button1=new JButton("1");
        button2=new JButton("2");
        button3=new JButton("3");
        button4=new JButton("4");
        button5=new JButton("5");
        button6=new JButton("6");
        button7=new JButton("7");
        button8=new JButton("8");
        button9=new JButton("9");
        buttonAdd=new JButton("+");
        buttonPoint = new JButton(".");
        buttonEqual=new JButton("=");

    //Create the panel with the GridLayout that will contain 12 buttons - 10 numeric ones, and button with the points
    //and the equal sign.
        pl = new JPanel ();
        GridLayout gl =new GridLayout(4,3);
        pl.setLayout(gl);
    //Add window controls to the panel pl.
        pl.add(button1);
        pl.add(button2);
        pl.add(button3);
        pl.add(button4);
        pl.add(button5);
        pl.add(button6);
        pl.add(button7);
        pl.add(button8);
        pl.add(button9);
        pl.add(buttonAdd);
        pl.add(buttonPoint);
        pl.add(buttonEqual);

    //Add the panel pl to the center area of the window
        windowContent.add("Center",pl);
    //Create the frame and set its content pane
        JFrame frame = new JFrame("Calculator");
        frame.setContentPane(windowContent);
    //set the size of the window to be big enough to accomodate all controls.
        frame.pack();
    //Finnaly, display the window
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        Calculator calc = new Calculator();
    }
}

Here is the exercise in exact wording:

Modify the class Calculator.java to keep all numeric buttons in the 10-element array declared as follows:

Buttons[] numButtons= new Buttons[10];

Replace 10 lines that start from

button0=new JButton("0");

with a loop that creates the buttons and store them in this array.

Ok, so I tried declaring the array with the Buttons[] numbuttons line, but that just gave me the error:

Multiple markers at this line
-Buttons can not be resolved to a type
-Buttons can not be resolved to a type

I instead tried this:

JButton[] buttons = new JButton[10]

And then added each button to the array like this:

buttons[0] = "button0";

Doing this didn't give me an error when I declared the array, but when I wrote the buttons[0] line, I got this error:

Syntax error on token "buttons",delete this token

So, I need help figuring out how to do this. Also, the book can be found here: http://myflex.org/books/java4kids/JavaKid811.pdf and the practice is on page 73. I apologize if I'm listing to much information. It's just because I'm very new to Java and I'm not sure what is necessary. Help is appreciated. Thanks.

回答1:

What you are doing is trying to set the array space to a string when you need a JButton.

You should be doing this

buttons[0] = new JButton("0");

instead of

buttons[0] = "button0";

EDIT:

I just did this

import javax.swing.*;

public class test {

    public static void main(String[] args) {
        JButton[] buttons = new JButton[10];

        buttons[0] = new JButton("0");

        System.out.println(buttons[0].getText());
    }

}

and got

0 

for an output so your error isn't in that line.

EDIT: Code

Calculator.java

import javax.swing.*;
import java.awt.GridLayout;
import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class Calculator {
    //Declaration of all calculator's components.
    JPanel windowContent;
    JTextField displayField;
    JButton buttons[];
    JButton buttonPoint;
    JButton buttonAdd;
    JButton buttonEqual;
    JPanel pl;

    //Constructor creates the components in memory and adds the to the frame using combination of Borderlayout.
    Calculator() {
        windowContent= new JPanel();
        buttons = new JButton[10];

    // Set the layout manager for this panel
        BorderLayout bl = new BorderLayout();
        windowContent.setLayout(bl);

    //Create the display field and place it in the North area of the window
        displayField = new JTextField(30);
        windowContent.add("North",displayField);

    //Create button field and place it in the North area of the window
        for(int i = 0; i < 10; i++) {
            buttons[i] = new JButton(String.valueOf(i));
        }

        buttonAdd=new JButton("+");
        buttonPoint = new JButton(".");
        buttonEqual=new JButton("=");

    //Create the panel with the GridLayout that will contain 12 buttons - 10 numeric ones, and button with the points
    //and the equal sign.
        pl = new JPanel ();
        GridLayout gl =new GridLayout(4,3);
        pl.setLayout(gl);
    //Add window controls to the panel pl.

        for(int i = 0; i < 10; i++) {
            pl.add(buttons[i]);
        }
        pl.add(buttonAdd);
        pl.add(buttonPoint);
        pl.add(buttonEqual);

    //Add the panel pl to the center area of the window
        windowContent.add("Center",pl);
    //Create the frame and set its content pane
        JFrame frame = new JFrame("Calculator");
        frame.setContentPane(windowContent);
    //set the size of the window to be big enough to accomodate all controls.
        frame.pack();
    //Finnaly, display the window
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        Calculator calc = new Calculator();
    }
}

回答2:

JButton[] buttons = new JButton[10]

The line above is correct, but I see two points of confusion:

1. This line:

   buttons[0] = "button0";
   

   should instead be as follows:

   buttons[0] = new JButton("button0");
   

   The reason is that in your code, you're trying to assign a String to buttons[0], instead of the expected JButton.

2. Ok, so I tried declaring the array with the Buttons[] numbuttons line, but that just gave me the error: Multiple markers at this line -Buttons can not be resolved to a type -Buttons can not be resolved to a type

   Buttons is not a standard Java class. Do a case-sensitive search for Buttons and replace all matches with JButton.

If you still have problems, please copy and paste the exact code for each variation that is not working.

回答3:

If I understand your problem, you need the loop to instantiate and store the JButtons.

for (int i=0; i<10; i++) {
    numButton[i] = new JButton(String.valueOf(i));
}

You need to convert the loop control variable into a String argument for the JButton constructor.

回答4:

you should use

buttons[0] = button0;

and not

buttons[0] = "button0";