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我應該修改程序來在GUI中顯示輸出。這主要是完成，除了一個領域。我無法展示的唯一領域是「補貨費」，它在DVD的子類中。 Inventory是主類，那麼DVD類和MovieTitle是DVD的子類。除了MovieTitle子類中的restockingFee之外，所有內容都可以正確顯示。例外情況是：如何將對象轉換爲不從其繼承的類型？
Exception in thread "main" java.lang.ClassCastException: inventoryProgram3.DVD cannot be cast to inventoryProgram3.MovieTitle
at inventoryProgram3.Inventory.myRedraw(Inventory.java:204)
at inventoryProgram3.Inventory.<init>(Inventory.java:149)
at inventoryProgram3.Inventory.main(Inventory.java:156)
如何從子類中調用restockfee方法？
這是Inventory類：
package inventoryProgram4;
import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.text.DecimalFormat;
import java.text.NumberFormat;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
@SuppressWarnings("serial")
public class Inventory extends JFrame implements ActionListener {
// declare instance attributes
private final JPanel buttonJPanel;
private final JPanel areaJPanel;
private final JButton buttons[];
private final BorderLayout layout;
private final Container pane;
private final JLabel l0;
private final JLabel l1;
private final JLabel l2;
private final JLabel l3;
private final JLabel l4;
private final JLabel l5;
private final JLabel totalInv;
// Text fields
static private JTextField t0;
static private JTextField t1;
static private JTextField t2;
static private JTextField t3;
static private JTextField t4;
static private JTextField t5;
static private JTextField totalInvValue;
static private DVD rb[];
static private int index;
static private DVD[] DVD;
public Inventory() { // constructor
super("DVD Inventory");
DVD = new DVD[5];
DVD[0] = new DVD("1", " Fast and Furious 6", 10, 15);
DVD[1] = new DVD("2", " The Matrix Reloaded", 8, 5);
DVD[2] = new DVD("3", " In Pursuit of Happiness", 8, 5);
DVD[3] = new DVD("4", " Darknet", 8, 5);
DVD[4] = new DVD("5", " Goonies", 8, 5);
DVD = sortDVD(DVD);
rb = DVD;
index = 0;
// setting layout
layout = new BorderLayout(25, 15);
setLayout(layout);
// getting pane
pane = getContentPane();
// set up areaJPanel
areaJPanel = new JPanel();
areaJPanel.setLayout(new GridLayout(0, 2));
// create area components
l0 = new JLabel(" DVD Title: ");
l1 = new JLabel(" Title Name: ");
l2 = new JLabel(" DVD's in Stock: ");
l3 = new JLabel(" DVD Price: ");
l4 = new JLabel(" Restock Fee: ");
l5 = new JLabel(" Inventory value: ");
totalInv = new JLabel(" Value of entire inventory: ");
totalInvValue = new JTextField(10);
totalInvValue.setEditable(false);
t0 = new JTextField(15);
t0.setEditable(false); // making text box non editable
t1 = new JTextField(15);
t1.setEditable(false);
t2 = new JTextField(10);
t2.setEditable(false);
t3 = new JTextField(10);
t3.setEditable(false);
t4 = new JTextField(10);
t4.setEditable(false);
t5 = new JTextField(10);
t5.setEditable(false);
// adding components
areaJPanel.add(l0); // n x 2 grid: label - textfield
areaJPanel.add(t0);
areaJPanel.add(l1);
areaJPanel.add(t1);
areaJPanel.add(l2);
areaJPanel.add(t2);
areaJPanel.add(l3);
areaJPanel.add(t3);
areaJPanel.add(t4);
areaJPanel.add(l4);
areaJPanel.add(t4);
areaJPanel.add(l5);
areaJPanel.add(t5);
areaJPanel.add(totalInv);
areaJPanel.add(totalInvValue);
// Adding four buttons: first, next, previous, last
// create array of buttons, size 4
buttons = new JButton[4];
// create a JPanel for the buttons
buttonJPanel = new JPanel();
// set panel layout to 1 row and columns = size of array
buttonJPanel.setLayout(new GridLayout(1, buttons.length));
// adding button "next"
buttons[1] = new JButton("Previous"); // instantiate button "next"
buttons[1].setActionCommand("Previous"); // identifies the action event
buttons[1].addActionListener(this); // add an action listener for button
buttonJPanel.add(buttons[1]); // add button to the button JPanel
// adding button "previous"
buttons[2] = new JButton("Next"); // instantiate button "previous"
buttons[2].setActionCommand("next"); // identifies the action event
buttons[2].addActionListener(this); // add an action listener for button
buttonJPanel.add(buttons[2]); // add button to the button JPanel
pane.add(buttonJPanel, BorderLayout.SOUTH); // place buttons at bottom of pane
pane.add(areaJPanel, BorderLayout.WEST);
myRedraw(); // showing first item
}// end constructor
public static void main(String args[]) {
// creating frame
Inventory panelFrame = new Inventory();
// close the frame on clicking X button
panelFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
// setting size
panelFrame.pack();
// setting visibility
panelFrame.setVisible(true);
}// end main method
static DVD[] sortDVD(DVD[] DVD) {
int in, out, NumOfEle = DVD.length;
for (out = 1; out < NumOfEle; out++) {
DVD temp = DVD[out];
in = out;
while (in > 0 && DVD[in - 1].getTitle().compareTo(temp.getTitle()) > 0) {
DVD[in] = DVD[in - 1];
--in;
}
DVD[in] = temp;
}
return DVD;
}
private static double calculateEntireInventory(DVD[] DVD1) {// method to return inventory total
double totalInventory = 0;
for (DVD DVD : DVD1) {
totalInventory += (DVD.getInventoryValue());
}
return totalInventory;
}
// method to display contents
static void myRedraw() {
// currency formatter
NumberFormat nf = new DecimalFormat("$#,##0.00");
// reference to object under consideration
DVD f = (DVD) rb[index];
double inventoryValue = f.getInventoryValue();
// set values for indicated object attribute
t0.setText(f.getTitle()); // item name
t1.setText(f.getItem() + ""); // item number
t2.setText(f.getStock() + ""); // item quantity
t3.setText(nf.format(f.getPrice())); // item price
// Problem here
/**
* Exception in thread "main" java.lang.ClassCastException: inventoryProgram3.DVD cannot be
* cast to inventoryProgram3.MovieTitle at
* inventoryProgram3.Inventory.myRedraw(Inventory.java:204) at
* inventoryProgram3.Inventory.<init>(Inventory.java:149) at
* inventoryProgram3.Inventory.main(Inventory.java:156)
**/
MovieTitle t = (MovieTitle) f;
t4.setText(nf.format(t.getRestockingFee())); // item re-stocking fee
// up above
t5.setText(nf.format(inventoryValue));
// up above
// total value of inventory
totalInvValue.setText(nf.format(calculateEntireInventory(DVD)));
}// end method
// action handler for button presses
public void actionPerformed(ActionEvent event) {
int last = rb.length - 1;
// get command string assigned to the pressed button.
String action = event.getActionCommand();
// set index based on which button was pressed.
if (action.equals("prev")) { // previous pressed
if (index == 0) {
index = last; // at first item, go to last item
} else {
--index; // go to previous item
}
} else {// next pressed
if (index == last) {
index = 0; // at last item, go to first time
} else {
++index; // go to next item
}
}
myRedraw();
}// end method
} // end program
這是DVD類：
package inventoryProgram3;
class DVD {
// Create local variable to capture data from parameter list of the constructor
private String thisItem;
private String thisTitle;
private double thisStock;
private double thisPrice;
// Create constructor to call this class from external classes
public DVD(String item, String title, double stock, double price) {
// Set local variables to values passed in via a class call
thisItem = item;
thisTitle = title;
thisStock = stock;
thisPrice = price;
} // end constructor
// getInventoryValue: calculates the total value of the inventory for the item
public double getInventoryValue() {
return getPrice() * getStock();
}
// set name
public void setTitle(String title) {
thisTitle = title;
}// end method setTitle
// return Title
public String getTitle() {
return thisTitle;
}// end method getTitle
// set Stock
public void setStock(double stock) {
thisStock = stock;
}// end method setStock
// return Stock
public double getStock() {
return thisStock;
}// end method get Stock
public void setPrice(double price) {
thisPrice = price;
}// end method setPrice
// return Price
public double getPrice() {
return thisPrice;
}// end method getPrice
public void setItem(String item) {
thisItem = item;
}// end method setItem
// return Item
public String getItem() {
return thisItem;
}// end method getItem
// calculate the inventory value
public double value() {
return thisPrice * thisStock;
}// end method value
}// end class
這是MovieTitle類：
package inventoryProgram3;
public class MovieTitle extends DVD {
private final double restockingFee = 0.05;
public MovieTitle(String item, String title, double stock, double price) {
super(item, title, stock, price);
}
public double getRestockingFee() {
return getPrice() * restockingFee;
}
}
可能重複http://stackoverflow.com/問題/ 4862960 /從超級類到子類的顯式投射） – Madhan