不能使用getSelectedIndex()因為串列為空-有解無憂

我正在研究一個學校專案，并試圖獲取串列中所選專案的位置和值。嘗試這樣做時，我收到錯誤“無法呼叫“javax.swing.JList.getSelectedIndex()”，因為“this.newList”為空”。我很確定這與單元渲染器實際上并未將專案添加到串列中有關，但我不確定如何修復它。


class OrderHandlePanel extends JPanel implements ActionListener {

private DefaultListModel newModel, cancelModel;
private JButton right, left;
private JList newList, cancelledList;

OrderHandlePanel() {

        newModel = new DefaultListModel();

        setLayout(new GridLayout(1,3));
        JPanel newOrders = new JPanel(new GridLayout(3,1));
        JLabel newOrder = new JLabel("New Order");
        JLabel orderAmount = new JLabel("Ordered Amt: ");
        JList newList = new JList(newModel);
        newList.setCellRenderer(new listCellRenderer());
        JScrollPane newScrollPane = new JScrollPane(newList);

        JPanel buttons = new JPanel();
        JButton right = new JButton("=>");
        JButton left = new JButton("<=");

        newOrders.add(newOrder);
        newOrders.add(newScrollPane);
        newOrders.add(orderAmount);
        right.addActionListener(this);
        left.addActionListener(this);
        right.setActionCommand("moveRight");
        left.setActionCommand("moveLeft");
        buttons.add(right);
        buttons.add(left);

        add(newOrders);
        add(buttons);
}

    private class listCellRenderer extends DefaultListCellRenderer {
        public Component getListCellRendererComponent(JList list, Object value, int index, boolean isSelected, boolean cellHasFocus) {
            super.getListCellRendererComponent(list, value, index, isSelected, cellHasFocus);
            Order order = (Order) value;
            String orderText = "<html>Name: "   order.getName() 
                  "<br/>Quantity: "   order.getQuantity() 
                  "<br/>Price"   order.getPrice() 
                  "<br/>Total Cost: "   order.getTotalCost()
                  "<br/> ";
            setText(orderText);
            return this;
        }
    }

    public void update() {
        newModel.clear();
        cancelModel.clear();
        for (byte i = 0; i < OrderContainer.getPosition(); i  ) {
            Order order = OrderContainer.getOrder(i);
            if (order.getType().equals("New"))
                newModel.addElement(order);
            else if (order.getType().equals("Cancelled"))
                cancelModel.addElement(order);
        }
    }
    
    @Override
    public void actionPerformed(ActionEvent e) {
        String actionCommand = ((JButton) e.getSource()).getActionCommand();
        
        switch (actionCommand) {
        case "moveRight":
            int newIndex = newList.getSelectedIndex();
            System.out.println(newList.getSelectedValue());
            break;
        case "moveLeft":
            int cancelIndex = cancelledList.getSelectedIndex();
            System.out.println(cancelledList.getSelectedValue());
            break;
        }
    }
}

uj5u.com熱心網友回復：

從 JList newList 中洗掉 Jlist。當您想將類變數設定為該新物件時，您正在定義一個區域變數。

因此

JList newList = new JList(newModel);

會變成

newList = new JList(newModel);

轉載請註明出處，本文鏈接：https://www.uj5u.com/ruanti/326956.html

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