在封闭作用域中定义的局部变量迭代必须是final或实际上是final

Question

public static void go(purchase joe) {
    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    SpinnerNumberModel NumberImput = 
            new SpinnerNumberModel(1.00,0.00,null,1.00);
    int iteration = 0;
    JSpinner spinner = new JSpinner(NumberImput);
    JLabel label = new JLabel("Please enter the price of the Item.");
    JButton button = new JButton("Press here to continue");
       button.addActionListener(new ActionListener(){  
            public void actionPerformed(ActionEvent e){  
                switch (iteration) {
                case 0:
                    joe.setPrice((double)NumberImput.getNumber());
                    label.setText("Plese enter the amount that you are buying");
                    NumberImput.setValue(1.00);
                    iteration += 1;
                    break;
                case 1:
                    joe.setAmount((int)NumberImput.getNumber());
                    label.setText("Plese enter the amount that you are buying");
                    break;
                
                
                
                }
            }  
            });  
    frame.setLayout(new FlowLayout());
    frame.add(label);
    frame.add(spinner);
    frame.add(button);
    frame.setSize(600, 500);
    frame.setExtendedState(JFrame.MAXIMIZED_BOTH); 
    frame.setVisible(true);
    joe.setPrice((double)NumberImput.getNumber());
}

Answer 1

您正在使用迭代+=1；。这是对局部变量的赋值。现在ActionListener对象的寿命比局部变量长；actionPerformed可以在方法的调用消失后调用。

为此，Java虚拟机向ActionListener添加了一个新变量iteration，即副本。为了防止分配给局部变量迭代会使另一个迭代过时，语言决定局部变量必须是常量，而不是分配给“有效的final”。

AtomicInteger iteration = new AtomicInteger();

... iteration.getAndIncrement();

但是，由于ActionPerformed，所以只需删除Iteration。您可以在ActionListener中使用一个字段。

    button.addActionListener(new ActionListener() {  
        int iter;

        @Override
        public void actionPerformed(ActionEvent e) {  
            switch (iter) {
            case 0:
                joe.setPrice(NumberImput.getNumber().doubleValue());
                label.setText("Please enter the amount that you are buying");
                NumberImput.setValue(1.00);
                iter++;
                break;
            case 1:
                joe.setAmount(NumberImput.getNumber().intValue());
                label.setText("Please enter the amount that you are buying");
                break;
            }
        }  
    });  

        @Override
        public void actionPerformed(ActionEvent e){