Java, JTextField .equals

Question

我正在尝试比较 JTextField 中的值作为验证部分，以确保如果所有 TextField 都没有值，用户将无法保存进度。

这是我的文本字段方法。

public void DrawBookAttributes() {
    lblBookName = new JLabel("Book Name :");
    lblBookName.setFont(getLblFont());
    lblBookName.setSize(140, 20);
    lblBookName.setLocation(280, 150);
    menuFrame.add(lblBookName);

    tfBookName = new JTextField();
    tfBookName.setSize(140, 20);
    tfBookName.setLocation(420, 150);
    menuFrame.add(tfBookName);

    lblBookID = new JLabel("Book ID :");
    lblBookID.setFont(getLblFont());
    lblBookID.setSize(140, 20);
    lblBookID.setLocation(280, 50);
    menuFrame.add(lblBookID);

    tfBookID = new JTextField();
    tfBookID.setSize(140, 20);
    tfBookID.setLocation(420, 50);
    tfBookID.setText("hello");
    tfBookID.setEditable(false);
    menuFrame.add(tfBookID);

    JLabel lblBookCost = new JLabel("Cost (£) :");
    lblBookCost.setFont(getLblFont());
    lblBookCost.setSize(140, 20);
    lblBookCost.setLocation(280, 200);
    menuFrame.add(lblBookCost);

    tfBookCost = new JTextField();
    tfBookCost.setSize(140, 20);
    tfBookCost.setLocation(420, 200);
    tfBookCost.addKeyListener(this);
    menuFrame.add(tfBookCost);

    lblBookYearOfPublication = new JLabel("Publication Date :");
    lblBookYearOfPublication.setSize(140, 20);
    lblBookYearOfPublication.setLocation(280, 250);
    lblBookYearOfPublication.setFont(getLblFont());
    menuFrame.add(lblBookYearOfPublication);

    JTextField tfBookYearOfPublication = new JTextField();
    tfBookYearOfPublication.setSize(140, 20);
    tfBookYearOfPublication.setLocation(420, 250);
    tfBookYearOfPublication.setText("DD-MM-YYYY");
    tfBookYearOfPublication.addActionListener(this);
    menuFrame.add(tfBookYearOfPublication);

    JLabel lblbookPublishngHouse = new JLabel("Publishing Orgaisation :");
    lblbookPublishngHouse.setSize(140, 20);
    lblbookPublishngHouse.setLocation(280, 300);
    lblbookPublishngHouse.setFont(getLblFont());
    menuFrame.add(lblbookPublishngHouse);

    JTextField tfBookPublishingHouse = new JTextField();
    tfBookPublishingHouse.setSize(140, 20);
    tfBookPublishingHouse.setLocation(420, 300);
    menuFrame.add(tfBookPublishingHouse);

    JLabel lblBookISBN = new JLabel("ISBN :");
    lblBookISBN.setSize(140, 20);
    lblBookISBN.setLocation(280, 350);
    lblBookISBN.setFont(getLblFont());
    menuFrame.add(lblBookISBN);

    JTextField tfBookISBN = new JTextField();
    tfBookISBN.setSize(140, 20);
    tfBookISBN.setLocation(420, 350);
    menuFrame.add(tfBookISBN);

    JLabel lblBookPages = new JLabel("Number of Pages :");
    lblBookPages.setSize(140, 20);
    lblBookPages.setLocation(280, 400);
    lblBookPages.setFont(getLblFont());
    menuFrame.add(lblBookPages);

    JTextField tfBookPages = new JTextField();
    tfBookPages.setSize(140, 20);
    tfBookPages.setLocation(420, 400);
    menuFrame.add(tfBookPages);

    menuFrame.setVisible(true);
}

这是验证部分

    public void EntryCheck() {

        if (tfBookName.getText().equals("")) {
            EntryChecker++;
            System.out.println(EntryChecker);
        }
        if (tfBookISBN.getText().equals("")) {
            EntryChecker++;
        }
        if (tfBookCost.getText().equals("")) {
            EntryChecker++;
        }
        if (tfBookPages.getText().equals("")) {
            EntryChecker++;
        }
        if (tfBookPublishingHouse.getText().equals("")) {
            EntryChecker++;
        }
        if (tfBookYearOfPublication.getText().equals("")) {
            EntryChecker++;
        }
        if (tfBookCost.getText().equals("DD-MM-YYY")) {
            EntryChecker++;
        }
        System.out.println(EntryChecker);
}

EntryChecker 是一个整数，每次当用户离开 TextField 而没有在其中输入信息时，我都会为其添加一个值。

每次调试程序时，我都会在此验证行中遇到 NullPointerException。

if (tfBookISBN.getText().equals("")) {
        EntryChecker++;

有人知道问题出在哪里以及如何解决吗？

谢谢！

Answer 1

如果-如您在评论中所说-您有

private JTextField tfBookISBN = new JTextField();

global 范围在你的 class，然后你必须删除那行

JTextField tfBookISBN = new JTextField();

来自 DrawBookAttributes()

如果它在声明时没有初始化（我认为它没有），你需要在 DrawBookAttributes() 但中保留该行作为初始化，而不是声明，如下所示:

tfBookISBN = new JTextField();

因为您正在该方法中创建一个新字段，并使全局字段未初始化，这会在 EntryCheck() 尝试访问该对象以调用 getText().

时创建 NPE

Java, JTextField .equals

Java, JTextField .equals

java

user-interface

swing

jtextfield