JTextField.setText() 在方法中不能作为数组工作

Question

这里是新的 Java 程序员，所以我在这里可能有一个基本问题。我正在制作一个 JTextField 数组。我想在公共方法中在类之外使用 setText，但它不起作用。但是，我可以在类中使用 setText（而不是在方法中）。我不确定为什么。这是一些作为 SSCCE 的代码，以显示我正在经历的事情。 导入 java.awt.BorderLayout； 导入 javax.swing.*;

public class testSetText extends JFrame 
{
    private JPanel          panel;
    private JTextField[]    arrayField;
    private JTextField      singleField;

    public testSetText()
    {
        // Specify an action for close button
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        // Make a panel
        panel = new JPanel();

        // Make array of JTextField components
        JTextField[] arrayField = new JTextField[2];
        for (int i = 0; i < 2; i++)
        {
            arrayField[i] = new JTextField(10);
            arrayField[i].setEditable(false);
            arrayField[i].setText("<>!");
            panel.add(arrayField[i]);
        }

        // Make just one JTextField component
        singleField = new JTextField(10);
        singleField.setText("Works here");
        panel.add(singleField);

        // Add to panel to frame
        add(panel);

        // Pack the contents of the window and display it
        pack();
        setVisible(true);

        // This will work!
        arrayField[0].setText("Array index in class");

        // This won't? Why?
        setInMethodWithArray();

        // Is this a problem with JTextField itself? Let me try a single element
        setInMethodWithSingleElement(); 

        // Hmmm so an element in an array of JTextFields can be addressed with setText in a class but not 
        // in a method in same class with same statement. But a single JTextField can be used in a method
        // by that same class. Why do arrays behave so differently?

        // EDIT: So I misunderstood, it does not work with a non array as well either!!
    }

    public void setInMethodWithArray()
    {
        arrayField[1].setText("This text will never show up");
    }

    public void setInMethodWithSingleElement()
    {
        //singleField.setText("Works here as non-array"); <--- before edit
        singleField.setText("nevermind, it does not work here either");
    }

    public static void main(String[] args)
    {
        new testSetText();
    }
}

Answer 1

您应该在类区域中声明arrayField[]，以便可以从setInMethodWithArray() 方法访问数组字段。

JTextField[] arrayField = new JTextField[2]; //class area

在构造函数中初始化它

 arrayField[i]= new JTextField(10);
 arrayField[i].setEditable(false);

---

是的，这是工作

arrayField[0].setText("Array index in class");

因为 arrayField 在构造函数范围内..并且您正在构造函数中访问 arrayField .所以它可以工作..

不会吧？为什么？

setInMethodWithArray();

因为方法setInMethodWithArray() 不能访问arrayField[] 因为它不在方法范围或类范围内。那是因为你已经在构造函数中声明了它，所以在构造函数代码块之后执行arrayField 不存在。它参考丢失，因为它是local variable ..

public void setInMethodWithArray()
{
    arrayField[1].setText("This text will never show up");
}

现在 set 可以访问 arratField[] 所以现在你的代码可以工作了

import java.awt.BorderLayout;
import javax.swing.*;

public class testSetText extends JFrame 
{
    private JPanel          panel;
    private JTextField      singleField;
    // Make array of JTextField components
    private JTextField[] arrayField = new JTextField[2];

    public testSetText()
    {
        // Specify an action for close button
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        // Make a panel
        panel = new JPanel();

        for (int i = 0; i < 2; i++)
        {
            arrayField[i] = new JTextField(10);
            arrayField[i].setEditable(false);
            arrayField[i].setText("<>!");
            panel.add(arrayField[i]);
        }

        // Make just one JTextField component
        singleField = new JTextField(10);
        singleField.setText("Works here");
        panel.add(singleField);

        // Add to panel to frame
        add(panel);

        // Pack the contents of the window and display it
        pack();
        setVisible(true);

        // This will work!
        arrayField[0].setText("Array index in class");

        // This won't? Why?
        setInMethodWithArray();

        // Is this a problem with JTextField itself? Let me try a single element
        setInMethodWithArray(); 

        // Hmmm so an element in an array of JTextFields can be addressed with setText in a class but not 
        // in a method in same class with same statement. But a single JTextField can be used in a method
        // by that same class. Why do arrays behave so differently?
    }

    public void setInMethodWithArray()
    {
        arrayField[1].setText("This text will never show up");
    }

    public void setInMethodWithSingleElement()
    {
        singleField.setText("Works here as non-array");
    }

    public static void main(String[] args)
    {
        new testSetText();
    }
}

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