java - 为什么我会收到不可转换的类型错误？

Question

如果我使用这个类:

public class BooleanTest {
    public static void main(String args[]) {
        final Object[] objarray = new Object[2];
        try {
            objarray[0] = "Hello World!";
            objarray[1] = false;
        } catch (NullPointerException e) {
        }
        boolean bool = (boolean) objarray[1];
    }
}

它工作正常，我可以毫无问题地分配 boolean。为什么在向我的用户询问密码时我不能做同样的事情？

final Object result[] = new Object[2];
try {
    java.awt.EventQueue.invokeAndWait(new Runnable() {
        @Override
        public void run() {
            JPanel panel = new JPanel();
            panel.setLayout(new GridLayout(3,0));
            JLabel label = new JLabel();

            label.setHorizontalAlignment(SwingConstants.LEADING);
            JTextField input = new JTextField();

            input.setHorizontalAlignment(SwingConstants.CENTER);
            JCheckBox checkbox = new JCheckBox("Pair with this device");
            checkbox.setHorizontalAlignment(SwingConstants.LEADING);
            panel.add(label);
            panel.add(input);
            panel.add(checkbox);
            if (wrong) {
                label.setText("Wrong password. Please enter the password from the other device:");
            } else {
                label.setText("Please enter the password from the other device:");
            }
            int response = JOptionPane.showConfirmDialog(SendGUI.this, panel, "Enter password", JOptionPane.OK_CANCEL_OPTION);
            if (response == JOptionPane.OK_OPTION) {
                result[0] = input.getText();
                result[1] = (boolean)checkbox.isSelected();
            } else {
                result[0] = null;
                result[1] = false;
            }
        }
    });
} catch (InterruptedException e) {
} catch (InvocationTargetException e) {
}
boolean pair = (boolean)result[1]; //inconvertible type, expected boolean found Object

据我所知，我在这两种情况下都做了同样的事情，但第一个示例编译正常，而第二个示例则没有。

Answer 1

您正在使用不同的编译器选项。你一定是。这两段代码都在 Java 7 规则下编译；两者都不会在 Java 6 规则下编译。例如，以您的第一代码(您说的为您编译的代码)为例:

c:\Users\Jon\Test>javac -source 1.7 BooleanTest.java

(No console output, i.e. no errors)

c:\Users\Jon\Test>javac -source 1.6 BooleanTest.java
warning: [options] bootstrap class path not set in conjunction with -source 1.6
BooleanTest.java:10: error: inconvertible types
        boolean bool = (boolean) objarray[1];
                                         ^
  required: boolean
  found:    Object
1 error
1 warning

编辑:我相信更改在 JLS(转换转换)的第 5.5 节中。

Java 7 version包括:

Casting contexts allow the use of one of:

• ...
• a narrowing reference conversion (§5.1.6) optionally followed by either an unboxing conversion (§5.1.8) or an unchecked conversion (§5.1.9)

JLS 3rd edition (基本上是 Java 5 和 6)包括:

Casting contexts allow the use of one of:

• ...
• a narrowing reference conversion (§5.1.6) optionally followed by an unchecked conversion

请注意此处缺少“拆箱转换”。