为什么 class 扩展 JTextField 不能实例化 JPasswordField？

Question

我有一个空的 class MyJTextField 扩展 JTextField，我希望将 MyJTextField 初始化为 JPasswordField，就像 JTextField 一样。

JTextField pass = new JPasswordField(); //no errors
but
MyJTextField pass = new JPasswordField();//"Type mismatch: cannot convert from JPasswordField to MyJTextField"

Answer 1

您的 MyJTextField 是 JTextField 的 child。而 JPasswordField 也是 JTextField 的 child。但是 MyJTextField 不是 JPasswordField 的 child，所以 MyJTextField 不是 JPasswordField，它们是兄弟。

    //you can do
    JTextField field1 = new JPasswordField(); //child is an instance of parent
    JTextField field2 = new MyJTextField(); //child is an instance of parent

    //you can't do
    JPasswordField field3 = new JTextField(); // parent is not instance of child
    MyJTextField field4 = new JTextField(); // parent is not instance of child
    JPasswordField field5 = new MyJTextField(); // siblings are not instances of each other
    MyJTextField field6 = new JPasswordField(); // siblings are not instances of each other

为什么 class 扩展 JTextField 不能实例化 JPasswordField？

Why can't a class extending JTextField instantiate a JPasswordField?

java

swing

instance

jtextfield

extending