Java 为什么可以'；扩展JTextField的类是否实例化JPasswordField？

我有一个空类MyJTextField扩展JTextField，我希望像JTextField一样，将MyJTextField初始化为JPasswordField

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

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您的MyJTextField是JTextField的子级。JPasswordField也是JTextField的孩子。但是MyJTextField不是JPasswordField的孩子，所以MyJTextField不是JPasswordField，他们是兄弟姐妹

    //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

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Thx，我没意识到/