为什么扩展JTextField的类不能实例化一个JPasswordField？

Question

我有一个空类MyJTextField延伸的JTextField，我想MyJTextField被初始化为JPasswordField中，喜欢的JTextField可以做。

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的子项。 JPasswordField也是JTextField的子项。但MyJTextField不是JPasswordField的子项，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