B a=new A(),A是父类，B是子类，为什么不能这么写，系统提示的错误是Type mismatch: cannot convert from A to B,这是一个类型转换的过程吗,求解 求解
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举个例子：
java中所有的类 的超类都是Object
难道 new Object(); 的 引用能随便使用一个 子类的引用吗？
例如 ： String str = new Object(); 这肯定不对呀
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你或许没有理解继承的含义；
例如：
图形是个类，矩形、三角形是它的子类；
你可以说矩形、三角形是图形，而不能反过来吧；
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理解有误呀！
在生活中都说儿子长的像爹，有人说爹长的像儿子吗
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B which is subclass of class A often carries more features besides those extends from A, in other words,A is subset of B.
= is an assignment symbol which requires the right side at least owns the whole features of the class on the left side.
Therefore,A a = new B() is correct whereas B b = new A() will fail to pass the compilation.
More vivid example, let us suppose that you need a book, your friend offer you a sheet of paper.Obviously your requirement cannot be catered,although a sheet of paper is a part of a book.But if your friend offer you a book and a sheet of paper.S/he can cater you. Perhaps it's not a suitable example, but it can make you easy to understand this question.
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B a=B(new A())
可以这么写 ，这样只能调用父类有的方法 ，子类中自己的方法是不能被调用的。