继承－java－箴言

1. A subclass inherits all the members (fields, methods, and nested classes) from its superclass. Constructors are not members, so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass.

注意：继承的内部类可以访问父类的所有field ，即使是private 的。

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2. The access specifier for an overriding method can allow more, but not less, access than the overridden method. For example, a protected instance method in the superclass can be made public, but not private, in the subclass.

注意：我们只能在子类中把访问限制放宽而不能缩窄。

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3.  You will get a compile-time error if you attempt to change an instance method in the superclass to a class method in the subclass, and vice versa.

注意：隐藏的意思是父类还是可以访问它自己的static 方法。你不能把父类中的static 方法在子类中变成实例方法，反之也一样。

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4. Within a class, a field that has the same name as a field in the superclass hides the superclass's field, even if their types are different. 

注意：子类中与父类同名的field 会把父类中的field 隐藏掉，即使类型不同。如果要访问父类中被隐藏掉的field ，只有通过super 干茧子。

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5. Note: If a constructor does not explicitly invoke a superclass constructor, the Java compiler automatically inserts a call to the no-argument constructor of the superclass. If the super class does not have a no-argument constructor, you will get a compile-time error. Object does have such a constructor, so if Object is the only superclass, there is no problem.

If a subclass constructor invokes a constructor of its superclass, either explicitly or implicitly, you might think that there will be a whole chain of constructors called, all the way back to the constructor of Object. In fact, this is the case. It is called constructor chaining, and you need to be aware of it when there is a long line of class descent.

注意：子类没有显示调用父类构造方法（在子类构造方法第一行），java 编译器就会默认调用父类的无参构造方法即super() 。如果父类没有无参构造方法，就会报错。调用构造方法时存在一条构造器链，一直调用到Object 。

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