可视化这一点的最简单方法是认为继承就像父/子关系。你可以有父 -> 子 -> 孙子等。当你有：class A {}class B extends A{}class C extends B{}C就像一个孙子A。这意味着C继承了所有的方法B，包括那些B本身继承自A.In OOP words,C **is**A` 的方法。但是，当你有class A {}class B extends A{}class C extends A{}C和B是兄弟类，这意味着它们都继承了A的方法，但它们彼此不兼容。在第一种情况下，这些是有效的：C c = new C();c.methodFromA(); //C inherits methods from A by being its grand-childc.methodFromB(); //C inherits methods from B by being its childc.methodFromC();然而，在第二种情况下，当两者都B直接C extends A：C c = new C();B b = new B();c.methodFromA(); //C inherits methods from A by being its childb.methodFromA(); //B inherits methods from A by being its childc.methodFromB(); //not allowedb.methodFromC(); //not allowedB但是，和之间没有直接关系C。这些是无效的：B b = new B();C c = new C();b = (B) c; //invalid, won't compileA b = b;c = (C) b; //will compile, but cause a ClassCastException at runtime