Generics

In general, if Foo is a subtype (subclass or subinterface) of Bar, and G is some generic type declaration, it is not the case that G<Foo> is a subtype of G<Bar>. This is probably the hardest thing you need to learn about generics, because it goes against our deeply held intuitions.

e.g.

List<String> ls = new ArrayList<String>();

List<Object> lo = ls; //error

 

注：假如定义Animal为一个类，Cat,Dog等都继承Animal,现在有一个ArrayList<Animal>的对象c，那么往c里放入任何Animal的对象或者继承Animal的类的对象都是合法的；但是要注意的是（正如前边所说），ArrayList<Dog>并不是ArrayList<Animal> 的子类，它们是完全不相关的两个类，将ArrayList<Dog>对象的引用赋值给ArrayList<Animal>对象的引用会造成编译错误（如上例子，又比如果一个method需要ArrayList<Animal>类型的参数，但是却传递进去一个ArrayList<Dog>类型的实参，这就会引起编译错误。