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正在读取Oracle Java docs。
Say you want to write a method that puts Integer objects into a list. To maximize flexibility, you would like the method to work on List, List, and List — anything that can hold Integer values.
To write the method that works on lists of Integer and the supertypes of Integer, such as Integer, Number, and Object, you would specify List<? super Integer>. The term List is more restrictive than List<? super Integer> because the former matches a list of type Integer only, whereas the latter matches a list of any type that is a supertype of Integer.
我试过这段代码，但它无法编译。

class MyNumber extends Number { ...  }

还有这个。

List<? super Integer> list = new ArrayList<>();
        list.add(1); // Ok
        list.add(new Object()); // Not Ok!

那么下界背后的想法是什么呢？
另外，另一件事引起了我的注意，使用上限，我们可以确保当我们调用超/父类上的方法时，子类必须拥有该方法，但使用下限，情况就不一样了，如下面的代码所示，所以同样，捕获在哪里，或者我们从下限中获得了什么？

Arrays.asList("one","two","three").stream().forEach(new Consumer<String>() {  // Accept it as a param.
    @Override
    public void accept(String s) {
        System.out.println(s.toLowerCase());  // OK!
    }
});

Arrays.asList("one","two","three").stream().forEach(new Consumer<Object>() {   //Accept it as a param.
    @Override
    public void accept(Object s) {
        System.out.println(s.toLowerCase()); // NOT OK! What is the idea then? 
    }
});