Stringbuffer lambda ApplicationContextAware学习回忆

1.StringBuffer中delete和deleteCharAt两个方法的区别：

delete(int a,int b):删除下标a到b的值，包含a但不包含b

deleteCharAt(int a):只删除下标为a的值

注意：下标从0开始数

2.lambda表达式：Collectors.groupingBy(参数1),返回值为map(key,value)，参数1对应key的值，value对应key下面的所有值。

如：List<Student> stus = new ArrayList<Student>();

Student stu1 = new Student(188,"xmd","a");

Student stu2 = new Student(188,"xmd","a");

Student stu3 = new Student(188,"xmd","b");

Student stu4= new Student(188,"xmd","b");

stus .add(stu1);

stus .add(stu2);

stus .add(stu3);

stus .add(stu4);

Map<String,LIst<Student>> map = stus.stream().collect(Collector.groupingBy(Student::getCode));

map的值为：

a:[Student{age=188, username='xmd', code='a'}, Student{age=188, username='xmd', code='a'}]

b:[Student{age=188, username='xmd', code='b'}, Student{age=188, username='xmd', code='b'}]

3.ApplicationContextAware作用

实现了ApplicationContextAware接口的类会自动调用setApplicationContext方法，实现了这个接口的bean，在容器初始化的时候，会自动的将ApplicationContext注入进来。