Stringbuffer lambda ApplicationContextAware學習回憶

1.StringBuffer中delete和deleteCharAt兩個方法的區別：

delete(int a,int b):刪除下標a到b的值，包含a但不包含b

deleteCharAt(int a):只刪除下標爲a的值

注意：下標從0開始數

2.lambda表達式：Collectors.groupingBy(參數1),返回值爲map(key,value)，參數1對應key的值，value對應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的值爲：

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

實現了ApplicationContextAware接口的類會自動調用setApplicationContext方法，實現了這個接口的bean，在容器初始化的時候，會自動的將ApplicationContext注入進來。