// Split String
        for (String word: "test test".split(" ")){
            System.out.println(word);

// Stringbuffer
        StringBuffer sb = new StringBuffer();
        sb.append("gavinfly");
        System.out.println("original stringbuffer==>"+sb);
        sb.append("fei");
        System.out.println("after append fei==>"+sb);
        sb.insert(5,"!!");
        System.out.println("insert from position 5=>"+sb);
        sb.delete(7,10);
        System.out.println("delete [7,10)==>"+sb);
        sb.reverse();
        System.out.println("reverse the string==>"+sb);
        sb.replace(0,3,"123");
        System.out.println("replace [0,3)==>"+sb);
        
        StringBuilder sb1 = new StringBuilder();
        for (String word : "test test".split(" ")) {
            if (sb1.length() > 0) {
                sb1.append(", ");
            }
            sb1.append(word);
        }
        System.out.println(sb1);
        
// 构造函数 => 给类对象赋初始
public class Person{
    
    String name;
    int age;
    Person(String name, int age){     
        this.name=name;
        this.age=age;
    }
    public static void main(String[] args){
        Person p = new Person("gavin",28);
        System.out.println("name :"+p.name+" age: "+p.age);
    }
}

//ArrayList
import java.util.ArrayList;

public class Main {

    public static void main(String[] args) {
        ArrayList<String> testArrayList = new ArrayList<String>();
        testArrayList.add("Google");
        testArrayList.add("hello");
        testArrayList.set(1,"Wiki");
        testArrayList.remove(0);
        System.out.println(testArrayList);
        System.out.println(testArrayList.size());
    }
}