Rookie learns Java calling package, String, ArrayList

1、package

package it.heima.loop3.loo1;


import it.heima.loop3.loo2.demo;
import java.util.Random;
import java.util.Scanner;

public class test {
    
    
    public static void main(String[] args) {
    
    
//        其他包里面的类
        demo d = new demo();
        d.print();

//        本包里的其他类
        demo11 d1 = new demo11();
        d1.printa();


//       java拓展库
//        java.long 不需要导入
        Scanner sc = new Scanner(System.in);
        Random r = new Random();

//        不同包下面的同名类，只能在import头部调用一个，倘若要用多个，就直接如下使用全名称就行；不必在import中写
        it.heima.loop.demo dd = new it.heima.loop.demo();
        dd.prints();
    }
}

2、String

package it.heima.loop4;

public class demo1 {
    
    
    public static void main(String[] args) {
    
    
        String st1 = "heihei";
        System.out.println(st1);

        String st2 = new String();
        System.out.println(st2);

        String st3 = new String("heihei");
        System.out.println(st3);

        char[] chars = new char[]{
    
    'a', 'b', 'h'};
        String st4 = new String(chars);
        System.out.println(st4);

        byte[] bytes = {
    
    97,98, 99};
        String st5 = new String(bytes);
        System.out.println(st5);
    }
}

output

heihei

heihei
abh
abc

1. What is String and what can it do?

Represents a string, which can be used to create an object to encapsulate string data and process it

2. How many ways does the string class create objects to encapsulate string data?

Method 1: Directly use double quotation marks "..."
Method 2: new String class, call the constructor to initialize the string object

ArrayList