Classes comumente usadas (StringBuffer / System / RunTime / Date)

Um, StringBuffer (segurança de thread)

Emenda falsa

public class Demo {
    
    
    public static void main(String[] args) {
    
    
        //字符串  String是一个不可改变的量
        String str = "听闻远方有你";
        str += "动身跋涉千里";
        str += "我吹过你吹过的风";
        str += "这算不算相拥";
        /*
        * 上面到底有几个字符串？？？
        * += ===》字符串拼接
        * 两个字符串的拼接不是在原本的字符串上面进行修改的。而是在内存的数据区重新创建了一个新的字符串常量
        *
        * */
    }
}

Splicing verdadeiro

append () anexa a string especificada a esta sequência de caracteres
insert (int offset, String str) insere a string nesta sequência de caracteres

public class Demo3 {
    
    
    public static void main(String[] args) {
    
    
        StringBuffer sb = new StringBuffer("舔狗日记");//字符序列
        System.out.println(sb);
        /*
         * append()  将指定的字符串附加到此字符序列
         * */
        sb.append("今天3月18号天气阴");
        sb.append("我给某某带了饭，某某说以后不用带了，我有男朋友了");
        sb.append("我说:我可以给你带两份");
        System.out.println(sb);//舔狗日记今天3月18号天气阴我给某某带了饭，某某说以后不用带了，我有男朋友了我说:我可以给你带两份
        /*
        * insert(int offset, String str)
            将字符串插入到此字符序列中。
        * */
        StringBuffer sb2 = new StringBuffer("012345");
        sb2.insert(0, 3);
        sb2.insert(4, "烤鸭");
        sb2.insert(5, new Demo1());
        System.out.println(sb2);//3012烤com.qfedu.commonTest.Demo1@1b6d3586鸭345
    }
}

É muito semelhante ao método abaixo da classe String
indexOf ()
toString ()
substring ()
replace

public class Demo4 {
    
    
    public static void main(String[] args) {
    
    

        StringBuffer sb = new StringBuffer("这里的方法和String类下面的方法很相似");
        System.out.println(sb.indexOf("String"));//6
        //StringBuffer转为String类型的数据
        String  s= sb.toString();
        System.out.println(sb);//这里的方法和String类下面的方法很相似
        System.out.println(s);//这里的方法和String类下面的方法很相似
        System.out.println(sb.substring(3));//方法和String类下面的方法很相似
        System.out.println(sb.substring(3, 8));//方法和St
        sb.replace(3,8, "呵呵哒");
        System.out.println(sb);//这里的呵呵哒ring类下面的方法很相似
    }
}

delete ()
deleteCharAt ()
reverse ()

public class Demo5 {
    
    
    public static void main(String[] args) {
    
    
        StringBuffer sb = new StringBuffer("仰天大笑出门去");
        sb.delete(2,5);
        System.out.println(sb);//仰天门去(去头不去尾)
        sb.deleteCharAt(0);
        System.out.println(sb);//天门去

        StringBuffer stringBuffer = new StringBuffer("我们都被carry过");
        System.out.println(stringBuffer.reverse());//翻转     过yrrac被都们我

    }
}

Em segundo lugar, sistema

import java.util.Properties;

/**
 * @author xue_yun_xiang
 * @create 2021-03-18-20:02
 */
public class Demo1 {
    
    
        public static void main(String[] args) {
    
    
            Properties pop = System.getProperties();
            //返回值是一个properties   属性类
            pop.list(System.out);
            System.out.println(pop.getProperty("os.name"));
            System.out.println(pop.getProperty("user.country"));
        }
}

Resultado de saída:

……
Windows 10
CN

Três, Tempo de Execução

import java.io.IOException;

/**
 * @author xue_yun_xiang
 * @create 2021-03-18-20:05
 */
public class Demo2 {
    
    
    public static void main(String[] args) throws IOException, InterruptedException {
    
    
        Runtime runtime = Runtime.getRuntime();
        Process exec = runtime.exec("C:\\Program Files (x86)\\Notepad++\\notepad++.exe");
        Thread.sleep(10000);
        exec.destroy();
    }
}

Quarta, Data

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;

/**
 * @author wangbo
 * @version 1.0
 * @date 2021/3/18 16:17
 */
public class Demo1 {
    
    
    public static void main(String[] args) {
    
    
        Calendar instance = Calendar.getInstance();
        int year = instance.get(Calendar.YEAR);
        int month = instance.get(Calendar.MONTH) + 1;
        int day = instance.get(Calendar.DAY_OF_MONTH);
        System.out.println(year + "-" + month + "-" + day );

        Date date = new Date();
        System.out.println(date);
        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("YYYY-MM-dd HH:mm:ss");
        String format = simpleDateFormat.format(date);
        System.out.println(format);
    }
}

2021-3-18
Thu Mar 18 20:14:41 CST 2021
2021-03-18 20:14:41