Java常用类

Posted 2021-11-07 Wecccccccc

常用类

包装类Wrapper的分类

包装类和基本数据的转换

01:

package TryCatchExercise;

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


        //手动装箱
      int n1 = 100;
        Integer integer = new Integer(n1);
        Integer integer1 = Integer.valueOf(n1);

        //手动拆箱
        int i = integer.intValue();

        //JKD5后，自动装箱
        int n2 = 200;
        Integer integer2 = n2;//底层使用的是 Integer.valueOf(n2)

        //自动拆箱
        int n3 = integer2;//底层仍然使用的是 intValue()方法

    }
}

小练习

包装类型和String类型的相互转换

01:

package TryCatchExercise;

public class WrapperVSString {
    public static void main(String[] args) {
        //包装类(Integer)->String
        Integer i = 100;
        //1.
        String str1 = i+"";

        //2.
        String str2 = i.toString();

        //3.
        String str3 = String.valueOf(i);

        //String -> Integer
        //1.
        String str4 = "12345";
        Integer i1 = Integer.parseInt(str4);

        //2.
        Integer integer = new Integer(str4);
        
        
    }
}

Integer类和Character类的常用方法

小练习

01:

package AbstractDemo01;

public class Test {
    public static void main(String[] args) {
        Integer i = new Integer(1);
        Integer j = new Integer(1);
        System.out.println(i==j);

        //底层Integer.valueOf()
        Integer n = 1;
        Integer m = 1;
        System.out.println(n==m);

        //底层Integer.valueOf()
        Integer x = 128;
        Integer y = 128;
        System.out.println(x==y);

//        @IntrinsicCandidate
//        public static Integer valueOf(int i) {
//            if (i >= IntegerCache.low && i <= IntegerCache.high)
//                return IntegerCache.cache[i + (-IntegerCache.low)];
//            return new Integer(i);
//        }
        

//        false
//        true
//        false
    }
}

02：

String类的理解和创建对象

两种创建String对象的区别

字符串的特性

String类的常用方法

01:

package StringDemo01;

import java.util.Locale;

public class StringMethod01 {
    public static void main(String[] args) {
        String str1 = "hello";
        String str2 = "Hello";
        System.out.println(str1.equals(str2));//false


        String username = "join";

        if ("join".equalsIgnoreCase(username))
        {
            System.out.println("join yes");//join yes
        }
        else
        {
            System.out.println("join no");
        }

        String username2 = "JoIn";

        if ("join".equalsIgnoreCase(username2))
        {
            System.out.println("join yes");//join yes
        }
        else
        {
            System.out.println("join no");
        }


        System.out.println("jack".length());//4


        String s1 = "wer@terwe@g";
        int idx = s1.indexOf('@');
        System.out.println(idx);//3

        s1 = "wer@asdasda@ad";
        idx = s1.lastIndexOf('@');
        System.out.println(idx);//11

        String name = "jackioio";
        System.out.println(name.substring(6));//io

        System.out.println(name.substring(0,3));//jac

        String s = "hello";
        System.out.println(s.toUpperCase());//HELLO

        System.out.println(s.toLowerCase());//hello

        String ss1 = "baoyu";
        ss1 = s.concat("jack").concat("jxuad").concat("dasda");
        System.out.println(ss1);//hellojackjxuaddasda

        ss1 = "jack and mike jack";
        ss1 = ss1.replace("jack","tom");
        System.out.println(ss1);//tom and mike tom


        String sss1 = "jack,miek,lll";
        String[] split = sss1.split(",");

        for (String v:split)
        {
            System.out.println(v);
//            jack
//                    miek
//            lll
        }


        String hhh = "hhkad\\\\asdas\\\\adas";
        String[] hhhs = hhh.split("\\\\\\\\");
        for (String v:hhhs)
        {
            System.out.println(v);

//            hhkad
//                    asdas
//            adas
        }

        String h1 = "dafsfsdf";
        char [] chs = h1.toCharArray();
        for (int i = 0;i< h1.length();i++)
        {
            System.out.println(chs[i]);
        }

        //1.如果长度相同，并且每个字符也相同，返回0
        //2.如果长度相同或者不相同，但是在进行比较时，可以区分大小就返回
//        if (c1 != c2)
//        {
//            return c1-c2;
//        }

        //3.如果前面的部分都相同，就返回str1.len-str2.len




        String a = "jack";
        String b = "apple";

        System.out.println(b.compareTo(a));//-9

        a = "jack1";
        b = "jack111";
        System.out.println(b.compareTo(a));//2

        String studentname = "john";
        int age = 10;
        double score = 98.3/3;
        char gender = '男';
        String info = "My name is"+name+"age = "+age+"score = "+score+"gender =  "+gender+"I want to make eyeryone happy";

        String formatStr = "my name is%s age = %d score = %.2f gender = %c ";

        String info2 =   String.format(formatStr,name,age,score,gender);
        System.out.println(info);

        System.out.println(info2);

//        My name isjackioioage = 10score = 32.766666666666666gender =  男I want to make eyeryone happy
//        my name isjackioio age = 10 score = 32.77 gender = 男

    }
}

StringBuffer类基本介绍

String VS StringBuffer

StringBuffer的构造器

01：

package StringBufferDemo01;

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


        //1.创建一个大小为16的char[],用于存放字符内容
        StringBuffer stringBuffer = new StringBuffer();

        //2.通过构造器指定char[]大小
        StringBuffer stringBuffer1 = new StringBuffer(100);

        //3.通过给一个String创建StringBuffer,char[]大小就是str.length()+16
        String hello = new String("hello");


        //String  ->  StringBuffer

        String str = "hello tom";
        //1.使用构造器
        //注意:返回的才是StringBuffer对象，对str本身没有影响
        StringBuffer stringBuffer2 = new StringBuffer(str);


        //2.使用append方法
        StringBuffer stringBuffer3 = new StringBuffer();
        StringBuffer append = stringBuffer3.append(str);


        //StringBuffer -> String
        //1.使用StringBuffer的toString方法
        StringBuffer hspedu = new StringBuffer("hspedu");
        String s = hspedu.toString();

        //2.使用构造器
        String s1 = new String(stringBuffer3);



    }
}

StringBuffer类常见方法

01：

package StringBufferMethod;

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


        StringBuffer hello = new StringBuffer("hello");

        //增
        hello.append(',');
        hello.append("zsf");
        hello.append("zhaom").append(100).append(true).append(10.5);
        System.out.println(hello);

        //删
        hello.delete(11,14);//[11,14)
        System.out.println(hello);

        //改
        hello.replace(9,11,"jack");//[9,11)
        System.out.println(hello);

        //插
        hello.insert(9,"mike");//插在坐标为9的前面
        System.out.println(hello);

        int length = hello.length();
        System.out.println(length);


    }
}

小练习

01:

02:

package StringBufferMethod;

public class StringBufferExercise01 {
    public static void main(String[] args) {

        String price = "123565467894.59";

        StringBuffer sb = new StringBuffer((price));

//        int i = sb.lastIndexOf(".");
//        sb = sb.insert(i-3,",");
//        System.out.println(sb);

        for (int i = sb.lastIndexOf(".");i-3 >0;i-=3)
        {
            sb = sb.insert(i-3,",");
        }

        System.out.println(sb);


    }
}

StringBuilder类结构剖析

Math类

01：

package StringBufferMethod;

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

        //1.abs 绝对值
        int abs = Math.abs(9);
        System.out.println(abs);


        //2.pow 求幂
        double pow = Math.pow(2,4);
        System.out.println(pow);

        //3.ceil 向上取整,返回>=该参数的最小整数
        double ceil = Math.ceil(-3.0001);
        System.out.println(ceil);

        //4.floor 向下取整,返回<=该参数的最大整数
        double floor = Math.floor(-4.999);
        System.outelasticsearch代码片段，及工具类SearchEsUtil.java

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