【JAVA】常用类

String

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String构造方法

package heima;

public class P119 {
    public static void main(String[] args) {
        String s1 = new String();
        System.out.println("s1:"+s1);

        char[] chs = {'a','b','c'};
        String s2 = new String(chs);
        System.out.println("s2:"+s2);

        byte[] bys = {97,98,99};
        String s3 = new String(bys);
        System.out.println("s3:"+s3);

        String s4 = "abc";
        System.out.println("s4:"+s4);
    }
}

String对象的特点

观察下列例子，体会两种String对象的特点不同

String的比较

String是对象，==比较的是地址值
比较数据值，使用方法：equals()

package heima;

public class P120 {
    public static void main(String[] args) {
        char[] chs = {'a','b','c'};
        String s1 = new String(chs);
        String s2 = new String(chs);

        String s3 = "abc";
        String s4 = "abc";

        System.out.println(s1 == s2);

        System.out.println(s1.equals(s2));
    }
}

案例–用户登录

package heima.P121;
import java.util.Scanner;
public class StringTest01 {
    public static void main(String[] args) {
        String username = "ludan";
        String password = "123456";

        for(int i = 0;i<3;i++){
            Scanner sc = new Scanner(System.in);

            System.out.println("请输入用户名");
            String name = sc.nextLine();

            System.out.println("请输入密码");
            String pwd = sc.nextLine();

            if(name.equals(username) && pwd.equals(password)){
                System.out.println("登录成功");
                break;
            }else{
                System.out.println("登录失败,你还有"+(2-i)+"次机会");
            }
        }
    }


}

nextLine忘了会看Scanner文章

案例–遍历字符串

charAt用法

package heima.P122;

import java.util.Scanner;

public class bl {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.println("请输入一个字符串: ");
        String line = sc.nextLine();

        System.out.println(line.charAt(0));
        System.out.println(line.charAt(1));
        System.out.println(line.charAt(2));
    }
}

length用法

package heima.P122;

import java.util.Scanner;

public class bl {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.println("请输入一个字符串: ");
        String line = sc.nextLine();

//        System.out.println(line.charAt(0));
//        System.out.println(line.charAt(1));
//        System.out.println(line.charAt(2));

        for(int i = 0;i<line.length();i++){
            System.out.println(line.charAt(i));
        }
    }
}

案例–统计字符次数

package heima;

import java.util.Scanner;

public class P123 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.println("请输入一个字符串");
        String line = sc.nextLine();

        int bigCount = 0;
        int smallCount = 0;
        int numberCount = 0;

        for(int i = 0;i<line.length();i++){
            char ch = line.charAt(i);

            if(ch >= 'A' && ch <='Z'){
                bigCount++;
            }else if(ch>='a' && ch<='z'){
                smallCount++;
            }else if(ch>='0' && ch<='0'){
                numberCount++;
            }
        }

        System.out.println("大写字母："+bigCount+"个");
        System.out.println("小写字母："+smallCount+"个");
        System.out.println("数字："+numberCount+"个");
    }
}

案例–拼接字符串

package heima;

public class P124 {
    public static void main(String[] args) {
        int[] arr = {1,2,3};

        String s = arrayToString(arr);

        System.out.println("s:"+s);
    }
    public static String arrayToString(int[] arr){
        String s = "";

        s+="[";

        for(int i = 0;i<arr.length;i++){
            if(i == arr.length-1){
                s+=arr[i];
            }else{
                s+=arr[i];
                s+=",";
            }
        } 
        s += ']';

        return s;
    }
}

输出：

案例–字符串反转

package heima;

import java.util.Scanner;

public class P125 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);//abc

        System.out.println("请输入一个字符串：");
        String line = sc.nextLine();

        String s = reverse(line);

        System.out.println("s:"+s);

    }

    public static String reverse(String s){
        String ss = "";

        for(int i = s.length()-1;i>=0;i--){
            ss += s.charAt(i);
        }

        return ss;
    }
}

通过帮助文档查看String中的方法

StringBuilder

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概述

StringBuilder构造方法

package heima;

public class P126 {
    public static void main(String[] args) {
        //创建一个空白可变字符串对象
        StringBuilder sb = new StringBuilder();
        System.out.println("sb:"+sb);
        System.out.println("sb.length():"+sb.length());

        //根据字符串内容创建可变字符串对象
        StringBuilder sb2 = new StringBuilder("hello");
        System.out.println("sb2:"+sb2);
        System.out.println("sb2.length():"+sb2.length());
    }
}1

StringBuilder的添加和反转方法

package heima;

public class P129 {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder();

        //append 添加数据，并返回对象本身
        StringBuilder sb2 = sb.append("hello");

        System.out.println("sb:"+sb);
        System.out.println("sb2:"+sb2);
        System.out.println(sb == sb2);

        //链式编程
        sb.append("hello").append("world").append("java").append(100);

        System.out.println("sb:"+sb);

        //reverse() 返回相反的字符序列
        sb.reverse();
        System.out.println("sb:"+sb);
    }
}

StringBuilder和String相互转换

package heima;

public class P130 {
    public static void main(String[] args) {
        //StringBuilder转换为String
        StringBuilder sb = new StringBuilder();
        sb.append("hello");

        String s = sb.toString();
        System.out.println(s);

        //String转换为StringBuilder
        String s1 = "hello1";

        StringBuilder sb1 = new StringBuilder(s1);

        System.out.println(sb1);

    }
}

案例升级–拼接字符串

使用StringBuilder、String、append

package heima;

public class P131 {
    public static void main(String[] args) {
        int[] arr = {1,2,3};

        String s = arrayToString(arr);

        System.out.println("s:"+s);
    }

    public static String arrayToString(int[] arr){
        StringBuilder sb = new StringBuilder();
        sb.append("[");

        for(int i = 0;i<arr.length;i++){
            if(i == arr.length-1){//到数组结尾时特殊处理
                sb.append(arr[i]);
            }else{
                sb.append(arr[i]).append(",");
            }
        }
        sb.append("]");

        String s = sb.toString();

        return s;


    }
}

append函数

append()：类似于尾插，就是给元素后面追加一个字符串

案例升级–反转字符串

package heima;

import java.util.Scanner;

public class P132 {

        public static void main(String[] args) {
            Scanner sc = new Scanner(System.in);

            System.out.println("请输入一个字符串：");
            String line = sc.nextLine();

            String s = myReverse(line);

            System.out.println("s:"+s);
        }
        public static String myReverse(String s){
            return new StringBuilder(s).reverse().toString();
        }
}