java字符串&字符串的使用&格式化字符串&正则&字符串生成器

public class Compound {
    public static void main(String args[]) {
        String  a = "good";
        System.out.println(a);

        char b[] = {'h','e','l','l','o'};
        String s = new String(b);
        System.out.println(a);

        char d[] = {'h','e','l','l','o'};
        String c = new String(d,2,3);
        System.out.println(c);
    }
}

public class Compound {
    public static void main(String args[]) {
        String  a = "good";
        System.out.println(a);

        char b[] = {'h','e','l','l','o'};
        String s = new String(b);
        System.out.println(a);

        char d[] = {'h','e','l','l','o'};
        String c = new String(d,2,3);
        System.out.println(c);
        System.out.println(s+' '+a);
        int myint = 1;
        System.out.println(s + ' ' + a + myint);
        String str = s + ' ' + a + myint;
        int size = str.length();
        System.out.println(size);

    }
}

public class LearnString {
    /**
     * 字符串的普通使用
     * @param args
     */
    public  static void  main(String[] args) {
        String str = "we are student";
        //字符串长度
        System.out.println(str.length());
        //字符串查找
        System.out.println(str.indexOf("a"));//第一次出现的位置
        System.out.println(str.lastIndexOf("e"));//最后一次出现的位置
        //获取指定位置的字符
        System.out.println(str.charAt(3));
        //截取字符串
        System.out.println(str.substring(3,3));
        //去除空格
        System.out.println(str.trim());
        //字符串替换
        System.out.println(str.replace("are","is"));
        //判断字符串的开始和结尾
        System.out.println(str.startsWith("we"));
        System.out.println(str.startsWith("wee"));
        System.out.println(str.endsWith("we"));
        System.out.println(str.endsWith("nt"));
        String str_a = "abC def";
        String str_b = "abc def";
        boolean b_bool = (str_a == str_b);
        System.out.println(b_bool);
        System.out.println(str_a.equals(str_b));
        System.out.println(str_a.equalsIgnoreCase(str_b));
        //字母大小写转换
        System.out.println(str_a.toLowerCase());
        System.out.println(str_b.toUpperCase());
        //字符串分割
        String str_arr[] = str_a.split(" ");
        for (String item : str_arr) {
            System.out.println(item);
        }




    }
}

import java.util.Date;

public class LearnString2 {
    /**
     * 格式化字符串
     * @param args
     */
    public static void main(String[] args) {
       Date date = new Date();
       String date_s = String.format("%tF",date);
       System.out.println(date_s);
    }
}

public class LearnString3 {
    /**
     * 使用正则表达式
     * @param args
     */
    public static void main(String[] args) {
        String regex = "\\w+@\\w+(\\.\\w{2,3})*\\.\\w{2,3}";

        String str1 = "www.aa@";
        String str2 = "www";
        String str3 = "[email protected]";
        if (str1.matches(regex)) {
            System.out.println(1+" "+true);
        }
        if (str2.matches(regex)) {
            System.out.println(2+" "+true);
        }
        if (str3.matches(regex)) {
            System.out.println(3+" "+true);
        }

    }
}

public class LearnString4 {
    /**
     * 字符串生成器
     */
    public static void main(String[] args) {
        String str = "";
        long start_time = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {
            str = str + i;
        }
        long end_time = System.currentTimeMillis();
        long time = end_time-start_time;
        System.out.println("String"+ time);
        System.out.println("String:" + str);
        StringBuilder builder = new StringBuilder();
        start_time = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {
            builder.append(i);
        }
        end_time = System.currentTimeMillis();
        time = end_time-start_time;
        System.out.println("Builder"+ time);
        System.out.println("Builder:"+ builder.toString());
    }
}