String和StringBuilder

import java.util.Collections;

import static com.sun.org.apache.xml.internal.security.keys.keyresolver.KeyResolver.length;
//java字符串不会改变,除非重新赋值
public class 字符串 {
    public static void main(String[] args) {
        String str = "44";
        int num = Integer.parseInt(str);
        str = String.valueOf(num);

        str = "dfa,sdf,adsf,asd,fasd,fas,df";
        String[] strings = str.split(",");
        str = String.join(",", strings);

        //左闭右开区间
        String string = str.substring(3,6);

        char c = string.charAt(1);
        int d = string.compareTo(string);
        String  ss = string.concat(string);
        boolean b = ss.contains("a");
        b = ss.startsWith("s");
        b = ss.endsWith("s");
        b = ss.isEmpty();

        b = ss.matches("a");

        ss = ss.replaceAll("sf" ,"asdf");
        ss = ss.replaceFirst("sf" ,"asdf");

        char[] chars = ss.toCharArray();

        ss = ss.toLowerCase();
        ss = ss.toUpperCase();
        ss = ss.trim();

        //StringBuilder

        StringBuilder stringBuilder = new StringBuilder("");
        stringBuilder.append("sdf");
        int aa = stringBuilder.capacity();
        int bb = stringBuilder.length();

        stringBuilder.delete(1, 2);//左闭右开

        stringBuilder.reverse();
        stringBuilder.deleteCharAt(1);
        stringBuilder.insert(0, "0000");

    }
}