@Test

    public void demo(){

        // 以下为String中的常用的方法及注释， 最常用的注释前有**标注

        String s = "abcdefg123456";

        // 返回下标对应的ASCII码

        int codePointAt = s.codePointAt(7);

        // **返回字符串s和字符串Q的，第一个不相等字符的ASCII码差值，如果查找结束发现都相等，则返回两个字符串的长度差值

        int compareTo = s.compareTo("Q");

        // 返回下标前一位对应的ASCII码

        int codePointBefore = s.codePointBefore(7);

        // 返回代码点长度

        int codePointCount = s.codePointCount(4, 7);

        // 返回从index=4处偏移到codePointOffSet=7之后的索引

        int offsetByCodePoints = s.offsetByCodePoints(4, 7);

        // 将字符串变成字节数组

        byte[] getBytes = s.getBytes();

        // **将字符串变成字符数组

        char[] toCharArray = s.toCharArray();

        // **返回指定索引出的字符

        char charAt = s.charAt(7);

        // **是否包含字符串"contains"

        boolean contains = s.contains("contains");

        // **将字符串s中的c全部替换为C

        String replaceAll = s.replaceAll("c", "C");

        // 将字符串中的1替换为一

        String replace = s.replace("1", "一");

        // **截取字符串s中的索引从0到3的字符

        String substring = s.substring(0, 3);

        // 截取字符串s中索引从4开始的字符

        String substring1 = s.substring(4);

        // 字符串s是否以6结尾

        boolean endsWith = s.endsWith("6");

        // 字符串是否以a开头

        boolean startsWith = s.startsWith("compareTo");

        // **将字符串s以字符c进行分割

        String[] split = s.split("c");

        // 将字符串s以字符c进行分割，分割成3份(如果最大能够分割到3份的话，如果最大不能分割到3份，则默认分割到最大份数)

        String[] split1 = s.split("c", 3);

        // 将字符串7890拼接到字符串s的后面，与+拼接字符串的区别是，+号左右的字符串都可以是null，而concat左右都不允许，否则会报空指针异常

        String concat = s.concat("7890");

        // 返回字符串是否相等，与equals区别，equals仅限于两个String类型字符串进行比较，而contentEquals可以比较类型为CharSequence的，

        // 而且如果比较的是StringBuffer还会加锁

        boolean contentEquals = s.contentEquals("contentEquals");

        // 比较两个字符串是否相等(忽略大小写)，

        boolean equalsIgnoreCase = s.equalsIgnoreCase("Abcdefg123456");

        // **比较两个字符串是否相等(不忽略大小写)

        boolean equals = s.equals("contentEquals");

        // 判断字符串是否为空

        boolean empty = s.isEmpty();

        // **获取字符串的长度

        int length = s.length();

        // **返回指定字符在字符串s中的索引，如果找不到返回-1

        int indexOf = s.indexOf("一");

        // 将字符串全部转换为小写

        String toLowerCase = s.toLowerCase();

        // 将字符串全部转换为大写

        String toUpperCase = s.toUpperCase();

        // 去除字符串前后的空格

        String trim = "  abc   ".trim();

        // 将指定数量的字符串(s,7890,AAA)以固定格式(,)进行拼接

        String join = String.join(",",s, "7890", "AAA");

        System.out.println("codePointAt = " + codePointAt);

        System.out.println("compareTo = " + compareTo);

        System.out.println("codePointBefore = " + codePointBefore);

        System.out.println("codePointCount = " + codePointCount);

        System.out.println("offsetByCodePoints = " + offsetByCodePoints);

        System.out.println("getBytes = " + Arrays.toString(getBytes));

        System.out.println("toCharArray = " + Arrays.toString(toCharArray));

        System.out.println("charAt = " + charAt);

        System.out.println("contains = " + contains);

        System.out.println("replaceAll = " + replaceAll);

        System.out.println("replace = " + replace);

        System.out.println("substring = " + substring);

        System.out.println("substring1 = " + substring1);

        System.out.println("endsWith = " + endsWith);

        System.out.println("startsWith = " + startsWith);

        System.out.println("split = " + Arrays.toString(split));

        System.out.println("split1 = " + Arrays.toString(split1));

        System.out.println("concat = " + concat);

        System.out.println("contentEquals = " + contentEquals);

        System.out.println("equalsIgnoreCase = " + equalsIgnoreCase);

        System.out.println("indexOf = " + indexOf);

        System.out.println("toLowerCase = " + toLowerCase);

        System.out.println("toUpperCase = " + toUpperCase);

        System.out.println("join = " + join);

        System.out.println("equals = " + equals);

        System.out.println("empty = " + empty);

        System.out.println("length = " + length);

        System.out.println("trim = " + trim);

    }

上述打印结果为:

codePointAt = 49

compareTo = 16

codePointBefore = 103

codePointCount = 3

offsetByCodePoints = 11

getBytes = [97, 98, 99, 100, 101, 102, 103, 49, 50, 51, 52, 53, 54]

toCharArray = [a, b, c, d, e, f, g, 1, 2, 3, 4, 5, 6]

charAt = 1

contains = false

replaceAll = abCdefg123456

replace = abcdefg一23456

substring = abc

substring1 = efg123456

endsWith = true

startsWith = false

split = [ab, defg123456]

split1 = [ab, defg123456]

concat = abcdefg1234567890

contentEquals = false

equalsIgnoreCase = true

indexOf = -1

toLowerCase = abcdefg123456

toUpperCase = ABCDEFG123456

join = abcdefg123456,7890,AAA

equals = false

empty = false

length = 13

trim = abc

以上为个人总结记录使用，希望能够帮助其他人，如有不足或错误欢迎大家指正。