创建实例：
String str = new String();
str = "String";
1、检测两个字符串是否相等，一定不要用 ==(内存位置相同) 来判断，用equals(值相等)如下例所示，返回值为 false
if(str.substring(2) == "ring"){
System.out.println("True");
}
else{
System.out.println("false");
}
2、char charAt(int index)：返回制定位置的代码单元
System.out.println("String.charAt:" + str.charAt(0));
//return "String.charAt:S"
3、int codePointAt(int index)：返回从指定位置开始或结束的代码点(ASCII码)
System.out.println("String.codePointAt:" + str.codePointAt(0));
//return "String.codePointAt:83";
4、int offsetByCodePoints(int startIndex, int cpCount)：返回从startIndex代码点开始，位移cpCount后的代码点索引
//注意：是 位移cpCount后 的代码点索引
System.out.println("String.offsetByCodePoints:" + str.offsetByCodePoints(0, 2));
//return "2";
5、String substring(int beginIndex)：返回指定起始位置至字符串末尾的字符串
substring(int beginIndex, int endIndex)：返回指定起始位置（含）到结束位置（不含）之间的字符串
System.out.println("String.substring:" + str.substring(str.offsetByCodePoints(0, 2)));
//return "String.substring:ring";
System.out.println("String.substring:" + str.substring(str.offsetByCodePoints(0, 2), str.offsetByCodePoints(0, 2)+1));
//return "String.substring:r";
6、int compareTo(String other)：如果字符串str在other之前，返回负数；如果str在other之后，返回正数，如果字符串相等，返回0
System.out.println("String.compareTo:" + str.compareTo("Show String"));
//return "12";
System.out.println("String.compareTo:" + str.compareTo("String"));
//return "0";
System.out.println("String.compareTo:" + str.compareTo("String nagitive"));
//return "-9";
7、boolean startsWith(String prefix)：如果字符串以prefix开头返回true，否则返回false
boolean endsWith(String suffix)：如果字符串以suffix结尾返回true，否则返回false
System.out.println("String.startsWith:" + str.startsWith("g"));
//return "String.startsWith:false";
System.out.println("String.endsWith:" + str.endsWith("g"));
//return "String.endsWith:true";
8、boolean equals(Object other)：如果字符串与other相等返回true，否则返回false
boolean equalsIgnoreCase(String other)：如果字符串与other相等（忽略大小写）返回true，否则返回false
System.out.println("String.equals:" + str.equals("string"));
//return "String.equals:false";
System.out.println("String.equalsIgnoreCase:" + str.equalsIgnoreCase("string"));
//return "String.equalsIgnoreCase:true";
9、int indexOf(String str)：返回指定字符串的索引位置
int indexOf(String str, int fromIndex)：返回从指定索引位置fromIndex开始的str的索引位置,如果没有返回-1
int indexOf(int cp)：cp----ASCII码对应的十进制码
int indexOf(int cp, int fromIndex)：----ASCII码对应的十进制码,formIndex字符串索引起始位置-----返回从指定索引位置fromIndex开始的str的索引位置,如果没有返回-1
System.out.println("String.indexOf:" + str.indexOf("i"));
//return "String.indexOf:3";
System.out.println("String.indexOf:" + str.indexOf("i", 1));
//return "String.indexOf:3";
System.out.println("String.indexOf:" + str.indexOf("i", 4));
//return "String.indexOf:-1";
System.out.println("String.indexOf:" + str.indexOf(83));//83:S
//return "String.indexOf:0";
System.out.println("String.indexOf222:" + str.indexOf(110, 0));//110:n
//return "String.indexOf:4";
10、int lastIndexOf(String str)：返回最后一个与str匹配的字符串的索引位置,如果没有返回-1
int lastIndexOf(String str, int fromIndex)：返回从指定索引位置fromIndex开始，最后一个与str匹配的字符串的索引位置,如果没有返回-1
int lastIndexOf(int cp)：cp----ASCII码对应的十进制码
int lastIndexOf(int cp, int fromIndex)：----ASCII码对应的十进制码,formIndex字符串索引起始位置-----返回从指定索引位置fromIndex开始的str的索引位置,如果没有返回-1
System.out.println("String.lastIndexOf:" + str.lastIndexOf("n"));
//return "String.lastIndexOf:4";
System.out.println("String.lastIndexOf:" + str.lastIndexOf("n", 3));
//return "String.lastIndexOf:-1";
System.out.println("String.lastIndexOf:" + str.indexOf(83));//83:S
//return "String.indexOf:0";
System.out.println("String.lastIndexOf:" + str.indexOf(110, 0));//110:n
//return "String.indexOf:4";
11、int length()：返回字符串长度
System.out.println("String.length:" + str.length());
//return "String.length:6";
12、String replace(CharSequence oldString, CharSequence newString):用newString替换字符串中的oldString
System.out.println("String.replace:" + str.replace("g", "gs"));
//return "String.replace:Strings";
13、String toLowerCase()：转换为小写字母
String toUpperCase()：转换为大写字母
System.out.println("String.toLowerCase:" + str.toLowerCase());
System.out.println("String.toUpperCase:" + str.toUpperCase());
14、String trim()：返回一个去除两头空格的新字符串
str = str.replace("g", "g ");
System.out.println("String.trim:" + str.trim());
//return "String.trim:String";