1
//判断字符串相等
2
String s = "abc";
3
if(s.equals("abc")){
4
  System.out.println("字符串相等");
5
}

1
String str = "hello";
2
char ch = str.charAt(0);  //'h'

1
str.length()  //字符串长度
2
arr.length  //数组长度
3
list.size()  //集合长度

1
String str = "hello world";
2
if(str.contains("world")){
3
  System.out.println("包含world");
4
}

1
String str = "Hello world";
2

3
str.startWith("Hello");  //是否以Hello开头
4
str.endWith("world");  //是否以world结尾

1
String str = "abcdef";
2
String s1 = str.substring(2);  //cdef
3
String s2 = str.substring(1,4);  //bcd

1
String str = "hello";
2

3
int index = str.indexOf("e");
4
//返回下标1；找不到返回-1

1
String str = "hello";
2
String newstr = str.replace("l","x");
3
System.out.println(newstr);  //hexxo

1
String str = " hello";
2
str = str.trim();  //"hello"

1
String str = "Hello";
2
System.out.println(str.toUpperCase());  //HELLO
3
System.out.println(str.toLowerCase());  //hello

1
char ch = '5';
2
if(Character.isDigit(ch)){
3
  System.out.println("是数字");
4
}

1
char ch = 'A';
2
if(Character.isLetter(ch)){
3
  System.out.println("是字母");
4
}

1
Character.isUpperCade(ch);  //是否大写
2
Character.isLowerCase(ch);  //是否小写

1
char ch = 'a';
2
char upper = Character.toUpperCase(ch);

1
char ch = 'a';
2
int num = ch - '0';

1
StringBuilder sb = new StringBuilder()'

1
result.append("hello");
2
System.out.println(result.toString());

1
StringBuilder sb = new StringBuilder("abc");
2
sb.reverse();
3
System.out.println(sb);  //cba

1
//判断回文
2
String str = "aba";
3

4
String str2 = new StringBuilder(str).reverse().toString();
5

6
if(str.equals(str2)){
7
  System.out.println("是回文");
8
}

1
StringBuilder sb = new StringBuilder("abc");
2
sb.deleteCharAt(1);  //ac

1
StringBuilder sb = new StringBuilder("hllo");
2
sb.insert(1,"e");//hello

1
StringBuilder sb = new StringBuilder("abc");
2
sb.setCharAt(1,"x");  //axc

1
StringBuilder sb = new StringBuilder();
2

3
sb.append("abc");
4

5
String result = sb.toString();