Java考试前的补天
String
判断字符串相等
//判断字符串相等
String s = "abc";
if(s.equals("abc")){
System.out.println("字符串相等");
}
取某个位置的字符
常用于遍历字符串
String str = "hello";
char ch = str.charAt(0); //'h'
长度
str.length() //字符串长度
arr.length //数组长度
list.size() //集合长度
判断是否包含
String str = "hello world";
if(str.contains("world")){
System.out.println("包含world");
}
判断开头结尾
String str = "Hello world";
str.startWith("Hello"); //是否以Hello开头
str.endWith("world"); //是否以world结尾
截取字符串
String str = "abcdef";
String s1 = str.substring(2); //cdef
String s2 = str.substring(1,4); //bcd
查找位置——indexOf
String str = "hello";
int index = str.indexOf("e");
//返回下标1；找不到返回-1
替换字符串——replace()
String str = "hello";
String newstr = str.replace("l","x");
System.out.println(newstr); //hexxo
去掉首尾空格——trim()
String str = " hello";
str = str.trim(); //"hello"
大小写转换
String str = "Hello";
System.out.println(str.toUpperCase()); //HELLO
System.out.println(str.toLowerCase()); //hello
char
判断是否是数字
char ch = '5';
if(Character.isDigit(ch)){
System.out.println("是数字");
}
判断是否是字母
char ch = 'A';
if(Character.isLetter(ch)){
System.out.println("是字母");
}
判断大小写字母
Character.isUpperCade(ch); //是否大写
Character.isLowerCase(ch); //是否小写
大小写转换
char ch = 'a';
char upper = Character.toUpperCase(ch);
字符转整数
char ch = 'a';
int num = ch - '0';
StringBuilder
创建StringBuilder
StringBuilder sb = new StringBuilder()'
追加内容
result.append("hello");
System.out.println(result.toString());
反转字符串——判断回文
StringBuilder sb = new StringBuilder("abc");
sb.reverse();
System.out.println(sb); //cba
//判断回文
String str = "aba";
String str2 = new StringBuilder(str).reverse().toString();
if(str.equals(str2)){
System.out.println("是回文");
}
删除——deleteCharAt()
StringBuilder sb = new StringBuilder("abc");
sb.deleteCharAt(1); //ac
插入——insert()
StringBuilder sb = new StringBuilder("hllo");
sb.insert(1,"e");//hello
修改——secCharAt()
StringBuilder sb = new StringBuilder("abc");
sb.setCharAt(1,"x"); //axc
最后要转String
StringBuilder sb = new StringBuilder();
sb.append("abc");
String result = sb.toString();