String类

String常用方法

示例代码：

public class Demo2 {
    public static void main(String[] args) {
        String string = "jav a";
        //        1. public int length()：返回字符串的长度
        System.out.println(string.length());
        //        2. public char charArt(int index)：根据下标获取字符
        System.out.println(string.charAt(string.length()-1));
        //        3. publiuc boolean contains(String str)：判断当前字符串中是否包含str。
        System.out.println(string.contains("java"));
        System.out.println(string.contains("ja"));
        System.out.println("----------------------------------");
        
        //        4.  public char[] toCharArray()：将字符串转换成数组
        System.out.println(Arrays.toString(string.toCharArray()));
        //        5. public int indexOf(String str)：查找str首次出现的下标，存在返回该下标，不存在返回-1。
        System.out.println(string.indexOf("a"));
        //        6. public int lastIndexOf(String str)：查找字符串在当前字符串中最后一次出现的下标索引。
        System.out.println(string.lastIndexOf("a"));
        System.out.println("----------------------------------");
        
        String s = "  QiuDajiang ";
        //        7. public String trim()：去掉字符串前后的空格
        System.out.println(s.trim());
        //        8. publiuc String toUpperCase()：将小写转换成大写  大写转为小写
        System.out.println(s.toUpperCase());
        System.out.println(s.toLowerCase());
        //        9. public boolean endWith(String str)：判断字符串是否以str结尾/开头
        System.out.println(s.endsWith("jiang"));
        System.out.println(s.startsWith(" "));
        System.out.println("----------------------------------");

        String s1 = "qiu da jiang zui shuai  le,mei ren bi ta shuai";
        //        10. public String replace(char oldChar,char newChar)：将旧字符串替换成新字符串
        System.out.println(s1.replace("i","x"));
        //        11. public String[] split(String str)：根据str拆分
        System.out.println(Arrays.toString(s1.split("[ ,]+")));
        System.out.println("----------------------------------");
        
        //补充  equals() 和 compareTo() 方法  compareTo()用于比较大小
        String s2 = "hello";
        String s3 = "HELLO";
        System.out.println(s2.equals(s3));
        System.out.println(s2.equalsIgnoreCase(s3)); //忽略大小写

        String s4 = "abc";  //a -> 97
        String s5 = "xyzasd";  //x -> 120
        System.out.println(s4.compareTo(s5));  //97-120 = -23   s4-s5

        String s6 = "abc";
        String s7 = "abcxyz";
        System.out.println(s6.compareTo(s7)); //-3   比较长度
    }
}

StringBuffer和StringBuilder

StringBuffer：

public class Demo2 {
    public static void main(String[] args) {
        StringBuffer stringBuffer = new StringBuffer();
        //1.append();   追加
        stringBuffer.append("java");
        System.out.println(stringBuffer.toString());
        stringBuffer.append("first");
        System.out.println(stringBuffer.toString());
        //2.insert();  添加
        stringBuffer.insert(0,"我在最前面");
        System.out.println(stringBuffer.toString());
        //3.replace();  替换
        stringBuffer.replace(0,5,"前面");
        System.out.println(stringBuffer.toString());
        //4.delete();  删除
        stringBuffer.delete(0,1);
        System.out.println(stringBuffer.toString());
        //清空
        stringBuffer.delete(0,stringBuffer.length());
        System.out.println(stringBuffer.toString());
    }
}

StringBuilder(效率比String高，比StringBuffer高)：

示例代码：

public class Demo2 {
    public static void main(String[] args) {
        //String效率低
//        String string = " ";
//        long start = System.currentTimeMillis();
//        for (int i = 0; i <99999 ; i++) {
//            string+=i;
//        }
//        long end = System.currentTimeMillis();
//        System.out.println(end-start);   //18293

        //StringBuilrder效率高
       StringBuilder stringBuilder = new StringBuilder();
        long start = System.currentTimeMillis();
        for (int i = 0; i <99999 ; i++) {
            stringBuilder.append(i);
        }
        long end = System.currentTimeMillis();
        System.out.println(end-start);   //8
    }
}