package demo;public class StringTest {public static void main(String[] args) {getCount("IamHandsome666");}public static void getCount(String str) {int upper = 0;int lower = 0;int digit = 0;for (int i = 0; i < str.length(); i++) {char c = str.charAt(i);if (c >= 65 && c <= 90) {upper++;} else if (c >= 97 && c <= 122) {lower++;} else if (c >= 48 && c <= 57) {digit++;}}System.out.println(upper);//2System.out.println(lower);//9System.out.println(digit);//3
    }
}

package demo;public class StringTest {public static void main(String[] args) {String string = toConvert("iAMhaNdSoMe");System.out.println(string);//输出：Iamhandsome
    }public static String toConvert(String str) {String first = str.substring(0,1);String after = str.substring(1);first = first.toUpperCase();after = after.toLowerCase();return first+after;}
}

package demo;public class StringTest {public static void main(String[] args) {System.out.println(getStringCount("Ilikejava,andjavaisthebest,java", "java"));}public static int getStringCount(String str, String key) {int count = 0;int index = 0;while ((index = str.indexOf(key)) != -1) {count++;str = str.substring(index+key.length());}return count;}
}
//输出：3

package demo;public class StringBufferDemo {public static void main(String[] args) {append();delete();insert();replace();reverse();toString_();}public static void append(){StringBuffer buffer = new StringBuffer();buffer.append(6);buffer.append("hello");System.out.println(buffer);//6hello
    }public static void delete(){StringBuffer buffer = new StringBuffer();buffer.append("helloIlikeJava");buffer.delete(1, 2);buffer.deleteCharAt(8);System.out.println(buffer);//hlloIlikJava
    }public static void insert(){StringBuffer buffer = new StringBuffer();buffer.append("java");buffer.insert(1, "Python");System.out.println(buffer);//jPythonava
    }public static void replace(){StringBuffer buffer = new StringBuffer();buffer.append("abcdefg");buffer.replace(2, 4, "H");System.out.println(buffer);//abHefg
    }public static void reverse(){StringBuffer buffer = new StringBuffer();buffer.append("abcdefg");buffer.reverse();System.out.println(buffer);//gfedcba
    }public static void toString_(){StringBuffer buffer = new StringBuffer();buffer.append("abcdefg");String string = buffer.toString();System.out.println(string);//输出一个String类型的abcdefg，即字符串
    }
}

public class StringBufferTest {public static void main(String[] args) {int[] arr = {4,1,4,56,7,8,76};System.out.println(toString(arr));}/** 目的：* int[] arr = {34,12,89,68};将一个int[]中元素转成字符串 * 格式 [34,12,89,68]*/public static String toString(int[] arr){//创建字符串缓冲区StringBuffer buffer = new StringBuffer();buffer.append("[");//数组遍历for(int i = 0 ; i < arr.length;i++){//判断是不是数组的最后一个元素if(i == arr.length-1){buffer.append(arr[i]).append("]");}else{buffer.append(arr[i]).append(",");}}return buffer.toString();}
}