求帮写几个JAVA小程序！急急急！

第一个不知道“回文”怎么理解，2,3,4都有了package test;

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

public class Test {

public static void main(String[] args) {
String str = "Hello,How are you?";
System.out.println(getPan(str));
System.out.println(getCount(str, 'l'));
System.out.println(change(1531858758));
}

/**
* 字符串中删去所有重复的字符，每种字符只保留一个
*/
public static String getPan(String str) {
Set set = new HashSet();
for (int i = 0; i < str.length(); i++) {
set.add(str.charAt(i));
}
String result = "";
Iterator it = set.iterator();
while (it.hasNext()) {
result = result + it.next();
}
return result;
}

/**
* 统计一个字符串中给定字符串出现的频率
*/
public static int getCount(String str, char c) {
int count = 0;
for (int i = 0; i < str.length(); i++) {
if (str.charAt(i) == c) {
count += 1;
}
}
return count;
}

/**
* 将一个表示十进制数的字符串转换为以逗号分隔的字符串，从右边开始每三个数字标一个逗号
*/
public static String change(int count) {
int i;
String result = "";
for (i = 1; count > 0; i++) {
int a = count % 10;
if (i % 3 == 0) {
result = "," + a + result;
} else {
result = a + result;
}
count = count / 10;
}
i = i - 1;
if (i % 3 == 0) {
return result.substring(1);
} else {
return result;
}
}
}