public class DemoTest {
public static void main(String[] args) throws IOException {
HashMap<String,String> hm = new HashMap<>();
BufferedReader br = new BufferedReader(new FileReader("E:\\1.txt"));
BufferedWriter bw = new BufferedWriter(new FileWriter("E:\\2.txt"));
String line;
while((line=br.readLine())!=null){
String[] arr = line.split("\\.");
//Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 1 这里是出错的地方
hm.put(arr[0],arr[1]);
}
for(String key:hm.keySet()){
String value = hm.get(key);
bw.write(key+"."+value);
}
bw.close();
br.close();
}
}

hm.put(arr[0],arr[1]);
//Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 1 这里是出错的地方 这个数组怎么越界了想了半天

因为你的line用“\.”分割后的长度只有1，换句话说就是你的line中不包含“\.”,明白了吗