Why this pojo program “readline” always return null?

Question

public static void main(String[] args) throws IOException {
    BufferedReader reader = new BufferedReader(new FileReader("day.txt"));
    BufferedWriter writer = new BufferedWriter(new FileWriter("day.txt"));
    System.out.println(reader.readLine());
}

The day.txt I have wrote some words before execute. If I change System.out.println with Writer, it will not be null. why?

Answer 1

You are overwriting the same file when you do new FileWriter("day.txt");

change your code to

BufferedReader reader = new BufferedReader(new FileReader("day.txt"));
BufferedWriter writer = new BufferedWriter(new FileWriter("day-new.txt"));
System.out.println(reader.readLine());

Answer 2

Create the writer instance after printing out to console. When the writer is initialized, the file is in use so you can't read it.