Reading and writing files

Input/Output and Streams in Java

Alex Liu

Software Development Engineer

Casting

Converting between different primitive data types
Widening casting - automatically for smaller -> larger size
- byte -> short -> char -> int -> long -> float -> double

// Widening casting - happens automatically
byte myByte = 8;
int myInt = myByte;

Narrowing casting - manual, larger -> smaller

// Narrowing casting - is manual
double myDouble = 3.14;
int myInt = (int) myDouble; // Note the (int)

Understanding specific casting for reading file

Casting from int to char: char data = (char) 123;

// Unicode value for character 'a'
int data = 97;
// Directly print the data in `int` format
System.out.print(data);
// Cast data from `int` to `char` and print
System.out.print((char)data);

97
a

¹ https://www.ascii-code.com/

Reading data from a file using FileReader

Import the FileReader class to enable file reading functionality

import java.io.FileReader;

Read the file, print character by character and then close the file

FileReader fr = new FileReader("example.txt");
int data = fr.read();
// Read and print each character
while (data != -1) {
    System.out.print((char) data);
    data = fr.read();
}
fr.close(); // Close the file to release resources

Efficient reading with BufferedReader

BufferedReader - import java.io.BufferedReader;
- Reads text files line by line
- More efficient for larger files

// Create a BufferedReader object
BufferedReader br = new BufferedReader(new FileReader("example.txt"));
String line;
// User .readLine() to read the file line by line
while ((line = br.readLine())!= null){
  System.out.println(line);
}
br.close();

Writing data using FileWriter

FileWriter:
- Overwrites existing content by default - use caution to prevent data loss!
Import FileWriter

import java.io.FileWriter;


// Create a new `FileWriter` Object
FileWriter fw = new FileWriter("example.txt");
// Write the data to file using `.write()
fw.write("Overwriting the file.");
fw.close()

Appending data using FileWriter

FileWriter
- Allows appending data to files by setting to append mode without overwriting existing content

// Initialize `FileWriter` with append mode
FileWriter fw = new FileWriter("example.txt", true);
// Append data to the file
fw.write("Appending data.");
fw.close()

Efficient writing with BufferedWriter

BufferedWriter - import java.io.BufferedWriter;
- Writes text more efficiently using larger chunks of text
- Minimizes write operations

// Create a BufferedWriter object
BufferedWriter bw = new BufferedWriter(new FileWriter("example.txt"));
bw.write("Writing data");

// .newLine() to add line breaks
bw.newLine(); 
bw.close();

Let's practice!

Input/Output and Streams in Java