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Java - Write and Read FilesJava - DirectoriesJava - IO StreamsJava - SerializationJava - Exceptions

Java - IO Streams

📌 What You Will Learn
  • What a stream is and how data flows
  • Byte streams vs character streams
  • Reading and writing text files easily
  • The modern File methods and Scanner input
  • Serialization — saving an object to a file

IO Streams is a core concept of the Java language. This lesson explains What is a Stream?, IO Class Map — Memory Aid and The Stream Family Tree with complete, runnable code examples, clear step-by-step explanations, and common mistakes to avoid with exam-style MCQs at the end.

What is a Stream?

A stream is a flow of data. It is like a pipe: data flows through it, from a source to a destination.

  • Input stream — brings data into the program (e.g., reading from keyboard or a file).
  • Output stream — carries data out of the program (e.g., writing to a file or the screen).

In Java, all input/output (IO) classes live in the java.io package. The two big families are byte streams and character streams:

PointByte streamsCharacter streams
Read/write whatBytes (images, videos, any file)Characters (text files)
Parent classesInputStream / OutputStreamReader / Writer
ExamplesFileInputStream, FileOutputStreamFileReader, FileWriter
Best forPictures, songs, zip files.txt, .csv, .html documents
In simple words: Byte streams handle raw bytes and character streams handle text. Use InputStream/OutputStream for images and videos, and Reader/Writer for .txt and .csv files.

IO Class Map — Memory Aid

Remember: text uses Reader/Writer, binary uses InputStream/OutputStream, whole objects use Object streams. Everything is under java.io.

Example02
JCode Cell
1TEXT -> Reader / Writer -> FileReader, FileWriter, BufferedReader
2BYTES -> InputStream / OutputStream -> FileInputStream, FileOutputStream
3WHOLE OBJECT -> ObjectOutputStream / ObjectInputStream

The Stream Family Tree

All IO classes fit a neat family tree. Learn the parents, and every child makes sense:

InputStream/OutputStream handle bytes (images, videos). Reader/Writer handle characters (text). ObjectInput/ObjectOutput handle whole objects (serialization). The Buffered* versions make IO faster by reading/writing in chunks.

Example03
JCode Cell
1BYTE streams CHARACTER streams
2InputStream Reader
3 |-- FileInputStream |-- FileReader
4 |-- BufferedInputStream |-- BufferedReader
5 |-- DataInputStream |-- InputStreamReader
6 |-- ObjectInputStream |-- StringReader
7 |-- ByteArrayInputStream
8 
9OutputStream Writer
10 |-- FileOutputStream |-- FileWriter
11 |-- BufferedOutputStream |-- BufferedWriter
12 |-- DataOutputStream |-- OutputStreamWriter
13 |-- ObjectOutputStream |-- PrintWriter
14 |-- PrintStream

RandomAccessFile — Read and Write Anywhere

A RandomAccessFile can both read and write, and can jump to any position (using a pointer). You choose the mode: "r" for read only, "rw" for read and write.

Example04
JCode Cell
1import java.io.RandomAccessFile;
2 
3class RandomDemo {
4 public static void main(String[] args) throws Exception {
5 RandomAccessFile raf = new RandomAccessFile("data.txt", "rw");
6 raf.writeUTF("Hello Java"); // write some text
7 raf.seek(0); // jump the pointer back to start
8 System.out.println(raf.readUTF()); // read it back
9 raf.close();
10 }
11}
Output
Hello Java
📝 Key Takeaways
  • A stream is a flow of data: input brings data in, output sends it out.
  • Byte streams (InputStream/OutputStream) for images/videos; character streams (Reader/Writer) for text.
  • Wrap FileReader in BufferedReader to read text line by line.
  • Scanner is the simplest way to read input from the keyboard.
  • The File class gives file information; it does not read data.
  • Serialization saves objects to files; transient fields are not saved.
  • Always close streams — or better, use try-with-resources.

🧠 Test Your Knowledge

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