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Java Input/Output

Theory

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Java Input/Output

Input/Output is about reading or writing data outside the program — files, bytes, or characters. Byte streams (InputStream/OutputStream) handle raw bytes. Character streams (Reader/Writer) handle text.

For Input/Output, always close resources. Prefer try-with-resources so close happens even if an error occurs. Buffered streams are faster for many small reads.

Input/Output viva: serialization turns an object into bytes. transient fields are skipped. Don’t serialize secrets without a plan.

Let's take this on the board with one tiny Main class — no extra files. Java Input/Output — output: Hello File. StringReader → StringWriter is the same read/write pattern as a real file, and this Main always runs. Start from main, go line by line, and stop at each print. That output is the proof for Input/Output.

Exam tip

Always mention closing resources in I/O answers.

Example

import java.io.*;
public class Main {
  public static void main(String[] args) throws Exception {
    String text = "Hello File";
    try (StringReader in = new StringReader(text);
         StringWriter out = new StringWriter()) {
      int ch;
      while ((ch = in.read()) != -1) out.write(ch);
      System.out.println(out.toString());
    }
  }
}

Java Input/Output — output: Hello File. StringReader → StringWriter is the same read/write pattern as a real file, and this Main always runs.

Short notes

DefInput/Output — I/O = read/write outside the program (file, bytes, text).
RuleInput/Output — byte streams for bytes, Reader/Writer for text, close with try-with-resources.
RememberInput/Output — buffered streams are faster for many small reads.
TrapInput/Output — forgetting to close, or serializing secrets.

import java.io.*; public class Main { public static void main(String[] args) throws Exception { String text = "Hello File"; try (StringReader in = new StringReader(text); StringWriter out = new StringWriter()) { int ch; while ((ch = in.read()) != -1) out.write(ch); System.out.println(out.toString()); } } }

Java Input/Output

Example

Short notes

Questions

Java Input/Output

Example

Short notes

Questions