PrintStream and PrintWriter ​

PrintStream is a FilterOutputStream that provides additional methods on top of the OutputStream interface for writing various data types:

• Write int: print(int)
• Write boolean: print(boolean)
• Write String: print(String)
• Write Object: print(Object) (which is equivalent to print(object.toString()))
• ...
• A corresponding set of println() methods, which automatically append a newline.

The commonly used System.out.println() actually uses PrintStream to print various data. Here, System.out is the default PrintStream provided by the system, representing standard output:

System.out.print(12345); // Outputs 12345
System.out.print(new Object()); // Outputs something like java.lang.Object@3c7a835a
System.out.println("Hello"); // Outputs Hello and moves to a new line

System.err is the standard error output provided by the system.

Compared to OutputStream, PrintStream adds a set of print()/println() methods that allow for easy printing of various data types. Another advantage is that it does not throw IOException, so you don't need to catch IOException when writing code.

PrintWriter ​

PrintStream ultimately outputs byte data, while PrintWriter extends the Writer interface, and its print()/println() methods output character data. The usage of both is almost identical:

import java.io.*;

public class Main {
    public static void main(String[] args) {
        StringWriter buffer = new StringWriter();
        try (PrintWriter pw = new PrintWriter(buffer)) {
            pw.println("Hello");
            pw.println(12345);
            pw.println(true);
        }
        System.out.println(buffer.toString());
    }
}

Summary ​

• PrintStream can accept various data types for output, making it convenient for printing data:
  • System.out is standard output.
  • System.err is standard error output.
• PrintWriter is output based on Writer.