• Java
    • JAXB Tutorial
      • What is JAXB
      • JAXB Marshalling Example
      • JAXB UnMarshalling Example
  • Spring Tutorial
    • Spring Core Tutorial
    • Spring MVC Tutorial
      • Quick Start
        • Flow Diagram
        • Hello World Example
        • Form Handling Example
      • Handler Mapping
        • BeanNameUrlHandlerMapping
        • ControllerClassNameHandlerMapping
        • SimpleUrlHandlerMapping
      • Validation & Exception Handling
        • Validation+Annotations
        • Validation+ResourceBundle
        • @ExceptionHandler
        • @ControllerAdvice
        • Custom Exception Handling
      • Form Tag Library
        • Textbox Example
        • TextArea Example
        • Password Example
        • Dropdown Box Example
        • Checkboxes Example
        • Radiobuttons Example
        • HiddenValue Example
      • Misc
        • Change Config file name
    • Spring Boot Tutorial
  • Hibernate Tutorial
  • REST Tutorial
    • JAX-RS REST @PathParam Example
    • JAX-RS REST @QueryParam Example
    • JAX-RS REST @DefaultValue Example
    • JAX-RS REST @Context Example
    • JAX-RS REST @MatrixParam Example
    • JAX-RS REST @FormParam Example
    • JAX-RS REST @Produces Example
    • JAX-RS REST @Consumes Example
    • JAX-RS REST @Produces both XML and JSON Example
    • JAX-RS REST @Consumes both XML and JSON Example
  • Miscellaneous
    • JSON Parser
      • Read a JSON file
      • Write JSON object to File
      • Read / Write JSON using GSON
      • Java Object to JSON using JAXB
    • CSV Parser
      • Read / Write CSV file
      • Read/Parse/Write CSV File – OpenCSV
      • Export data into a CSV File
      • CsvToBean and BeanToCsv – OpenCSV

JavaInterviewPoint

Java Development Tutorials

Java – How System.out.println() really work?

August 31, 2015 by javainterviewpoint Leave a Comment

This is one of most important interview question asked and probably this will be the most executed statement in Java. Many developers doesn’t know how System.out.println() actually works. In this article we will learn how it actually works.

Lets now break them into small pieces and for better understanding.

Role of Dot operator

We all know that in Java we use the Dot operator to call the methods or variable of a class. Here ‘out’ is the mysterious part as we doesn’t whether it is a variable or method ?. ‘out’ cannot possibly be a method as we don’t have a parenthesis() at the end and we are not calling like “System.out(“JIP”).println”, so from this we can be sure that out is a variable and not a method.

What type of variable is ‘out’ ?

Now we have come to a conclusion that ‘out’ is a variable, now comes the question what type of variable is ‘out’ is it static variable or instance variable ? We all know that only static variables can be called with the class name directly, same is happening here also System is a class in java.lang package we are using it to call ‘out’ variable.  Moreover ‘out’ cannot be a instance variable as we not creating any instance for calling it.

In the System class, ‘out’ is declared like below

public final static PrintStream out = nullPrintStream();

out is a static final reference of the PrintStream class.

How ‘out’ is initialized ?

We may think that nullPrintStream() method initialized ‘out’ reference, but it is not the case the nullPrintStream() simply returns null or throws an NullPointerException.

private static PrintStream nullPrintStream() throws NullPointerException {
    if (currentTimeMillis() > 0) {
        return null;
    }
    throw new NullPointerException();
}

then how ‘out’ is initialized ? When the JVM is initialized, the method initializeSystemClass() is called which initializes the out variable through setOut() method.

FileOutputStream fdOut = new FileOutputStream(FileDescriptor.out);
setOut0(new PrintStream(new BufferedOutputStream(fdOut, 128), true))

Finally how System.out.println() method works ?

Lets put all together

    • ‘out’ is a static final reference of the PrintStream class declared in the System class.
    • ‘out’ is initialized through the setOut() method which is called inisde initializeSystemClass() method of the System class.
    • Finally about println(), it is the method which is declared inside the PrintStream class
public class PrintStream extends FilterOutputStream 
{
 //out object is inherited from FilterOutputStream class
 public void println() {
 ...}
}

Other interesting articles which you may like …

  • JVM Architecture – Understanding JVM Internals
  • Object and Object Class in Java
  • Difference between JDK, JRE and JVM
  • Components of Java Development Kit (JDK)
  • What is a Class in Java with Example
  • How to open .class file in Java
  • How to Set Classpath for Java in Windows
  • ClassNotFoundException Vs NoClassDefFoundError
  • How HashMap works in Java
  • How to make a class Immutable in Java
  • Polymorphism in Java – Method Overloading and Overriding
  • Types of polymorphism in Java
  • Types of Inheritance in Java
  • Java does not supports Multiple Inheritance Diamond Problem?
  • Java Method Overloading Example
  • Java Constructor Overloading Example
  • Java this keyword | Core Java Tutorial
  • Java super keyword

Filed Under: Core Java, Java Tagged With: How System.out.println() works, Java, out, System, System class, System.out.println()

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Java Basics

  • JVM Architecture
  • Object in Java
  • Class in Java
  • How to Set Classpath for Java in Windows
  • Components of JDK
  • Decompiling a class file
  • Use of Class.forName in java
  • Use Class.forName in SQL JDBC

Oops Concepts

  • Inheritance in Java
  • Types of Inheritance in Java
  • Single Inheritance in Java
  • Multiple Inheritance in Java
  • Multilevel Inheritance in Java
  • Hierarchical Inheritance in Java
  • Hybrid Inheritance in Java
  • Polymorphism in Java – Method Overloading and Overriding
  • Types of Polymorphism in java
  • Method Overriding in Java
  • Can we Overload static methods in Java
  • Can we Override static methods in Java
  • Java Constructor Overloading
  • Java Method Overloading Example
  • Encapsulation in Java with Example
  • Constructor in Java
  • Constructor in an Interface?
  • Parameterized Constructor in Java
  • Constructor Chaining with example
  • What is the use of a Private Constructors in Java
  • Interface in Java
  • What is Marker Interface
  • Abstract Class in Java

Java Keywords

  • Java this keyword
  • Java super keyword
  • Final Keyword in Java
  • static Keyword in Java
  • Static Import
  • Transient Keyword

Miscellaneous

  • newInstance() method
  • How does Hashmap works internally in Java
  • Java Ternary operator
  • How System.out.println() really work?
  • Autoboxing and Unboxing Examples
  • Serialization and Deserialization in Java with Example
  • Generate SerialVersionUID in Java
  • How to make a class Immutable in Java
  • Differences betwen HashMap and Hashtable
  • Difference between Enumeration and Iterator ?
  • Difference between fail-fast and fail-safe Iterator
  • Difference Between Interface and Abstract Class in Java
  • Difference between equals() and ==
  • Sort Objects in a ArrayList using Java Comparable Interface
  • Sort Objects in a ArrayList using Java Comparator

Follow

  • Coding Utils

Useful Links

  • Spring 4.1.x Documentation
  • Spring 3.2.x Documentation
  • Spring 2.5.x Documentation
  • Java 6 API
  • Java 7 API
  • Java 8 API
  • Java EE 5 Tutorial
  • Java EE 6 Tutorial
  • Java EE 7 Tutorial
  • Maven Repository
  • Hibernate ORM

About JavaInterviewPoint

javainterviewpoint.com is a tech blog dedicated to all Java/J2EE developers and Web Developers. We publish useful tutorials on Java, J2EE and all latest frameworks.

All examples and tutorials posted here are very well tested in our development environment.

Connect with us on Facebook | Privacy Policy | Sitemap

Copyright ©2023 · Java Interview Point - All Rights Are Reserved ·