What s the meaning of System out println in Java

Question

Is this static println function in out class from System namespace    namespace System     class out       static println           How can I interpret this name  And where in JRE this function is defined  In java lang System java lang Object

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System out println  Hello World       System  It is the name of standard class that contains objects that encapsulates the standard I O devices of your system    It is contained in the package java lang  Since java lang package is imported in every java program by default therefore java lang package is the only package in Java API which does not require an import declaration    out The object out represents output stream i e Command window and is the static data member of the class System    So note here System out  System -Class    amp   out- static object i e why its simply referred to by classname and we need not create any object     println The println   is method  of out object that takes the text string as an argument and displays it to the standard output i e on monitor screen    Note System   -Class out      -static Object println   -method    Remember a function  in java function is called method  always  has the format function

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From the javadoc about System  here s what the doc says   public final class System extends Object  The System class contains several useful class fields and methods  It cannot be instantiated  Among the facilities provided by the System class are standard input  standard output  and error output streams  access to externally defined properties and environment variables  a means of loading files and libraries  and a utility method for quickly copying a portion of an array   Since  JDK1 0   Regarding System out  public static final PrintStream out  The  standard  output stream class Prinstream  belongs to java io package  This stream is already open and ready to accept output data   When the JVM is initialized  the method initializeSystemClass   is called that does exactly what it   s name says     it initializes the System class and sets the out variable  The initializeSystemClass   method actually calls another method to set the out variable     this method is called setOut    Typically this stream corresponds to display output or another output destination specified by the host environment or user    Regarding println     class PrintStream  public void println        For simple stand-alone Java applications  a typical way to write a line of output data is   System out println data

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System is a class of java lang package  out is an object of PrintStream class and also static data member of System class  print   and println   is an instance method of PrintStream class  it is provide soft output on console

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No  Actually out is a static member in the System class  not as in  NET   being an instance of PrintStream  And println is a normal  overloaded  method of the PrintStream class   See http   download oracle com javase 6 docs api java lang System html out   Actually  if out err in were classes  they would be named with capital character  Out Err In  due to the naming convention  ignoring grammar

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System out println        in Java code is translated into JVM  Looking into the JVM gave me better understanding what is going on behind the hood    From the book Programming form the Java Virtual Machine  This code is copied from https   github com ymasory programming-for-the-jvm blob master examples HelloWorld j   This is the JVM source code     class public HelloWorld  super java lang Object   method public static main  Ljava lang String  V  limit stack 2  limit locals 1   getstatic java lang System out Ljava io PrintStream    ldc  Hello  world    invokevirtual java io PrintStream println      Ljava lang String  V   return   end method  end class   As  The JVM doesn t permit byte-level access to memory  the out object in type Ljava io PrintSteram  is stored in a stack with getstatic JVM command   Then the argument is pushed on the stack before called a method println of the java io PrintStream class from an instance named out  The method s parameter is  Ljava lang String   and output type is void  V

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System is a class in java lang package        out is a static object of PrintStream class in java io package        println   is a method in the PrintStream class

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Check following link   http   download oracle com javase 1 5 0 docs api java lang System html  You will clearly see that   System is a class in the java lang package   out is a static member of the System class  and is an instance of java io PrintStream   println is a method of java io PrintStream   This method is overloaded to print message to output destination  which is typically a console or file

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System out println   System is a class in the java lang package   out is a static data member of the System class and references a variable of the PrintStream class

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System is a class  that has a public static field out   So it s more like  class System        public static PrintStream out     class PrintStream       public void println         This is a slight oversimplification  as the PrintStream class is actually in the java io package  but it s good enough to show the relationship of stuff

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System  is predefined class of java lang package   out  is a static member of printStream class and its connect with console   Println  is a method of printstream class and its not a static

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System - class which is final in nature  public final class System    Belongs to java lang package  out - static reference variable of type PrintStream  println   - non static method in PrintStream class  PrintStream belongs to java io package   To understand it better you can visit   How System out println   Works In Java

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It is quite simple to understand the question  but to answer it we need to dig deeper in to Java native code     System is static class and cannot be instantiated out is a reference variable defined in System println   is the method used to print on standard output    A brief and nice explanation is always welcome on this as we can learn much from this single line of statement itself

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System out println      System is the class  out is a variable in the System class and it is a static and variable type is PrintStream   Here is the out variable in System class   public final static PrintStream out   null    You can see implementation of System here   println   is a overloaded method in PrintStream class   PrintStream includes three overloaded printing methods  those are    print   println   printf     You can see implementation of PrintStream here   You cannot instantiate System class and it is child class of Object and the Object is the father superclass  of every classes including classes that you defined   Here is what the oracle docs says      public final class System extends Object      The System class contains several useful class fields and methods  It   cannot be instantiated       Among the facilities provided by the System class are standard input    standard output  and error output streams  access to externally   defined properties and environment variables  a means of loading files   and libraries  and a utility method for quickly copying a portion of   an array       Since        JDK1 0   If you donot know what is meant by instantiate  read this questioh  It is C  question but the concept is same   Also  What is the difference between an Instance and an Object   If you donot know what is meant by overload read this quesiotn

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System is a class in java lang package   out is the static data member in System class and reference variable of PrintStream class   Println   is a normal  overloaded  method of PrintStream class

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println and print are the two overloaded methods which belong to the PrintStream class   To access them we need an instance of this class   A static property called out of type PrintStream is created on the System class   Hence to access the above methods we use the following statements   System out println  foo    System out print  foo

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System is a class in java lang package  And out is a PrintStream object  Nice explanation   http   lazy-geeks blogspot in 2015 01 what-is-systemoutprintln html

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Because out is being called with the System class name itself  not an instance of a class  an object   So out must be a static variable belonging to the class System  out must be instance of a class  because it is invoking the method println        the System class belongs to java lang package class System       public static final PrintStream out    class PrintStream       public void println

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System out println     High level Understanding  For understanding this we need to recall few basics of java    dot     operator in java  In java    dot operator  is used only to call methods or variables  So we can say out is either method or variable  Methods in java   we know methods always have parenthesis           after method name  So out cannot be a method in java  So out its a variable and println   is a method  Class name in java  Class name should start with Capital letter ideally in java  So System is a class    Now with basic knowledge of java we know     System is a Class out is a Variable println   is a method   Lets get more in details    out variable  static or instance    called using class name  so we know its static variable of System class  but its calling a method println   method so    out    is an object of the reference type PrintStream    the System class belongs to java lang package  class System     public static final PrintStream out              the Prinstream class belongs to java io package  class PrintStream  public void println

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System is the java class   out is the instance and also static member of PrintStream   println is the method of PrintStream

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