How does System out print work

Question

I have worked with Java for a quite a long time  and I was wondering how the function System out print   works   Here is my doubt   Being a function  it has a declaration somewhere in the io package  But how did Java developers do that  since this function can take in any number of arguments and any argument types no matter how they are arranged  e g   System out print  Hello World    System out print  My name is    foo   System out print  Sum of     a    and     b    is     c   System out print  Total USD is     usd     No matter what is the datatype of variables a  b  c  usd  foo or how they are passed  System out print   never throws an error   For me  I have never worked on any project where the requirement was like this  Provided  if I get a requirement like this  I really don t know how to solve it   Can anyone explain to me how it s done

User · Accepted Answer

System out is just an instance of PrintStream  You can check its JavaDoc  Its variability is based on method overloading  multiple methods with the same name  but with different parameters    This print stream is sending its output to so called standard output     In your question you mention a technique called variadic functions  or varargs   Unfortunately that is not supported by PrintStream print  so you must be mistaking this with something else  However it is very easy to implement these in Java  Just check the documentation     And if you are curious how Java knows how to concatenate non-string variables  foo    1   true   myObj  it is mainly responsibility of a Java compiler    When there is no variable involved in the concatenation  the compiler simply concatenates the string  When there is a variable involved  the concatenation is translated into StringBuilder append chain  There is no concatenation instruction in the resulting byte code  i e  the   operator  when talking about string concatenation  is resolved during the compilation   All types in Java can be converted to string  int via methods in Integer class  boolean via methods in Boolean class  objects via their own  toString        You can check StringBuilder s source code if you are interested     UPDATE  I was curious myself and checked  using javap  what my example System out println  foo    1   true   myObj  compiles into  The result   System out println new StringBuilder  foo1true   append myObj  toString

User · Answer

Its all about Method Overloading   There are individual methods for each data type in println   method  If you pass object       Prints an Object and then terminate the line  This method calls at first String valueOf x  to get the printed object s string value  then behaves as though it invokes print String  and then println      If you pass Primitive type      corresponding primitive type method calls   if you pass String         corresponding println String x  method calls

User · Answer

Evidently  the compiler was made in a confusing way although the compiler developers thought they added some smartness  The true smartness they should really add is to look entire argument and interpret   operator consistently  For example  System out println 1 2  hello  3 4   should output 3hello7 instead of 3hello34

User · Answer

I think you are confused with the printf String format  Object    args  method  The first argument is the format string  which is mandatory  rest you can pass an arbitrary number of Objects   There is no such overload for both the print   and println   methods

User · Answer

The scenarios that you have mentioned are not of overloading  you are just concatenating different variables with a String   System out print  Hello World     System out print  My name is    foo    System out print  Sum of     a    and     b    is     c     System out print  Total USD is     usd     in all of these cases  you are only calling print String s  because when something is concatenated with a string it gets converted to a String by calling the toString   of that object  and primitives are directly concatenated  However if you want to know of different signatures then yes print   is overloaded for various arguments

User · Answer

You can convert anything to a String as long as you choose what to print  The requirement was quite simple since Objet toString   can return a default dumb string  package classname       object number   If your print method should return an XML or JSON serialization  the basic result of toString   wouldn t be acceptable  Even though the method succeed   Here is a simple example to show that Java can be dumb  public class MockTest   String field1   String field2   public MockTest String field1 String field2   this field1 field1  this field2 field2        System out println new MockTest  a   b      will print something package Mocktest 3254487   Even though you only have two String members and this could be implemented to print  Mocktest 3254487  field1   a   field2   b      or pretty much how it appears in the debbuger

User · Answer

It is a very sensitive point to understand how to work System out print  If the first element is String then plus    operator works as String concate operator  If the first element is integer plus    operator works as mathematical operator   public static void main String args          System out println  String    8   8     String88     System out println 8   8   String      16String

User · Answer

ikis  firstly as  Devolus said these are not multiple aruements passed to print    Indeed all these arguments passed get  concatenated to form a single String  So print   does not teakes multiple arguements  a  k  a  var-args   Now the concept that remains to discuss is how print   prints any type of the arguement passed to it    To explain this - toString   is the secret   System is a class  with a static field out  of type PrintStream  So you re calling the println Object x  method of a  PrintStream   It is implemented like this    public void println Object x       String s   String valueOf x      synchronized  this           print s          newLine             As wee see  it s calling the String valueOf Object  method  This is implemented as follows    public static String valueOf Object obj       return  obj    null     null    obj toString        And here you see  that toString   is called     So whatever is returned from the toString   method of that class  same gets printed   And as we know the toString   is in Object class and thus inherits a default iplementation from Object   ex  Remember when we have a class whose toString   we override and then we pass that ref variable to print  what do you see printed  - It s what we return from  the toString

User · Answer

Even though it look as if System put print      take a variable number of arguments it doesn t  If you look closely  the string is simply concatenated and you can do the same with any string  The only thing that happens is  that the objects you are passing in  are implicitily converted to a string by java calling the toString   method   If you try to do this it will fail   int i   0  String s   i  System out println s     Reason is  because here the implicit conversion is not done   However if you change it to  int i   0  String s        i  System out println s     It works and this is what happens when using System put print      as well   If you want to implement a variable number of arguments in java to mimimc something like C printf you can declare it like this   public void t String s  String     args        String val   args 1       What happens here is that an array of Strings is passed in  with the length of the provided arguments  Here Java can do the type checking for you   If you want truly a printf then you have to do it like this   public void t String s  Object     args        String val   args 1  toString        Then would you have to cast or interpret the arguments accordingly

[java] How does System.out.print() work?

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