What's the meaning of System.out.println in Java?

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?

Solution 1:

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).

Solution 2:

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.

Solution 3:

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();
//...
}

Solution 4:

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.

Solution 5:

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");