#system.err

First Java “Hello World” Program

First Java “Hello World” Program

Here is the first Java program “Hello World” program for basic understanding about the Java Syntax and execution of a program.

public class HelloWorldFirstProgram { public static void main(String[] args) { // Prints the string to the console. System.out.println("Hello World!"); } }

Output

Hello World !

See Also :

How to execute a Java…

Java: System.out Vs System.in Vs System.err classes and methods.

The java.lang.System class, having three static members (in, out and err) that pre-initialize at JVM startup that’s what no need to initialize. These classes also known as Stream classes.

Out of these System.out is most frequent use by programmers to write output on the console.

System Stream Classes

Here is a list of System Stream classes objects:

System.in

System.out

System.err

System.in

Syste…

original source : http://tutorials.jenkov.com/java-io/system-in-out-error.html

The 3 streams System.in, System.out, and System.err are also common sources or destinations of data. Most commonly used is probably System.out for writing output to the console from console programs.

These 3 streams are initialized by the Java runtime when a JVM starts up, so you don't have to instantiate any streams yourself (although you can exchange them at runtime).

System.in

System.in is an InputStream which is typically connected to keyboard input of console programs. System.in is not used as often since data is commonly passed to a command line Java application via command line arguments, or configuration files. In applications with GUI the input to the application is given via the GUI. This is a separate input mechanism from Java IO.

System.out

System.out is a PrintStream. System.out normally outputs the data you write to it to the console. This is often used from console-only programs like command line tools. This is also often used to print debug statements of from a program (though it may arguably not be the best way to get debug info out of a program).

System.err

System.err is a PrintStream. System.err works like System.out except it is normally only used to output error texts. Some programs (like Eclipse) will show the output to System.err in red text, to make it more obvious that it is error text.

Simple System.out + System.err Example:

Here is a simple example that uses System.out and System.err:

try {  InputStream input = new FileInputStream("c:\\data\\...");  System.out.println("File opened..."); } catch (IOException e){  System.err.println("File opening failed:");  e.printStackTrace(); }

Exchanging System Streams

Even if the 3 System streams are static members of the java.lang.System class, and are pre-instantiated at JVM startup, you can change what streams to use for each of them. Just set a new InputStream for System.in or a new OutputStream for System.out or System.err, and all further data will be read / written to the new stream.

To set a new System stream, use one of th emethods System.setIn(), System.setOut() or System.setErr(). Here is a simple example:

OutputStream output = new FileOutputStream("c:\\data\\system.out.txt"); PrintStream printOut = new PrintStream(output); System.setOut(printOut);