Set a system property : System Properties « Development « Java Tutorial

public class Main {
  public static void main(String[] argv) throws Exception {
    String previousValue = System.setProperty("", "newValue");

6.2.System Properties
6.2.1.Java system properties
6.2.2.Using System.getProperty(String key) to retrieve system properties
6.2.3.Accessing a single System Properties: user.dir
6.2.4.Getting the Default System Properties
6.2.5.Setting System Properties
6.2.6.Create a user-defined property or change the value of the current property
6.2.7.Determine operating system using System class
6.2.8.Get Java specification version using System class
6.2.9.Get system properties using System class
6.2.10.Loading Native Code
6.2.11.Set a system property
6.2.12.Setting the Value of a System Property from the Command Line: add -D option to the java command when running your program.
6.2.13.Get Java Runtime Environment (JRE) version
6.2.14.Get user home directory name
6.2.15.Get operating system name and version
6.2.16.Get file separator symbol
6.2.17.Get path / classpath separator
6.2.18.Get Java Home directory
6.2.19.Get username of system current user
6.2.20.Clear system property: The System.clearProperty() method available in Java 1.5
6.2.21.Get operating system temporary directory / folder
6.2.22.Get Environment Variables
6.2.23.Terminate a Java application
6.2.24.System.setProperty("apple.laf.useScreenMenuBar", "true")
6.2.25.Emit a beep by printing the ASCII Bell character to the console
6.2.26.Returns a default value if the specified property does not exist