Miscellaneous System Methods

The System class provides several methods that provide miscellaneous functionality including copying arrays and getting the current time.
Copying Arrays
Use System's arraycopy() method to efficiently copy data from one array into another. The arraycopy() method requires five arguments:
public static
    void arraycopy(Object source,
                   int srcIndex,
                   Object dest,
                   int destIndex,
                   int length,
The two Object arguments indicate the array to copy from and the array to copy to. The three integer arguments indicate the starting location in each the source and the destination array, and the number of elements to copy. This diagram illustrates how the copy takes place:

The following program uses arraycopy() to copy some elements from the copyFrom array to the copyTo array.
class ArrayCopyTest {
    public static void main(String[] args) {
        byte[] copyFrom = { 'd', 'e', 'c', 'a', 'f', 'f', 'e', 'i', 'n', 'a', 't', 'e', 'd' };
        byte[] copyTo = new byte[7];

        System.arraycopy(copyFrom, 2, copyTo, 0, 7);
        System.out.println(new String(copyTo, 0));
    }
}
The arraycopy() method call in this example program begins the copy at element number 2 in the source array--remember that array indices start at 0 so that the copy begins at the array element 'c'. The arraycopy() method call puts the copied elements into the destination array beginning at the first element (element 0) in the destination array copyTo. The copy copies 7 elements: 'c', 'a', 'f', 'f', 'e', 'i', and 'n'. Effectively, the arraycopy() method takes the "caffein" out of "decaffeinated", like this:

Note that the destination array must be allocated before you call arraycopy() and must be large enough to contain the data being copied.
Getting the Current Time
The currentTimeMillis() method returns the current time in milliseconds since 00:00:00, January 1, 1970. The currentTimeMillis() method is commonly used during performance tests: get the current time, perform the operation that you want to time, get the current time again--the difference in the two time samples is roughly the amount of time that the operation took to perform.
Often in graphical user interfaces the time difference between mouse clicks is used to determine whether a user double clicked. The following applet uses currentTimeMillis() to compute the number of milliseconds between two mouse clicks. If the time period between the clicks is smaller than 200 milliseconds, the two mouse clicks are interpreted as a double mouse click.

Your browser doesn't understand the APPLET tag. Here's a screenshot of the TimingIsEverything applet that you would see running here if it did:

Here's the source for the TimingIsEverything applet:
import java.awt.Graphics;

public class TimingIsEverything extends java.applet.Applet {

    public long firstClickTime = 0;
    public String displayStr;

    public void init() {
        displayStr = "Double Click Me";
    }
    public void paint(Graphics g) {
        g.drawRect(0, 0, size().width-1, size().height-1);
        g.drawString(displayStr, 40, 30);
    }
    public boolean mouseDown(java.awt.Event evt, int x, int y) {
        long clickTime = System.currentTimeMillis();
        long clickInterval = clickTime - firstClickTime;
        if (clickInterval < 200) {
            displayStr = "Double Click!! (Interval = " + clickInterval + ")";
            firstClickTime = 0;
        } else {
            displayStr = "Single Click!!";
            firstClickTime = clickTime;
        }
        repaint();
        return true;
    }
}
You could use the return value from this method to compute the current date and time. However, you'll probably find that it's more convenient to get the current date and time from the Date class.

Note: Previous versions of the System class support two other time-related methods besides the currentTimeMillis() method: currentTime() and nowMillis(). These two methods are now obsolete--you should use currentTimeMillis() instead. currentTime() and nowMillis() are no longer supported.
Exiting the Runtime Environment
To exit the Java interpreter, call the System.exit() method. You pass an integer exit code to the exit() method, and the interpreter exits with that exit code.
System.exit(-1);
Note: The exit() method causes the Java interpreter to exit, not just your Java program--you should use this function with caution.

Security consideration: Invocation of the exit() method is subject to security restrictions. So depending on the browser that an applet is running in, a call to exit() from within an applet will likely result in a SecurityException. See Understanding Applet Capabilities and Restrictions for information about the security restrictions placed on applets.
Setting and Getting the Security Manager

The security manager is an object that enforces a certain security policy for a Java application. You can set the current security manager for your applications using System's setSecurityManager() method and you can retrieve the current security manager using System's getSecurityManager() method.
Providing Your Own Security Manager discusses security managers in detail and shows you how to create and install your own security manager using these two System methods.

Security consideration: The security manager for an application can be set only once. Typically, a browser sets its security manager during its startup procedure. Thus, most of the time, applets cannot set the security manager because it's already been set. A SecurityException will result if your applet attempts to do so. See Understanding Applet Capabilities and Restrictions for information about the security restrictions placed on applets.

See also

java.lang.SecurityManager

Using System Resources