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Trail: Essential Java Classes
Lesson: Accessing System Resources

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. For more information, refer to Copying Arrays (in the Essential Java Classes trail).

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.

Note: If you don't see the applet running above, you need to install Java Plug-in, which happens automatically when you install the J2SE JRE or SDK. We strongly recommend that you install the latest version; at least 1.3.1 is required for all our applets. You can find more information in the Java Plug-in home page.
Here's the source (in a .java source file) for the TimingIsEverything applet:
import java.awt.Graphics;
import java.awt.Dimension;

import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;

public class TimingIsEverything extends java.applet.Applet {

    public long firstClickTime = 0;
    public String displayStr;

    public void init() {
        displayStr = "Double Click Me";
        addMouseListener(new MyAdapter());
    public void paint(Graphics g) {
        g.drawRect(0, 0, getSize().width-1, getSize().height-1);
        g.drawString(displayStr, 40, 30);
    class MyAdapter extends MouseAdapter {
        public void mouseClicked(MouseEvent evt) {
            long clickTime = System.currentTimeMillis();
            long clickInterval = clickTime - firstClickTime;
            if (clickInterval < 300) {
                displayStr = "Double Click!! (Interval = "
			     + clickInterval + ")";
                firstClickTime = 0;
            } else {
                displayStr = "Single Click!!";
                firstClickTime = clickTime;

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 (in the API reference documentation) 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.

Exiting the Runtime Environment

To exit the Java interpreter, call the System.exit method. The interpreter exits with the integer exit code that you specify to the exit method.
Note: The exit method causes the Java interpreter to exit, not just your Java program. Use this function with caution.

Security consideration:  Invocation of the exit method is subject to security restrictions. 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 Security Restrictions (in the Essential Java Classes trail) 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. An application's security manager is a java.lang.SecurityManager (in the API reference documentation) object.

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 Security Restrictions (in the Essential Java Classes trail) for information about the security restrictions placed on applets.

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