Java Tips – Series III
Přehled: In this ‘Java Tips’ series, we have mainly worked on the hands-on coding part. The objective of this series is to help you understand the coding part and implementation details. We have tried to cover various topics along with coding examples. You will also like our Java series I a Java series II
1) How to display a clock using an applet?
A: The example has used String class and Calendar class to display the applet clock. Calendar class gives the value of ‘Hours’, ‘Minutes’ and ‘second’.
import java.awt *.;
import java.applet.*;
import java.applet.*;
import java.awt *.;
import java.util.*;
public class ClockApplet extends Applet implements Runnable{
Thread t,t1;
public void start(){
t = new Thread(tento);
t.start();
}
public void run(){
t1 = Thread.currentThread();
zatímco(t1 == t){
repaint();
zkusit{
t1.sleep(1000);
}
úlovek(InterruptedException e){}
}
}
public void paint(Graphics g){
Calendar cal = new GregorianCalendar();
String hour = String.valueOf(cal.get(Calendar.HOUR));
String minute = String.valueOf(cal.get(Calendar.MINUTE));
String second = String.valueOf(cal.get(Calendar.SECOND));
g.drawString(hour + “:” + minute + “:” + second, 20, 30);
}
}
Again you have to run the applet in a browser or applet viewer to check the output.
2) How to create an event listener in Applet?
A: Following example demonstrates how to create a basic Applet having buttons to add & subtract two numbers. Methods used here are addActionListener () to listen to an event(click on a button) & Tlačítko() constructor to create a button.
import java.applet.*;
import java.awt.event.*;
import java.awt *.;
public class EventListeners extends Applet
implements ActionListener{
TextArea txtArea;
String Add, Subtract;
int i = 10, j = 20, sum =0,Sub=0;
public void init(){
txtArea = new TextArea(10,20);
txtArea.setEditable(false);
přidat(txtArea,”center”);
Button b = new Button(“Add”);
Button c = new Button(“Subtract”);
b.addActionListener(tento);
c.addActionListener(tento);
přidat(b);
přidat(C);
}
public void actionPerformed(ActionEvent e){
sum = i + j;
txtArea.setText(“”);
txtArea.append(“i = “+ já + “\t” + “j = ” + j + “\n”);
Button source = (Tlačítko)e.getSource();
jestliže(source.getLabel() == “Add”){
txtArea.append(“Sum : ” + sum + “\n”);
}
jestliže(já >j){
Sub = i – j;
}
jiný{
Sub = j – já;
}
jestliže(source.getLabel() == “Subtract”){
txtArea.append(“Sub : ” + Sub + “\n”);
}
}
}
3) How to open a link in a new window using Applet?
A: This example demonstrates how to open a particular webpage from an applet in a new window using showDocument() method with second argument as “_blank” .
import java.applet.*;
import java.awt *.;
import java.net.*;
import java.awt.event.*;
public class testURL_NewWindow extends Applet
implements ActionListener{
public void init(){
String link_Text = “google”;
Button b = new Button(link_Text);
b.addActionListener(tento);
přidat(b);
}
public void actionPerformed(ActionEvent ae){
Button source = (Tlačítko)ae.getSource();
String link = “http://www.”+source.getLabel()+”.com”;
zkusit {
AppletContext a = getAppletContext();
URL url = new URL(link);
a.showDocument(url,”_blank”);
}
úlovek (MalformedURLException e){
System.out.println(e.getMessage());
}
}
}
4) How to read a file using Applet?
A: The following code snippet demonstrates how to read a file using an Applet using openStream() method of URL. There are different ways to read a stream like buffered stream, customized buffered stream etc. Here we have used buffered stream to read the data.
import java.applet.*;
import java.awt *.;
import java.io.*;
import java.net.*;
public class readFileApplet extends Applet{
String fileToRead = “test1.txt”;
StringBuffer strBuff;
TextArea txtArea;
Graphics g;
public void init(){
txtArea = new TextArea(100, 100);
txtArea.setEditable(false);
přidat(txtArea, “center”);
String prHtml = this.getParameter(“fileToRead”);
jestliže (prHtml != null) fileToRead = new String(prHtml);
readFile();
}
public void readFile(){
String line;
URL url = null;
zkusit{
url = new URL(getCodeBase(), fileToRead);
}
úlovek(MalformedURLException e){}
zkusit{
InputStream in = url.openStream();
BufferedReader bf = new BufferedReader
(new InputStreamReader(in));
strBuff = new StringBuffer();
zatímco((line = bf.readLine()) != null){
strBuff.append(line + “\n”);
}
txtArea.append(“File Name : ” + fileToRead + “\n”);
txtArea.append(strBuff.toString());
}
úlovek(IOException e){
e.printStackTrace();
}
}
}
5) How to create different shapes using Applet?
A: This example demonstrates how to create an applet which will have a line, an Oval & a Rectangle using drawLine(), drawOval() and drawRect() methods of Graphics class. In java Graphics class is widely used to draw diagrams and those are used in pie chart, bar diagram etc.
import java.applet.*;
import java.awt *.;
public class Shapes extends Applet{
int x=300,y=100,r=50;
public void paint(Graphics g){
g.drawLine(30,300,200,10);
g.drawOval(x-r,y-r,100,100);
g.drawRect(400,50,200,100);
}
}
The above code sample will produce the following result in a java enabled web browse
A line, Oval & a Rectangle will be drawn in the browser.
6) How to use swing applet in JAVA?
A: Swing is an important component in java to produce UI interfaces. These swing components are also used in applet to make the user interface.Following example demonstrates how to go use Swing Applet in JAVA by implementing ActionListener & by creating JLabels.
import javax.swing. *;
import java.applet.*;
import java.awt *.;
import java.awt.event.*;
public class SApplet extends Applet implements ActionListener {
TextField input,output;
Label label1,label2;
Button b1;
JLabel lbl;
int num, sum = 0;
public void init(){
label1 = new Label(“please enter number : “);
přidat(label1);
label1.setBackground(Color.yellow);
label1.setForeground(Color.magenta);
input = new TextField(5);
přidat(input);
label2 = new Label(“Sum : “);
přidat(label2);
label2.setBackground(Color.yellow);
label2.setForeground(Color.magenta);
output = new TextField(20);
přidat(output);
b1 = new Button(“Add”);
přidat(b1);
b1.addActionListener(tento);
lbl = new JLabel(“Swing Applet Example. “);
přidat(lbl);
setBackground(Color.yellow);
}
public void actionPerformed(ActionEvent ae){
zkusit{
num = Integer.parseInt(input.getText());
sum = sum num;
input.setText(“”);
output.setText(Integer.toString(sum));
lbl.setForeground(Color.blue);
lbl.setText(“Output of the second Text Box : ”
+ output.getText());
}
úlovek(NumberFormatException e){
lbl.setForeground(Color.red);
lbl.setText(“Invalid Entry!”);
}
}
}
7) How to write to a file using Applet?
A: In the following example java text area has been used to enter the user input. After entering the input it has been written in a file in local file system. The File () constructor has been used to create the file.
import java.io.*;
import java.awt *.;
import java.awt.event.*;
import javax.swing. *;
import java.applet.Applet;
import java.net.*;
public class WriteFile extends Applet{
Button write = new Button(“WriteToFile”);
Label label1 = new Label(“Enter the file name:”);
TextField text = new TextField(20);
Label label2 = new Label(“Write your text:”);
TextArea area = new TextArea(10,20);
public void init(){
přidat(label1);
label1.setBackground(Color.lightGray);
přidat(text);
přidat(label2);
label2.setBackground(Color.lightGray);
přidat(area);
přidat(write,BorderLayout.CENTER);
write.addActionListener(new ActionListener (){
public void actionPerformed(ActionEvent e){
new WriteText();
}
}
);
}
public class WriteText {
WriteText(){
zkusit {
String str = text.getText();
jestliže(str.equals(“”)){
JOptionPane.showMessageDialog(nula,
“Please enter the file name!”);
text.requestFocus();
}
jiný{
File f = new File(str);
jestliže(f.exists()){
BufferedWriter out = new
BufferedWriter(nové FileWriter(F,skutečný));
jestliže(area.getText().rovná(“”)){
JOptionPane.showMessageDialog
(nula,”Please enter your text!”);
area.requestFocus();
}
jiný{
out.write(area.getText());
jestliže(f.canWrite()){
JOptionPane.showMessageDialog(nula,
“Text is written in “+str);
text.setText(“”);
area.setText(“”);
text.requestFocus();
}
jiný{
JOptionPane.showMessageDialog(nula,
“Text isn’t written in “+str);
}
out.close();
}
}
jiný{
JOptionPane.showMessageDialog
(nula,”File not found!”);
text.setText(“”);
text.requestFocus();
}
}
}
úlovek(Exception x){
x.printStackTrace();
}
}
}
}
8) How to print summation of n numbers?
A: Following example demonstrates how to add first n natural numbers by using the concept of stack. Here the static Stack class is used to hold the numbers. The custom stackAddition () method is used to add the numbers into the stack.
import java.io.IOException;
public class AdditionStack {
static int num;
static int ans;
static Stack theStack;
public static void main(Řetěz[] args) hází IOException {
num = 50;
stackAddition();
System.out.println(“Sum=” + ans);
}
public static void stackAddition() {
theStack = new Stack(10000);
ans = 0;
zatímco (num > 0)
{
theStack.push(num);
–num;
}
zatímco (!theStack.isEmpty())
{
int newN = theStack.pop();
ans = newN;
}
}
}
class Stack {
private int maxSize;
private int[] data;
private int top;
public Stack(int s) {
maxSize = s;
data = new int[maxSize];
top = -1;
}
public void push(int p) {
data[++top] = p;
}
public int pop() {
return data[top–];
}
public int peek() {
return data[top];
}
public boolean isEmpty() {
return (top == -1);
}
}
The above code sample will produce the following result.
Sum=1225
9) How to get the first and the last element of a linked list?
A: Following example shows how to get the first and last element of a linked list with the help of linkedlistname.getFirst() and linkedlistname.getLast() of LinkedList class. All of the stack/queue/deque operations could be easily recast in terms of the standard list operations. They’re included here primarily for convenience, though they may run slightly faster than the equivalent List operations.
import java.util.LinkedList;
public class Hlavní {
public static void main(Řetěz[] args) {
LinkedList lList = new LinkedList();
lList.add(“100”);
lList.add(“200”);
lList.add(“300”);
lList.add(“400”);
lList.add(“500”);
System.out.println(“First element of LinkedList is :
” + lList.getFirst());
System.out.println(“Last element of LinkedList is :
” + lList.getLast());
}
}
The above code sample will produce the following result.
First element of LinkedList is: 100
Last element of LinkedList is: 500
10) How to add an element at first and last position of a linked list?
A: Following example shows how to add an element at the first and last position of a linked list by using addFirst() and addLast() method of Linked List class.
import java.util.LinkedList;
public class Hlavní {
public static void main(Řetěz[] args) {
LinkedList lList = new LinkedList();
lList.add(“1”);
lList.add(“2”);
lList.add(“3”);
lList.add(“4”);
lList.add(“5”);
System.out.println(lList);
lList.addFirst(“0”);
System.out.println(lList);
lList.addLast(“6”);
System.out.println(lList);
}
}
The above code sample will produce the following result.
1, 2, 3, 4, 5
0, 1, 2, 3, 4, 5
0, 1, 2, 3, 4, 5, 6
