HashTable

 

• HashTable is synchronized.
• Iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn't.
• Hashtable doesn’t allow nulls

 

Below is a HashTable Example showing how collections are manipulated using a HashTable

 // Demonstrates the Hashtable class of the java.util package.
 import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.util.Enumeration;
import java.util.Hashtable;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.JTextField;

 public class HashTableDemo extends JFrame 
 {

	 public HashTableDemo()
	 {
		 super( " Hashtable Sourcecode Example" );

		 final JLabel jlbStatus = new JLabel();
		 final Hashtable hashTable = new Hashtable();
		 final JTextArea display = new JTextArea( 4, 20 );
		 display.setEditable( false );

		 JPanel jplNorth = new JPanel();
		 jplNorth.setLayout( new BorderLayout() );
		 JPanel jplNorthSub = new JPanel();
		 jplNorthSub.add( new JLabel("Name (Key)"));
		 final JTextField jtfFirstName = new JTextField(8);
		 jplNorthSub.add( jtfFirstName );

		 jplNorthSub.add( new JLabel("Phone No"));
		 final JTextField jtfPhone = new JTextField(8);
		 jplNorthSub.add( jtfPhone );
		 jplNorth.add( jplNorthSub, BorderLayout.NORTH );
		 jplNorth.add( jlbStatus, BorderLayout.SOUTH );

		 JPanel jplSouth = new JPanel();
		 jplSouth.setLayout( new GridLayout( 2, 5 ) );
		 JButton jbnAdd = new JButton("Add");
		 jbnAdd.addActionListener( new ActionListener() {
			 public void actionPerformed( ActionEvent e ){
				 String strNum = jtfPhone.getText().trim();
				 String strName = jtfFirstName.getText().trim();
				 if((strNum!= null && strNum.equals("")) || 
						(strName != null &&strName.equals(""))){
					 JOptionPane.showMessageDialog(HashTableDemo.this, 
					"Please enter both Name and Phone No");

				 	return;
				 }
				 int num = 0;
				 try{
					 num = Integer.parseInt(strNum);	 
				 }catch(NumberFormatException ne){
					 ne.printStackTrace();
				 }
				 
				 EmployeeDetails emp = new EmployeeDetails(strName , num );
				 Object val = hashTable.put( strName, emp );

				 if ( val == null )
					 jlbStatus.setText( "Added: " + emp.toString() );
				 else
					 jlbStatus.setText( "Added: " + emp.toString() + 
							 "; Replaced: " + val.toString() );
			 }
		 } );
		 jplSouth.add( jbnAdd );

		 JButton jbnGet = new JButton( "Get" );
		 jbnGet.addActionListener(	 new ActionListener(){
			 public void actionPerformed( ActionEvent e ){
			    Object val = hashTable.get( jtfFirstName.getText().trim() );

				 if ( val != null )
				    jlbStatus.setText( "Get: " + val.toString() );
				 else
			            jlbStatus.setText( "Get: " + jtfFirstName.getText() 
							 	+ " not in table" );
			 }
		 } );
		 jplSouth.add( jbnGet );

		 JButton jbnRemove = new JButton( "Remove Name" );
		 jbnRemove.addActionListener( new ActionListener(){
			 public void actionPerformed( ActionEvent e ){
			    Object val = hashTable.remove( jtfFirstName.getText().trim() );

				 if ( val != null )
				    jlbStatus.setText( "Remove: " + val.toString() );
				 else
					 jlbStatus.setText( "Remove: " 

					+ jtfFirstName.getText() + " not in table" );
			 }
		 } );
		 jplSouth.add( jbnRemove );

		 JButton jbnIsEmpty = new JButton( "Empty ?" );
		 jbnIsEmpty.addActionListener( new ActionListener() {
			 public void actionPerformed( ActionEvent e ){
				 jlbStatus.setText( "Empty: " + hashTable.isEmpty() );
			 }
		 } );
		 jplSouth.add( jbnIsEmpty );

		 JButton jbnContains = new JButton( "Contains key" );
		 jbnContains.addActionListener( new ActionListener(){
			 public void actionPerformed( ActionEvent e ){
				 jlbStatus.setText( "Contains key: " 

								+ hashTable.containsKey
			( jtfFirstName.getText().trim() ) );
			 }
		  } );
		  jplSouth.add( jbnContains );

		 JButton jbnClear = new JButton( "Clear table" );
		 jbnClear.addActionListener( new ActionListener(){ 
			 public void actionPerformed( ActionEvent e ){
				 hashTable.clear();
				 jlbStatus.setText("HashTable Emptied");
			 }
		 } );
		 jplSouth.add( jbnClear );

		 JButton jbnDisplay = new JButton( "List objects" );
		 jbnDisplay.addActionListener( new ActionListener(){
			 public void actionPerformed( ActionEvent e ){
				 StringBuffer buf = new StringBuffer();
				 for ( Enumeration enum = hashTable.elements(); 

							enum.hasMoreElements(); )
					 buf.append( enum.nextElement() ).append( '\n' );
					 display.setText( buf.toString() );
			 }
		 } );
		 jplSouth.add( jbnDisplay );

		 JButton jbnKeys = new JButton( "List keys" );
		 jbnKeys.addActionListener( new ActionListener(){
			 public void actionPerformed( ActionEvent e ){
				 StringBuffer buf = new StringBuffer();

				 for ( Enumeration enum = hashTable.keys(); 

								enum.hasMoreElements(); )
				 buf.append(enum.nextElement() ).append( '\n' );

				 JOptionPane.showMessageDialog( null, buf.toString(), 
						 "Display Keys of HashTable ", 

							JOptionPane.PLAIN_MESSAGE );
			 }
		 } );
		 jplSouth.add( jbnKeys );
		 Container c = getContentPane();
		 c.add( jplNorth, BorderLayout.NORTH );
		 c.add( new JScrollPane( display ), BorderLayout.CENTER );
		 c.add( jplSouth, BorderLayout.SOUTH );

		 setSize( 540, 300 );
		 setVisible(true);
	 }

	 public static void main( String args[] ){
		 HashTableDemo hashTableDemo = new HashTableDemo();

		 hashTableDemo.addWindowListener( new WindowAdapter(){
			 public void windowClosing( WindowEvent e ){
				 System.exit( 0 );
			 }
		 } );
	 }
 }

 class EmployeeDetails{
	 private String name;
	 private int phoneNp;

	 public EmployeeDetails( String fName, int phNo ){
		 name = fName;
		 phoneNp = phNo;
	 }

	 public String toString(){ 
		 return name + " " + phoneNp; 
	 }
 }

     

Output

 

Download HashTableDemo.java