Java Hashtable

Java HashTable is synchronized.

Iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn’t.

Java Hashtable doesn’t allow nulls

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

Please Note that It must be Compiled in Java 1.4.

// Demonstrates the Hashtable class of the java.util package.

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