Using Hashtables

How to use Java Hashtable objects in your code.

About this technique	Details
Category	Core Programming Techniques> Java
Description	You'll learn about: Creating a Hashtable Adding data to a Hashtable Retrieving a value from a Hashtable with a key Removing selected data from a Hashtable Clearing an entire Hashtable
	You can run this technique code from: Database: Examples3_Java Form: frmHashtable NOTE   First make sure that database is running on your localhost SilverStream Server
Related reading	See the chapter on coding Java for SilverStream applications in the Programmer's Guide

This example creates a Hashtable, adds values to it, retrieves values back from it, and clears it. You can enter a key and value pair into the Hashtable. The Hashtable contents are automatically displayed in list boxes. You can then remove a selected pair, clear the entire Hashtable, or find the value associated with a selected key.

Creating a Hashtable   

The following code illustrates how to create a new Hashtable.

  // Declarations under General 
  Hashtable m_hashtable = new Hashtable(); 

Notes about the code

• A new instance of Hashtable is declared in the General section.

Adding data to a Hashtable   

The following code illustrates how to add a new key and value pair to a Hashtable. In the example, the key and value are obtained from entry fields.

  private void handle_btnAddToHashtable_actionPerformed(ActionEvent 
    evt) 
    { 
        // Get the new key 
        String sKey = fldKey.getText(); 
        // Get the new value 
        String sValue = fldValue.getText();       
        // Make sure we have a key and a value 
        if (sKey.equals("")) 
        { 
          // No key entered, so display an error message,  
          // then return 
          agDialog.showMessage("Hashtable error", "You must  
              enter a key."); 
          fldKey.requestFocus(); 
          return; 
        } 
        if (sValue.equals("")) 
        { 
          // No value entered, so display an error message,  
          // then return 
          agDialog.showMessage("Hashtable error", "You must  
             enter a value."); 
          fldValue.requestFocus(); 
          return; 
        } 
        // Add the new key/value combination to the Hashtable 
        m_hashtable.put(sKey, sValue);     
        // Add the new key to the Key list box 
        lbKeys.addItem(sKey); 
        // Add the new value to the Value list box 
        lbValues.addItem(sValue); 
       
        // Clear the Key data-entry field 
        fldKey.setText(""); 
        // Clear the Value data-entry field 
        fldValue.setText("");     
        // Set the focus to the Key data-entry field 
        fldKey.requestFocus(); 
    } 

Notes about the code

• First, the code uses getText() to obtain the new user-entered key and value pair.

• The code checks to see if either the new key or new value is empty. If so, agDialog.showMessage() is called to display an error message.

• The code adds the key and value pair to the Hashtable with put().

• The code shows the new entries in the key and value list boxes by calling addItem(). This is for display purposes only and does not affect the Hashtable.

• Finally, the entry fields are cleared to make room for future entries.

Retrieving a value from a Hashtable with a key   

The following code illustrates how to retrieve a value from a Hashtable based on the key that was stored with it.

  private void handle_btnGetValue_actionPerformed(ActionEvent evt) 
    { 
        // Get the index of the selected key in the Key list box 
        int iIndex = lbKeys.getSelectedIndex();     
        // Got a valid index? 
        if (iIndex >= 0) 
        { 
          // Yes, so get the key String 
          String sKey = lbKeys.getItemAt(iIndex); 
          // Get the value for the selected key 
          String sValue = (String) m_hashtable.get(sKey);     
          // Display the value for the selected key 
          fldGetValue.setText(sValue); 
        } 
      } 

Notes about the code

• The code gets the string value for the desired key by calling getItemAt() on the list box using the selected index, which can be obtained by calling getSelectedIndex().

• Then, the code calls get() on the Hashtable using the key string as the argument. This will return the associated value that is stored in the Hashtable.

• The value is changed to a string and is displayed in a field with setText().

Removing selected data from a Hashtable   

This code illustrates how to remove a key and value pair from a Hashtable, based on the key.

  private void handle_btnRemove_actionPerformed(ActionEvent evt) 
    { 
        // Get the index of the selected key in the Key list box 
        int iIndex = lbKeys.getSelectedIndex();     
        // Got a valid index? 
        if (iIndex >= 0) 
        { 
          // Yes, so get the key String 
          String sKey = lbKeys.getItemAt(iIndex);         
          // Remove the selected key from the Key list box 
          lbKeys.removeItemAt(iIndex);         
          // Remove the corresponding value from the Value list box 
          lbValues.removeItemAt(iIndex);        
         // Remove the key/value pair from the Hashtable 
          m_hashtable.remove(sKey); 
        } 
     } 

Notes about the code

• First, getSelectedIndex() is called to find out which row to remove.

• The code then calls getItemAt() on the selected index to get the string for the key associated with that row.

• The key and value are visually removed from the list boxes with removeItemAt(). This is for display purposes only and does not affect the Hashtable.

• Finally, the code actually removes the pair from the Hashtable by calling remove(), using the key string as the argument.

Clearing an entire Hashtable   

The following code illustrates how to clear all the data from a Hashtable.

  private void handle_btnClearHashtable_actionPerformed(ActionEvent evt) 
    { 
        // Remove every entry from the Key list box 
        for (int i = lbKeys.getItemCount() - 1; i >= 0; i--) 
        { 
          lbKeys.removeItemAt(i); 
        } 
       // Remove every entry from the Value list box 
        for (int i = lbValues.getItemCount() - 1; i >= 0; i--) 
        { 
          lbValues.removeItemAt(i); 
        } 
        // Clear the Hashtable 
        m_hashtable.clear(); 
    } 

Notes about the code

• The code uses for loops to remove each row that is displayed in the key and value list boxes. The getItemCount() method gives the number of rows to loop through.

• Within a for loop, the index is calculated by subtracting one from the row count. The index is then passed to removeItemAt(). Removing rows from the list boxes is for display purposes only and does not affect the Hashtable.

• Finally, the code clears the entire Hashtable by calling clear() on it.