本文将主要讲述RecordFilter和RecordEnumeration的用法，这在整个Record Management System中都是至关重要的。由于本人觉得RecordComparator和RecordFilter类似并且用出相对小些，因此不再这里做介绍了。我们依然是通过一个示范的应用程序说明如何使用这两个接口。
RecordFilter的定义非常的简单，他只定义了一个方法那就是boolean matches(byte[] data)。
public interface RecordFilter {
boolean matches( byte[] recordData );
}
使用起来也非常简单，我们只需要实现这个方法并根据需要返回boolean类型的值就可以了，通常我们在查找纪录的时候使用这个接口，把它作为一个参数传递给enumerateRecords()方法，例如
RecordEnumeration records = rs.enumerateRecords(new RecordEntryFilter(key),null,false)，首先我们看看这个方法的参数，第一个参数是RecordFilter，它就是用来筛选数据库中的纪录的，筛选的条件就是我们上面定义的方法，boolean matches(byte[] data),第二个参数是RecordComparator，它是对选择出来的数据进行排序的，如果你不指定的话就按照默认的顺序排序。第三个参数是boolean类型的，如果是true的话，那么record会跟踪rms中的数据变化，这是比较昂贵的开销，我一般都是用false的。得到records后我们可以进行很多有用的操作，具体的方法有：
public interface RecordEnumeration {
void destroy();
boolean hasNextElement();
boolean hasPreviousElement();
boolean isKeptUpdated();
void keepUpdated( boolean keepUpdated );
byte[] nextRecord() throws InvalidRecordIDException,
RecordStoreNotOpenException,
RecordStoreException;
int nextRecordId() throws InvalidRecordIDException;
int numRecords();
byte[] previousRecord() throws InvalidRecordIDException,
RecordStoreNotOpenException,
RecordStoreException;
int previousRecordId() throws InvalidRecordIDException;
void rebuild();
void reset();
}
其中标记的方法很常用应该记住。我们实现RecordFilter的时候通常是会写成一个类的内部类，这非常普遍也很合理。我下面的程序依然使用这样的方法，
private static class RecordEntryFilter implements RecordFilter
{
private String key;
public RecordEntryFilter(String key) {
this.key = key;
}
public boolean matches(byte[] data)
{
try
{
return RecordEntry.matches(data, key);
} catch (IOException e)
{
e.printStackTrace();
return false;
}
}
}
在例子用我们向RMS中存入几个数据实体，它包括一个username和一个phonenumber字段。我们写入和读出字段的时候可以使用系列之二中的序列化机制。
import java.io.*;
/*
* Created on 2004-7-13
*
* TODO To change the template for this generated file go to
* Window - Preferences - Java - Code Style - Code Templates
*/
/**
* @author E2412C
*
* TODO To change the template for this generated type comment go to Window -
* Preferences - Java - Code Style - Code Templates
*/
public class RecordEntry
{
private String userName;
private String phoneNum;
public RecordEntry() {
}
public RecordEntry(String userName, String phoneNum) {
this.userName = userName;
this.phoneNum = phoneNum;
}
/**
* @return Returns the phoneNum.
*/
public String getPhoneNum()
{
return phoneNum;
}
/**
* @param phoneNum
* The phoneNum to set.
*/
public void setPhoneNum(String phoneNum)
{
this.phoneNum = phoneNum;
}
/**
* @return Returns the userName.
*/
public String getUserName()
{
return userName;
}
/**
* @param userName
* The userName to set.
*/
public void setUserName(String userName)
{
this.userName = userName;
}
public byte[] serialize() throws IOException
{
ByteArrayOutputStream baos = new ByteArrayOutputStream();
DataOutputStream dos = new DataOutputStream(baos);
dos.writeUTF(userName);
dos.writeUTF(phoneNum);
return baos.toByteArray();
}
public static RecordEntry deserialize(byte[] data) throws IOException
{
ByteArrayInputStream bais = new ByteArrayInputStream(data);
DataInputStream dis = new DataInputStream(bais);
RecordEntry record = new RecordEntry();
record.userName = dis.readUTF();
record.phoneNum = dis.readUTF();
return record;
}
public static boolean matches(byte[] data,String key) throws IOException
{
ByteArrayInputStream bais = new ByteArrayInputStream(data);
DataInputStream dis = new DataInputStream(bais);
return dis.readUTF().equals(key);
}
public String toString()
{
return userName+":"+phoneNum;
}
}
我们本文要讲述的另一个重点是如何使用RecordEnumeration，相关的代码写在了一个RecordModel类里面如下：
import java.io.IOException;
import javax.microedition.rms.*;
/*
* Created on 2004-7-13
*
* TODO To change the template for this generated file go to
* Window - Preferences - Java - Code Style - Code Templates
*/
/**
* @author E2412C
*
* TODO To change the template for this generated type comment go to Window -
* Preferences - Java - Code Style - Code Templates
*/
public class RecordModel
{
private RecordStore rs;
private boolean firstTime = true;
public static final String NAME = "record";
private RecordEntry[] record = { new RecordEntry("ming", "12345"),
new RecordEntry("pain", "123456"),
new RecordEntry("linux", "234566"),
new RecordEntry("mingtian", "3456677") };
private static class RecordEntryFilter implements RecordFilter
{
private String key;
public RecordEntryFilter(String key) {
this.key = key;
}
public boolean matches(byte[] data)
{
try
{
return RecordEntry.matches(data, key);
} catch (IOException e)
{
e.printStackTrace();
return false;
}
}
}
public RecordModel() {
try
{
rs = RecordStore.openRecordStore(NAME, true);
} catch (RecordStoreException e)
{
e.printStackTrace();
}
if(firstTime)
{
init();
firstTime = false;
}
}
public void init()
{
try
{
for (int i = 0; i < record.length; i++)
{
byte[] data = record[i].serialize();
rs.addRecord(data, 0, data.length);
}
} catch (IOException e)
{
e.printStackTrace();
} catch (RecordStoreException e)
{
e.printStackTrace();
}
}
public RecordEntry[] getRecord(String key)
{
try
{
if (rs.getNumRecords() > 0)
{
RecordEnumeration records = rs.enumerateRecords(
new RecordEntryFilter(key), null, false);
int length = records.numRecords();
if (length == 0)
{
return new RecordEntry[0];
} else
{
System.out.println(length);
RecordEntry[] record = new RecordEntry[length];
for (int i = 0; i < length; i++)
{
record[i] = RecordEntry.deserialize(rs
.getRecord(records.nextRecordId()));
}
return record;
}
} else
{
return new RecordEntry[0];
}
} catch (RecordStoreException e)
{
e.printStackTrace();
return new RecordEntry[0];
} catch (IOException e)
{
e.printStackTrace();
return new RecordEntry[0];
}
}
}
其中标记为灰色的代码是我们应该重点掌握的，下面是RecordFilterMIDlet类的代码：
import javax.microedition.lcdui.*;
import javax.microedition.midlet.MIDlet;
import javax.microedition.midlet.MIDletStateChangeException;
/*
* Created on 2004-7-13
*
* TODO To change the template for this generated file go to
* Window - Preferences - Java - Code Style - Code Templates
*/
/**
* @author E2412C
*
* TODO To change the template for this generated type comment go to Window -
* Preferences - Java - Code Style - Code Templates
*/
public class RecordFilterMIDlet extends MIDlet implements ItemStateListener
{
private Display display;
private Form mainForm;
private TextField textField;
private RecordModel model;
/*
* (non-Javadoc)
*
* @see javax.microedition.midlet.MIDlet#startApp()
*/
protected void startApp() throws MIDletStateChangeException
{
// TODO Auto-generated method stub
display = Display.getDisplay(this);
mainForm = new Form("Test");
model = new RecordModel();
textField = new TextField("input:", null, 20, TextField.ANY);
mainForm.append(textField);
mainForm.setItemStateListener(this);
display.setCurrent(mainForm);
}
/*
* (non-Javadoc)
*
* @see javax.microedition.midlet.MIDlet#pauseApp()
*/
protected void pauseApp()
{
// TODO Auto-generated method stub
}
/*
* (non-Javadoc)
*
* @see javax.microedition.midlet.MIDlet#destroyApp(boolean)
*/
protected void destroyApp(boolean arg0) throws MIDletStateChangeException
{
// TODO Auto-generated method stub
}
public void itemStateChanged(Item item)
{
if (item == textField)
{
String input = textField.getString();
RecordEntry[] record = model.getRecord(input);
if (record.length == 0)
{
System.out.println("no record");
} else
{
for (int i = 0; i < record.length; i++)
{
System.out.println(record[i]);
}
}
}
}
}
为了方便我没有在手机界面上显示，而是输出到控制台。下面是在input内输入ming的时候在控制台打印出来的结果：
no record
no record
no record
1
ming:12345
你可能想把以输入字段为开头的记录筛选出来，就像在通讯录中查询一样。那么你可以在Filter中添加一个int类型的type来告诉实体中的matches()方法应该如何做。这样就可以筛选出你所需要的记录了：）
还是那句话，希望对大家有用！本来还想在系列5中介绍我自己实现的一个个人通信录，程序已经写完，大概有3k的代码吧，基本覆盖了前面所讲述的专题。由于我准备投稿，因此决定不继续写Record Management System的内容了。
