Java heap space

Hello,
i write a program that creates many hashTable; it counts the frequency of each word in some document s(that it's very large). So there are many words... This is a part of code. Do you know how come overcome this problem (in general) ?

thanks

Code:

..............................
HashMap<String, Double> frequency = null;
  for (every document )  {
        frequency = new HashMap<String, Double>();
         //select a word and count
        frequecny.put( myWord , new Integer (count ) );
}
.................................................................

Code:

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
        at java.util.HashMap.resize(Unknown Source)
        at java.util.HashMap.addEntry(Unknown Source)
        at java.util.HashMap.put(Unknown Source)
        at MyClass.setting(MyClass.java:71)
        at filter.main(filter.java:19)

public String read(String filesDir, String file){
StringBuilder fileData = new StringBuilder(1000);
try {
BufferedReader reader = new BufferedReader( new FileReader( new File(filesDir + "//" + file) ));
try {
char[] buf = new char[1024];
int numRead = 0;
while((numRead=reader.read(buf)) != -1){
fileData.append(buf, 0, numRead);
}
} finally { reader.close(); }
} catch (IOException e) {
System.out.println("file not found");
System.exit(-1);
}
return fileData.toString();