I have this code here:
it uses text files and hash tables to keep some student records:
can somebody help me with the 'FindEntry' Method, i don't really get why it doesn't work....
public class HashTable {
public static class StudentRecord implements Serializable {
public String name,family;
public int stNr,flag;
public static final int SIZE=68;//record size
public StudentRecord() {
name=" ";family=" ";stNr=0;flag=0;
}
public StudentRecord(String name,String family,int stNr,int flag){
this.name=name;
this.family=family;
this.stNr=stNr;
this.flag=flag;
}
public StudentRecord(int key,String value){
stNr=key;
setValue(value);
}
public String getValue(){
return (family+"#"+name+"#");
}
public void setValue(String value){
String t[]=new String[2];
int i=0;
StringTokenizer tokens=new StringTokenizer(value,"#");
while(tokens.hasMoreTokens()){
t[i]=tokens.nextToken();
i++;
}
this.family=t[0];
this.name=t[1];
}
public void write(RandomAccessFile file)throws IOException{
writeName(file,name);
writeName(file,family);
file.writeInt(stNr);
file.writeInt(flag);
}
public void writeName(RandomAccessFile file,String str)throws IOException{
StringBuffer buffer=null;
if(str!=null)
buffer=new StringBuffer(str);
else
buffer=new StringBuffer(15);
buffer.setLength(15);
file.writeChars(buffer.toString());
}
public void read(RandomAccessFile file)throws IOException{
name=readName(file);
family=readName(file);
stNr=file.readInt();
flag=file.readInt();
}
public String readName(RandomAccessFile file)throws IOException{
char name[]=new char[15];
for(int i=0;i<name.length;i++)
name[i]=file.readChar();
return new String(name).replace('\0',' ');
}
}
protected static class DefaultEqualityTester implements EqualityTester {
....
}
protected int n = 0;
protected int N;
File filename1=new File("D:\\b.txt");
RandomAccessFile file=new RandomAccessFile(filename1,"rw");//readwrite
File filename2=new File("D:\\c.txt");
RandomAccessFile file2=new RandomAccessFile(filename2,"rw");
protected EqualityTester T; // the equality tester
protected int scale, shift; // the shift and scaling factors
public HashTable() throws FileNotFoundException, IOException {
N =1023; // default capacity
T = new DefaultEqualityTester(); // use the default equality tester
java.util.Random rand = new java.util.Random();
scale = rand.nextInt(N-1) + 1;
shift = rand.nextInt(N);
StudentRecord record=new StudentRecord();
for(int i=0;i<N;i++)
record.write(file);
}
public HashTable(int bN, EqualityTester tester) throws FileNotFoundException, IOException {
N = bN;
T = tester;
java.util.Random rand = new java.util.Random();
scale = rand.nextInt(N-1) + 1;
shift = rand.nextInt(N);
StudentRecord record=new StudentRecord();
for(int i=0;i<N;i++)
record.write(file);
}
protected int [B]findEntry[/B](Object key) throws InvalidKeyException, IOException {
int avail = 0;
checkKey(key);
int i = hashValue(key);
int j = i;
StudentRecord currentRecord=new StudentRecord();
do {
file.seek(i*StudentRecord.SIZE);
if (currentRecord == null) return -i - 1; // entry is not found
if (currentRecord.flag ==0) { // bucket is deactivated
avail = i; // remember that this slot(سوراخ) is available
i = (i + 1) % N;
}
else if (T.isEqualTo(key,currentRecord.stNr)) // we have found our entry
return i;
else // this slot is occupied--we must keep looking
i = (i + 1) % N;
} while (i != j);
return -avail - 1; // entry is not found
}
public Object get (Object key) throws InvalidKeyException, IOException {
StudentRecord currentRecord=new StudentRecord();
int i = findEntry(key); // helper method for finding a key
if (i < 0) return null; // there is no value for this key
file.seek(i*StudentRecord.SIZE);
return currentRecord.getValue(); // return the found value in this case
}
public void put (int key, String value) throws InvalidKeyException, IOException {
StudentRecord currentRecord = new StudentRecord(key, value);
if (n >= N/2) rehash(); // rehash to keep the load factor <= 0.5
int i = findEntry(key); //find the appropriate spot for this entry
if (i < 0) { // this key does not already have a value
file.seek((-i-1)*StudentRecord.SIZE); // convert to the proper index
currentRecord.write(file);
n++;
// there was no previous value
}
else { // this key has a previous value
file.seek(i*StudentRecord.SIZE);
currentRecord.write(file);
}
}
public void remove (Object key) throws InvalidKeyException, IOException {
// StudentRecord currentRecord=new StudentRecord();
int i = findEntry(key); // find this key first
if (i < 0) // nothing to remove System.out.println("this item not found!");
else{
StudentRecord currentRecord=new StudentRecord();
while(true){
do{
currentRecord.read(file);
}while(currentRecord.stNr!=key);
currentRecord.flag=0;
}
// file.seek(i*StudentRecord.SIZE);
// currentRecord.flag=0;
}
n--;
}
..
.
.
.thanks