Exception in thread "Thread-4" java.lang.NullPointerException? - multithreading

Question

i have a problem with this exception.
I'm trying to make a function to display data from database on a table, every keystroke i made at jtextfield. So its like the table automatically refreshed with new data everytime i type in the jtextfield.
Here's the code :
First i have this variable :
private Statement stmt;
List<Barang> dataSBarang =new LinkedList();
boolean searchBarang=true;
Searching sBarang;
And this is how i call the function :
private void inputkodeTFMouseClicked(java.awt.event.MouseEvent evt){
sBarang = new Searching( stmt, dataSBarang, modelDetail, tabelDetailBarang, inputkodeTF, searchBarang);
sBarang.start();
}
And this is the Searching Object
public class Searching extends Thread{
private Statement stmt;
private List<Barang> dataBarang;
private JTable jTabelBarang;
private JTextField tf;
private boolean cari;
private DefaultTableModel modelBarang;
public Searching(Statement stmt, List<Barang> dataBarang, DefaultTableModel tm, JTable jTabelBarang, JTextField tf, boolean cari){
this.stmt=stmt;
this.dataBarang=dataBarang;
this.modelBarang=tm;
this.jTabelBarang=jTabelBarang;
this.tf=tf;
this.cari=cari;
}
#Override
public void run(){
String temp="";
while(cari==true){
//System.out.println("jalan");
try{
String masukan = tf.getText();
System.out.println(masukan);
if(!masukan.equals("")&&!masukan.equals(temp)){
clearTableBarang();
//System.out.println("Mencari "+ masukan);
ResultSet rs = stmt.executeQuery("select kode_barang, nama_barang, jumlah_stok, " +
"minimal_stok, harga_jual, deskripsi_barang from BARANG WHERE (kode_barang LIKE " +
"'"+masukan+"%')");
System.out.println(rs);
while(rs.next()){
String kode_barang = rs.getString ("kode_barang");
String nama_barang = rs.getString ("nama_barang");
int jumlah_stok = rs.getInt("jumlah_stok");
int minimal_stok = rs.getInt("minimal_stok");
int harga_jual = rs.getInt("harga_jual");
String deskripsi_barang = rs.getString ("deskripsi_barang");
//System.out.println(kode_barang+" "+deskripsi_barang);
dataBarang.add(new Barang(kode_barang,nama_barang,jumlah_stok,minimal_stok,harga_jual,deskripsi_barang));
((DefaultTableModel) jTabelBarang.getModel()).insertRow(jTabelBarang.getRowCount(), new Object[]{kode_barang, nama_barang, jumlah_stok, minimal_stok, harga_jual, deskripsi_barang});
}
temp = masukan;
}
else if(masukan.equals("")&&!masukan.equals(temp)) {
clearTableBarang();
showTableBarang();
temp = masukan;
}
} catch(SQLException s){s.printStackTrace();}
catch(ArrayIndexOutOfBoundsException s){s.printStackTrace();}
try {
sleep(500);
} catch(InterruptedException e){}
}
}
public void clearTableBarang(){
int numrows = modelBarang.getRowCount();
for(int i = numrows - 1; i >=0; i--){
modelBarang.removeRow(i);
}
dataBarang.clear();
}
public void showTableBarang(){
try{
ResultSet rs = stmt.executeQuery("select kode_barang, nama_barang, jumlah_stok, minimal_stok, harga_jual, deskripsi_barang from barang");
while(rs.next()){
String kode_barang = rs.getString ("kode_barang");
String nama_barang = rs.getString ("nama_barang");
int jumlah_stok = rs.getInt("jumlah_stok");
int minimal_stok = rs.getInt("minimal_stok");
int harga_jual = rs.getInt("harga_jual");
String deskripsi_barang = rs.getString ("deskripsi_barang");
//System.out.println(kode_barang+" "+deskripsi_barang);
dataBarang.add(new Barang(kode_barang,nama_barang,jumlah_stok,minimal_stok,harga_jual,deskripsi_barang));
((DefaultTableModel)jTabelBarang.getModel()).insertRow(jTabelBarang.getRowCount(), new Object[]{kode_barang, nama_barang, jumlah_stok, minimal_stok, harga_jual, deskripsi_barang});
}
} catch(SQLException s){s.printStackTrace();}
}
public void delay(){
try {
sleep(1000000000);
} catch(InterruptedException e){}
}
}
This is the error :
Exception in thread "Thread-4" java.lang.NullPointerException
at mypkg.Searching.run(Searching.java:47)
FYI : Line 47 is pointing to
ResultSet rs = stmt.executeQuery("select kode_barang, nama_barang, jumlah_stok, " +
"minimal_stok, harga_jual, deskripsi_barang from BARANG WHERE (kode_barang LIKE " +
"'"+masukan+"%')");
Please help me solve the problem. Thank you very much. :D

Answer 1

NullPointerExceptions are the most easy ones to fix with a debugger. Just place a breakpoint on that line and see what is null.
If the line you posted is correct, you do not even need a debugger since the only thing that can throw the exception is stmt which will be null.
Note:
It is a good thing to run your DB query in a separate Thread to avoid blocking the UI. However, in your case you are updating the UI from that Thread which is not allowed and will cause weird issues. All Swing components must be accessed on the Event Dispatch Thread (EDT). Consult the Swing concurrency tutorial for more information
I do hope you are not starting up a separate Thread on each keystroke in the textfield as you indicated in your question. Looking at the code in the Thread, you remove all elements from the table and then re-add rows. So if a users types in 5 characters at a normal typing speed, you will launch 5 threads which most likely run all at the same time (since a DB connection might not be that fast if your network is lagging). That means that with your current code 5 Threads are, at the same time, removing the table model and adding rows. Even if you put all the Swing code on the EDT (see my first point), you still end up with 5 threads posting runnables on the EDT messing with your table model. I do not know what the resulting table model will be, but probably not what you want

Answer 2

Clearly stmt is null.
You declare it here:
private Statement stmt;
List<Barang> dataSBarang =new LinkedList();
boolean searchBarang=true;
Searching sBarang;
But I don't see any initialization code anywhere, so most likely it remains null for this method call:
private void inputkodeTFMouseClicked(java.awt.event.MouseEvent evt){
sBarang = new Searching( stmt, dataSBarang, modelDetail, tabelDetailBarang, inputkodeTF, searchBarang);
sBarang.start();
}