GTT row not found on delete but found on insert

I have never used GTT under Oracle and Java.

I want to use it to manage some datas, I want to add some forecasting of products and I wish to remove any existing row before inserting looking it by ID

Here is my GTT

CREATE GLOBAL TEMPORARY TABLE "M_PRODUCT_FORECAST" 
(	
"M_PRODUCT_FORECAST_ID" NUMBER(10,0) NOT NULL ENABLE, 
"FORECAST_MODE" CHAR(1 BYTE) DEFAULT 'D' NOT NULL ENABLE, 
"FORECAST_QTY" NUMBER DEFAULT 1 NOT NULL ENABLE, 
CHECK (forecast_mode IN ('D','P')) ENABLE, 
CONSTRAINT "M_PRODUCT_FORECAST_KEY" PRIMARY KEY ("M_PRODUCT_FORECAST_ID", "FORECAST_MODE") ENABLE
) ON COMMIT PRESERVE ROWS ;

Then here is my Java code

private void addProductToForecast() {
Integer 	productId 		= (Integer) productSearchEditor.getValue();
BigDecimal 	qty 			= qtyNumberBox.getValue();
String 		mode 			= developementCheckbox.isSelected()?"D":"P";
PreparedStatement 	ps 							= null;
try {
String deletePreviousForcastIfExists = 	  "	DELETE FROM  "+M_PRODUCT_FORECAST
+ " 	   WHERE "+M_PRODUCT_FORECAST_ID+" = ? "
+ " 	   AND 	 "+FORECAST_MODE+"         = ?";
ps = DB.prepareStatement(deletePreviousForcastIfExists, null);
ps.setInt(1, productId);
ps.setString(2, mode);
int update = ps.executeUpdate();
log.info(deletePreviousForcastIfExists + "return update value :: "+update);
} catch (SQLException e) {
e.printStackTrace();
}finally {
Util.closeCursor(ps,null);
ps = null; 
}
try {
String insertIntoForcast = " INSERT INTO " +M_PRODUCT_FORECAST
+" VALUES(?,?,?)";
ps = DB.prepareStatement(insertIntoForcast, null);
ps.setInt(1, productId);
ps.setString(2, mode);
ps.setBigDecimal(3, qty);
int update = ps.executeUpdate();
log.info(insertIntoForcast + "return update value :: "+update);
} catch (SQLException e) {
e.printStackTrace();
} finally {
Util.closeCursor(ps, null);
ps = null;
}
}

NB

While debugging the first remove update value return 0, I guess this means it doesn't find any row to be deleted, however the insert is rising the violation of unique key exception which means there is data.

>java.sql.SQLIntegrityConstraintViolationException: ORA-00001: violation de contrainte unique (M_PRODUCT_FORECAST_KEY)

First of all, I would suggest to use merge instead of delete+insert:

merge into M_PRODUCT_FORECAST m
using (
   select :id as id, 
          :mode as mode, 
          :qty as qty 
   from dual) n
on (m.M_PRODUCT_FORECAST_ID = n.id and m.FORECAST_MODE = n.mode)
when matched then update
     set m.M_PRODUCT_FORECAST_ID = n.mode
when not matched then
     insert
     values (n.id,n.mode,n.qty)

In addition, I would check what exactly is coming into db. I mean bind variables and their types, just to confirm that there is no implicit datatype conversion or something like that