Birt Global Integer

I have a Birt Report which read some stuff from a database. After that i want to increment a global Integer for every Detailrow that is loaded. So far i have initialized a global Integer with the following lines:开发者_运维百科

importPackage(Packages.java.lang);
reportContext.setPersistentGlobalVariable("minTotalPlus", new Integer(0));

After that i added the following line into a field in my detail row:

reportContext.setGlobalVariable("minTotalPlus",  new Integer reportContext.getGlobalVariable("minTotalPlus")) + 1);

When i preview the report i get an "java.lang.NumberFormatException: null" which means that the global variable is null. Why is that so? How could I fix that?

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Dont' declare variables like that in the initialize method declare the like the following

materiales=0;
tools=0;

then in the fetch method use the following

tools++;

...etc.