If you change an element you got from the array list, it should be update in the list as well.

1.     lines[currentPage].add(new Line(0, 0, 0, 0,
2.     lineWidth, globalColor, "", drawSphere, globalColorFill )); // add
3.     
4.     // get last one and print   
5.     Line line = (Line) lines[currentPage].get( lines[currentPage].size()-1 );
6.     println("Before: " + line.boxPos1.x);
7. 
   
8.     // change last one --- this doesn't work ---- how can I do this right??
9.     ((Line) lines[currentPage].get( lines[currentPage].size()-1 )).boxPos1=lineBackup.boxPos1;
10.     ((Line) lines[currentPage].get( lines[currentPage].size()-1 )).boxSize1=lineBackup.boxSize1;
11.    
12.     // test
13.     line.boxPos1.x=-1111;
14. 
    
15.     // get last one again and print (prints -1111 instead of lineBackup.boxPos1 )
16.     line = (Line) lines[currentPage].get( lines[currentPage].size()-1 );   
17.     println("After: " + line.boxPos1.x);