A Buffer is an object that represents a zone of memory that temporarily stores data items. For simplicity, we assume that each data item is a float. A Buffer has a limited capacity, corresponding to the number of data items it can store. Buffer objects support the following functionalities:

• creation: that, given an integer c, creates a Buffer object with capacity c containing 0 data items;
• remainingCapacity: that returns the remaining capacity of the Buffer;
• numDataItems: that returns the number of data items present in the Buffer;
• addDataItem: that, given a data item d (represented by a float), adds the data item to the end of the Buffer, if the Buffer has sufficient remaining capacity to store the data item; otherwise it does nothing;
• extractDataItem: that, given an integer i between 0 and numDataItems()-1, returns the data item in position i and eliminates it from the Buffer; the positions of all data items following the one in position i are decreased by 1; if the integer i is not between 0 and numDataItems()-1, a message is printed, the buffer is not modified, and 0 (as a float) is returned;
• countOccurrences: that, given a data item d, returns the number of times that such a data item is present in the Buffer;
• whichPositions: that, given a data item d, returns an array of integers of proper size containing the positions in the Buffer of the occurrences of the data item d.

Part 1. Write a Java class Buffer to represent Buffer objects.

Solution: representation of the objects, skeleton of the class, class Buffer

Part 2. Realize a static method client of the class Buffer that, given a string f that represents the name of a file containing a line with an integer c and a certain number of successive lines each containing a float, returns a Buffer object with initial capacity c and containing as data items the floats present in the file in the order in which they are read from the file.

Solution

Part 3. Describe the notions of local variable and of instance variable, and illustrate specifically the concepts of scope and lifetime.