A Buffer is an object that represents a zone of memory that temporarily stores data items. A Buffer has a limited capacity, i.e., it can store data only as long as there is still free memory in the zone represented by the buffer. For simplicity, we assume that each data item in a Buffer is represented by a string and that the amount of memory that it uses corresponds to the length of the string. 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 string), adds the data item to the end of the Buffer, if the Buffer has sufficient remaining capacity to store the data item; otherwise it throws a "Buffer full" exception;
• extractDataItem: that, given an integer i (between 0 and numDataItems()-1), returns the data item in position i and eliminates it from the Buffer;
• 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 string, returns a Buffer object with initial capacity c and containing as data items the strings 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.