FURTHER PROGRAMMING Lecture 5

1. FURTHER PROGRAMMINGLecture 5 • A list class • Using 2D arrays • Null pointers again • The debugger MSc IT Further Programming Lecture 5

2. Why Arrays • Arrays • Some programming languages only provide arrays as a form of list • Fast for accessing elements directly • Slow for adding and deleting elements if we want to keep the list in a sorted order • This comes next week • There are lots of alternative ways of storing lists • Algorithms and Data Structures elective module MSc IT Further Programming Lecture 5

3. A LIST OF ITEMS CLASS • What will the instance variables be? • What will the constructor do? • What methods do we need? See L5IntArray and HoursOfSun for example of a class which contains a list of items in an array MSc IT Further Programming Lecture 5

4. Size of array • Arrays in Java must be created with a fixed size • Either we refuse to add additional items when the array is full • Or we copy the entire array and recreate a bigger area when required • Our approach will be • To assume that we can realistically create an array that will be big enough • However, always to check that there is room in the array and produce an error message if necessary MSc IT Further Programming Lecture 5

5. hoursOfSun Points to details 1 3 0 9 8 6 0 ASSIGNING MEMORY TO AN ARRAY • When an array is declared with no values and no size, a small amount of space is allocated to it to hold a ‘pointer’ • The pointer is the address in memory of the full details for the object, and is initially null • Memory is only allocated once the array is initialised hoursOfSun Points nowhere MSc IT Further Programming Lecture 5

6. NULL POINTER EXCEPTION • You can get a null pointer exception by not initialising the array in the HoursOfSun class • Either by omitting this line in the constructorhoursOfSun = new int [7]; • I then ran the program : error when I try to refer to a speific item in the array in the addEntry method. sunHours[numInList] = number; java.lang.NullPointerException at HoursOfSun.addEntry(HoursOfSun.java:31) at L3cIntArray.main(L3cIntArray.java:46) MSc IT Further Programming Lecture 5

7. We will not use • We will not be using these java classes in Further Programming • Array : methods which allow you to create arrays and get and set the elements • Arrays : methods for sorting and searching • ArrayList : provides methods to add, delete, return values etc • System.arraycopy : probably not required by the coursework • We are teaching arrays from first principles using Java, rather than Java programming MSc IT Further Programming Lecture 5

8. Multiple arrays • You can have arrays of many dimensions. The next example shows 2D arrays • In object oriented programming, quite often we do not need 2D arrays as these may be better modelled as arrays of objects which are themselves arrays • Here, could have an array of Weeks, where each week is an array of sunshine hours for each day of the week • The above comment is also true for parallel arrays covered later • However, we are covering ‘basic programming concepts’ which is why we are doing parallel arrays and multi-D arrays MSc IT Further Programming Lecture 5

9. 2 D arrays : notation • Declare a 2D array with the rows first, then the columns int [][] sunshine = new int [4][7]; MSc IT Further Programming Lecture 5

10. 2D arrays contd • Each element needs to have the row and the column specified sunshine[1][5] = 9; pw.println(sunshine[weekIndex][dayIndex]); • Use nested loops to traverse complete array • Use meaningful indexes e.g. rowIndex, columnIndex, weekIndex, dayIndex rather than iand j. MSc IT Further Programming Lecture 5