Wednesday, February 14, 2007

More On Arrays

Review of Arrays

  • Declaring
    • double[] testmark;
  • Assign Space
    • testMark = new double[5]
  • Use Elements in Array
    • testMark[3] = 720;
  • an array is a table of values stored in memory
  • the array name (testMark) is a reference to the array, just like object references
  • to refer the ith member of the array
    • testMark[i]
  • Very Important
    • array indices start at zero, not one.
  • use testMark.length
    • length is the keyword to get the length of the array
  • arrays are objects
    • they should be treated as such
  • because we can change the elements of an array and because arrays are objects we have to treat them as mutable objects
    • deep copy, defensive copy
  • look again at testMark

double[]copyMarks; // create a copy of testMarks
copyMarks = testmarks //BAD!!!
  • both references (copyMarks and testMarks) point to the same thing
  • remember this also happens with object references
  • how do we create a copy of an array?
    1. allocate new space
    2. individually copy elements

testMark = new double[5]
int i;
for (i=0, i < testMark.length, i++)
{
copyMark[i] = testMark[i];
}
  • Summary
    • Arrays are mutable objects which means Defensive copying
    • copy on object containing an array
      • deep copy
    • so i do deep copy test mark
    • testMark == copyMark
      • this is not the same reference
    • unfortunately equals() does not exist for arrays
    • there exist java utilities for array equals
      • java.util.arrays
        • equals
        • fill
        • sort

2 comments:

Anonymous said...

hey Jake,
is this Friday, Feb 9th lecture??

Jake deVos said...

Yeah i Believe the Arrays part is, i checked my notes and i beleive i missed this part when i originally posted, either way it's very important so i think i'm going to leave it as a seperate post