Pitfalls to Avoid: Here are things to do and concepts to keep clear

+ read and follow *all* instructions *and* comments!

+ assignment is "destination = result", *not* "result = destination"

+ $==$ versus $=$

+ $return$ versus $print$

+ *declaration* versus *assignment* versus *instantiation*:
  
    $int x;$    versus $x = y;$
    $Person p;$ versus $p = q;$ versus $new Person(...)$.
    $int[] a;$  versus $a = b;$ versus $new int[] { 2, 3, 4 }$.

+ values versus non-values:
  + methods can return only values
  + variables can contain only values
  + objects are not values
  + references to objects are values
  + variables are not values
  + there are never references to variables

+ (re)initialize variables in an inner loop:

  incorrect:

    // try to print sum of each row:
    // i = next row to process, j = next column to process in row i
--> sum = 0;
    for (int i = 0; i < a.length; i++) {
        for (int j = 0; j < a[i].length; j++) sum += a[i][j];
        System.out.println(sum);
    }

  correct:

    // try to print sum of each row:
    // i = next row to process
    // j = next column to process in row i
    // sum = sum so far of row i = sum of a[i][..j-1]
    for (int i = 0; i < a.length; i++) {
-->     sum = 0;
        for (int j = 0; j < a[i].length; j++) sum += a[i][j];
        System.out.println(sum);
    }

+ write a specification-comment for each method, briefly explaining
  *what* the method does in terms of all its parameters, but usually
  not explaining *how* the method does it.

+ write a loop "invariant" or "storyline" for loops, briefly
  explaining what the loop does in terms of its variables (see example
  above).  often a clear picture is a good idea.  this is to help both
  you and us understand the code.

+ inheritance: when choosing which implementation of a method to run,
  *always* start searching from the specific class of the *object*,
  *not* from the declared class of the reference.

+ visibility with inheritance: what is visible to a method depends on
  its *implementation-specific* class, *not* the specific class of the
  object.

+ instance versus static methods, e.g. method $overlap$ on Prelim 3.

+ Matlab indices start from 1, Java indices start from 0