How it works
A test plan is written from the specification, not from the implementation, so it can catch what the implementation got wrong.
Write the empty, single, typical, and boundary cases before the code, then run them after.
Common error
Testing only the path you had in mind while coding, which confirms your assumptions rather than checking them.
Worked example
Java snippets run inside a main method. Add import java.util.*; when collection types are used.
Java
int[] values = {-4, -1, -9};
int best = values[0];
for (int value : values) best = Math.max(best, value);
System.out.println(best);Output
-1A negative-only test distinguishes a correct maximum implementation from one incorrectly initialized to zero. Record expected and observed results, isolate the cause, and rerun both the failing test and earlier successful cases.
Exercises
0.3.1 · Trace
Determine the exact output and explain the state changes.
Java
int[] a = {1, 2, 3};
int sum = 0;
for (int i = 0; i < a.length - 1; i++) sum += a[i];
System.out.println(sum);Show solution
Output
3The final index is excluded. A full traversal should use i < a.length.
0.3.2 · Explain
Give boundary tests for a method counting positive array elements.
Show solution
Test an empty array, a single positive value, a single zero, all negatives, and mixed values. Expected counts include 0 for empty and zero-only input.