What is a stable sort? Which common sorts are stable?

Stable sorting

A sort is stable if equal keys retain original relative order. Matters when sorting by one key then another (sort by name after sorting by department).

• Stable: merge sort, insertion sort, bubble sort, TimSort.
• Unstable: quick sort, heap sort (typical implementations).

// Java Collections.sort on objects is stable (TimSort)
students.sort(Comparator.comparing(Student::getGrade)
    .thenComparing(Student::getName));