ArraysSorting

Comparator & Comparable

Custom sort logic in Java — intuition-first. How to write any comparator from first principles.

The ONE Rule

compare(a, b) answers: should a come before b?

💡

negative → a before b | zero → same | positive → b before a

Process — English first, code second

Say what "a before b" means in plain English
Express that as math
Return negative when true

Common Patterns

Ascending

Java

Collections.sort(list, (a, b) -> Integer.compare(a, b));

⚠

Never use a - b directly — integer overflow. Always Integer.compare(a, b).

Descending

Java

Collections.sort(list, (a, b) -> Integer.compare(b, a));

Sort by absolute value

Java

// [-10, 6, 2, -6, 1, 10] → [1, 2, 6, -6, -10, 10]
Collections.sort(list, (a, b) -> Integer.compare(Math.abs(a), Math.abs(b)));

a=5,  b=10  |5|-|10|  = -5 < 0  → 5 before 10  ✓
a=-6, b=2   |-6|-|2|  =  4 > 0  → 2 before -6  ✓
a=-6, b=6   |-6|-|6|  =  0      → same          ✓

Multi-key Sorting

Java

Collections.sort(list, (a, b) -> {{
    int cmp = a.name.compareTo(b.name);
    if (cmp != 0) return cmp;
    return Integer.compare(a.age, b.age);
}});

Comparable vs Comparator

Feature	Comparable	Comparator
Logic lives	Inside class	Outside class
Method	`compareTo(other)`	`compare(a, b)`
Orderings	Only ONE	As many as you want
Modify class?	YES	NO

✓

One natural order → Comparable. Multiple orderings or can't modify class → Comparator.

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