Predefined & static methods — cheat sheet

From: fundamentals/10-predefined-methods.md, fundamentals/11-static.md. Detailed tables live in the fundamentals files — this is the conceptual summary worth being able to explain out loud.

static

• Marks a field/method as belonging to the class, not to individual instances — one shared copy, not one per object.
• A static method is called on the class itself (ClassName.method()), can't use this, and can't access non-static (instance) fields directly.
• Common uses seen here: a shared counter across all instances of a class (private static int count); utility classes like Collections/Math that are never instantiated, just called via the class name.
• Course guidance: avoid reaching for static in your own classes until you have more experience — it's easy to end up with shared mutable state across every instance (the same bug pattern as [[java-oop/02-mutability]]) and it tends to make code harder to test (can't substitute/mock a static call the way you can an injected instance).

String methods (immutable — every method returns a new string)

length(), isEmpty(), equals/equalsIgnoreCase, startsWith/contains/endsWith, toUpperCase/toLowerCase, trim(), substring(start, end) (end exclusive), replaceAll(target, replacement), split(separator) (returns String[], wrap with List.of(...) for a List).

Random

Create one instance, reuse it. nextInt() (any int), nextInt(n) (0 to n-1), nextDouble() (0.0-1.0), nextBoolean(). Ranged roll pattern: min + random.nextInt((max - min) + 1).

Object methods every class inherits

• toString() — string representation; default is unhelpful (class name + memory address) unless overridden.
• equals(other) — default is identity/reference comparison unless overridden.
• compareTo(other) — ordering; 0/negative/positive for equal/smaller/bigger.
• Because every class is an Object, generic code (collections, comparisons) can work across arbitrary types by only knowing about Object's methods.

Collections / Math static utilities

• Collections.reverse/sort/shuffle(list) — mutate the list in place, so it must be mutable (e.g. ArrayList, not List.of(...)); copy first with new ArrayList<>(immutableList) if needed.
• Math.ceil/Math.floor (round, return double), Math.abs (works for int or double).

Type conversion

• Integer.valueOf(String) / Double.valueOf(String) — parse a numeric string; throw if the input isn't valid for that type.
• String.valueOf(Object) — converts anything to a string; never throws.