Learn to code, one question at a time

Part 1 of 5 of the OOPS! course: build a Parking Lot in Java and C#, with the full code, a practice task and its solution. You will: Find the classes in a problem: nouns become classes, details become fields, verbs become methods. Write a class with fields and a constructor, and create objects with new. Overload a constructor and chain it with this(...). Give one object a reference to another (a ParkingSpot holds a Vehicle).

Part 2 of 5 of the OOPS! course: build a Parking Lot in Java and C#, with the full code, a practice task and its solution. You will: Make fields private and check every value in the constructor. Read data through getters (Java) or read-only properties (C#), with no setters. Let methods guard their own rules: park() refuses a taken spot, leave() never crashes. The three uses of this, and static: one ticket counter shared by the whole class.

Part 3 of 5 of the OOPS! course: build a Parking Lot in Java and C#, with the full code, a practice task and its solution. You will: Turn Vehicle into a parent class, with Car and Bike as child classes (extends in Java, : in C#). Call the parent's constructor first: super(...) in Java, : base(...) in C#. Use the IS-A test, and keep fields private, or protected for child classes. Remove the type check: each kind of vehicle passes its own rate to the parent.

Two classic warm-up problems: reverse a string in place with two pointers, and test a palindrome while ignoring case and punctuation.

Count how often each character appears with Map of Character to Integer and merge, then print the most frequent first.

A short program that shows which strings share the pooled object, which are new objects, and what intern() does.