Collections and Generics

When an array is no longer enough

Arrays are convenient when the size of a data set is known in advance and simple index-based access is enough. But in real programs, data is often added and removed, you may need to check quickly whether a value is present, store key-value pairs, or choose a structure for a particular way of working. That is what the Java standard library collections are for.

In this section, we will explore the main collection interfaces and implementations, along with generics — the mechanism that lets a collection know in advance what type of values it stores. Instead of one universal container, Java offers several structures with different rules and strengths.

What this section includes

• Generics. We will see how a type parameter makes containers safer and removes unnecessary casts.
• Collection Hierarchy. We will look at the overall collection interface structure and understand how its main branches differ.
• List. We will work with sequences where element order and index-based access matter.
• Set. We will explore sets in which every element must be unique.
• Queue and Deque. We will learn to work with queues and structures that can be accessed from one or both ends.
• Map. We will move to storing data by keys, where a value is found by a meaningful identifier rather than by position.
• ArrayList and LinkedList. We will compare two List implementations and see why the chosen structure affects the nature of operations.
• HashMap. We will see how hashing helps find a value by key quickly.
• HashSet. We will look at how the same hashing idea is used for fast storage of unique elements.
• Comparable and Comparator. We will learn to define the natural order of objects and separate comparison rules.
• Iterator and Iterable. We will see the mechanism behind sequential traversal of elements.
• Collections Class. We will bring together useful ready-made collection operations: sorting, rearranging, finding extremes, and protective wrappers.

How the topics fit together

First, we need a common language for types — generics. Then we will look at collection interfaces and work through their main models one by one: a sequence, a set, a queue, and a mapping from keys to values. After that, it becomes much clearer why different implementations exist and why the same interface can hide very different internal structures.

At the end of the section, we will add object comparison, element traversal, and ready-made standard-library utilities. After that, collections should stop looking like a set of unrelated classes: you will understand which abstraction to choose and what to expect from code that works with it.