Generics in Java parameterize classes, interfaces and methods by reference types so reusable code can retain compile-time type checks. For example, List<String> describes a list whose elements are strings, and retrieving one has static type String. Java generic types are generally invariant: List<Integer> is not a subtype of List<Number>. Wildcards express related producer or consumer views. Type erasure means a parameterized class usually has one shared runtime class rather than a fresh class for each type argument. Primitive values use wrapper types in ordinary generic collections.^[ref]

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Java arrays are fixed-length objects. A two-dimensional array is an array of row-array references, so rows can have different lengths and must be allocated separately if not allocated by the initial expression.

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A protected method is accessible to code in its package and, subject to the cross-package receiver restriction, to subclasses elsewhere. It can expose a subclass customization hook without being a public client operation. Protected access is broader than subclass-only access.

// Destination: computer-science.bigb

A class declared final cannot be subclassed. This can protect an implementation contract against overriding in an untrusted subclass. It does not by itself make the class immutable, and it rules out subclass-based customization.

// Destination: computer-science.bigb

A field declared private is hidden from ordinary external source-level access. Library methods can enforce invariants and change the representation without exposing storage to clients. Returning a mutable internal object can still undermine this encapsulation in object-oriented programming.

// Destination: computer-science.bigb

An interface specifies a type contract that independent classes can implement. Code accepting the interface can operate on multiple implementations, avoiding dependence on one concrete class. A class may implement several interfaces despite having only one direct class superclass.

// Destination: computer-science.bigb

Online text: docs.oracle.com/en/java/javase/25/docs/api/java.base/java/lang/Object.html

Cited passages: Object, hashCode general contract and equals contract: equal objects must have equal hash codes; unequal objects need not.