Java Glossary: 57 Core Terms, Defined

Plain-English definitions of the Java language and JVM vocabulary — from JVM, JDK and bytecode to record, stream and virtual thread. Every entry is a standalone page with a short definition, a minimal code example, and links to the deeper guide.

📚 57 terms ⚡ One page per term 🧭 A–Z index 🆕 Java 17 & 21

A

Abstract class — An abstract class in Java cannot be instantiated directly. It declares abstract methods that subclasses must implement and can also hold state and concrete methods.
Abstraction — Abstraction in Java means hiding complexity behind a simple interface. Achieved through interfaces and abstract classes. Callers depend on "what", not "how".
Annotation — An annotation in Java is metadata attached to code with the @ symbol. Used by frameworks (Spring, JPA, JUnit) and the compiler (@Override, @Deprecated).
ArrayList — ArrayList is the default Java List implementation. Backed by a dynamically resizing array. O(1) get, amortised O(1) append, O(n) arbitrary insert/remove.
Autoboxing — Autoboxing in Java automatically converts primitives to wrappers (int → Integer) and back. Silent and convenient, but watch out for NullPointerException on unboxing.

B

Bytecode — Java bytecode is the platform-independent instruction set that the javac compiler emits. The JVM executes bytecode via interpretation and JIT compilation.

C

Class — A class in Java is a blueprint that declares fields (state) and methods (behaviour). Objects are instances of a class, created with the new keyword.
ClassLoader — A ClassLoader in Java loads .class files into the JVM at runtime. The hierarchy (Bootstrap → Platform → App) enables class isolation and custom loading.
Classpath — The classpath tells the JVM where to find .class files and JARs at runtime. Set it with -cp, CLASSPATH env var, or module-path for JPMS modules.
Collection — A Collection in Java is any group of objects — List, Set, Queue. The root interface java.util.Collection provides add, remove, contains and iteration.
Constructor — A constructor is a special method that runs when an object is created with new. Same name as the class, no return type, used to initialise fields.

E

Encapsulation — Encapsulation hides an object's internal state behind a controlled public interface. Private fields, public getters/setters, invariant enforcement.
Enum — An enum in Java is a type with a fixed set of named constants. Can hold methods and fields, implement interfaces, and is type-safe by design.
Exception — An Exception in Java is a throwable that signals an error or unexpected condition. Two categories: checked (must be declared) and unchecked (RuntimeException).

F

final (keyword) — final in Java has three meanings: final variable (cannot be reassigned), final method (cannot be overridden), final class (cannot be subclassed).
Functional interface — A functional interface in Java has exactly one abstract method. It can be implemented with a lambda or method reference. Examples: Runnable, Function, Predicate.

G

Garbage collector (GC) — The garbage collector is the JVM subsystem that reclaims memory of unreachable objects. Modern collectors: G1 (default), ZGC, Shenandoah, Parallel GC.

H

HashMap — HashMap is Java's default Map implementation. Stores key-value pairs in a hash table. Average O(1) get/put. Not thread-safe — use ConcurrentHashMap for concurrency.
Heap (JVM memory) — The heap is the JVM memory region where all Java objects live. Managed by the garbage collector. Size tuned with -Xms and -Xmx JVM flags.

I

Immutable object — An immutable object in Java cannot change state after construction. Achieved by making fields final, providing no setters, and ensuring defensive copies.
Inheritance — Inheritance in Java lets a subclass extend a superclass using extends. The subclass inherits fields and methods and can override or add its own.
Interface — An interface in Java defines a contract: a set of method signatures that implementing classes must provide. Supports default and static methods since Java 8.
Iterator — An Iterator in Java lets you traverse a collection one element at a time with hasNext() and next(). Used by the enhanced for loop under the hood.

J

JAR (Java Archive) — A JAR file is a zipped Java package: .class files, resources, and a MANIFEST.MF. Executable JARs include a Main-Class entry and run with java -jar.
JavaBean — A JavaBean is a Java class following conventions: no-arg constructor, private fields, public getters and setters, and Serializable. Used by JavaBeans spec and many frameworks.
JDK (Java Development Kit) — The JDK is the full Java developer toolkit: the JVM, the standard library, the javac compiler and diagnostic tools. Required to compile and build Java code.
JRE (Java Runtime Environment) — The JRE bundles the JVM and the standard Java class library. It is enough to run Java programs but not to compile them. Largely superseded by full JDK…
JVM (Java Virtual Machine) — The JVM is the runtime that executes Java bytecode. It provides memory management, JIT compilation and platform independence — the foundation of "write once, run anywhere".

L

Lambda capture / closure — A Java lambda captures effectively-final local variables from the enclosing scope. It cannot capture or mutate non-final locals.
Lambda expression — A lambda in Java is an anonymous function expression introduced in Java 8. Used to implement functional interfaces concisely, e.g. (a, b) -> a + b.
List — A List in Java is an ordered collection that allows duplicates and index-based access. Main implementations: ArrayList (default), LinkedList.

M

Map — A Map in Java associates keys with values. HashMap is the default, LinkedHashMap preserves insertion order, TreeMap keeps keys sorted.
Method — A method is a named block of code in a Java class with a return type and optional parameters. Methods expose the behaviour of objects.
Module (JPMS) — A module in Java (since Java 9) is a higher-level grouping of packages with explicit dependencies and exports. Declared in module-info.java.

N

NullPointerException — NullPointerException (NPE) is thrown when your code dereferences a null reference. Java 14+ helpful NPE messages pinpoint the exact null variable.

O

Object — An object in Java is a concrete instance of a class — a chunk of heap memory holding the class's fields, accessed through a reference variable.
Optional — Optional (Java 8+) is a container that explicitly encodes "value present or absent". Used as return type to avoid returning null and NullPointerException.

P

Package — A Java package is a namespace for classes. Declared with package com.example; and mapped to a matching directory structure on disk.
POJO (Plain Old Java Object) — A POJO is a simple Java class with no special framework-imposed constraints. Just fields, constructors, and getter/setter methods. Coined in contrast to EJB.
Polymorphism — Polymorphism in Java means one reference type can refer to many concrete types. Runtime method dispatch, interface-based decoupling, method overloading.
Primitive type — Java has 8 primitive types: byte, short, int, long, float, double, char, boolean. They hold a value directly, not a reference — unlike wrapper classes.

R

Record — A record (Java 16+) is a compact class syntax for immutable data carriers. The compiler generates constructor, accessors, equals, hashCode and toString.
Reflection — Reflection in Java (java.lang.reflect) lets code inspect and modify classes, methods and fields at runtime. Powers frameworks like Spring, Jackson and JUnit.

S

Sealed class — Sealed classes (Java 17+) restrict which classes may extend them. Enables exhaustive pattern matching and algebraic-data-type style domain modelling.
Serialization — Serialization in Java converts an object to a byte stream for storage or transmission. Built-in via Serializable and ObjectOutputStream — but JSON is preferred today.
Set — A Set in Java is a collection that disallows duplicates. HashSet is the default, LinkedHashSet keeps insertion order, TreeSet keeps elements sorted.
static (keyword) — static in Java marks a field or method as belonging to the class, not to instances. One copy shared by all. Used for utility methods and constants.
Stream (Stream API) — The Stream API (Java 8+) lets you express data pipelines declaratively: filter, map, reduce, collect. Not a collection — a lazy, one-shot sequence of operations.
String pool — The String pool is a special area in the JVM heap where String literals are interned (deduplicated). Enables a.equals(b) shortcut for identical literals.
StringBuilder — StringBuilder is a mutable sequence of characters. Use it to concatenate many strings efficiently — avoids creating a new String per += operation.
synchronized — The synchronized keyword in Java enforces mutual exclusion on an object monitor. Prevents race conditions on shared mutable state.

T

this (keyword) — this in Java refers to the current object instance. Used to disambiguate fields from parameters, call other constructors, or pass the current object.
Thread — A Thread in Java is a path of execution. Create one with new Thread(runnable) or virtual threads (Java 21). ExecutorService is the modern preferred API.

V

var (local type inference) — var (Java 10+) is a reserved type name for local variable type inference. The compiler infers the type from the initialiser. Only for local variables.
Varargs — Varargs (variable arguments, Java 5+) let a method accept any number of arguments of the same type. Syntax: String... names. Used by printf, Arrays.asList.
volatile — volatile in Java guarantees that reads and writes to a variable are visible across threads. No locking — lighter than synchronized but narrower guarantees.

W

Wrapper class — Wrapper classes in Java box a primitive type into an object: Integer wraps int, Double wraps double. Used in collections and generics via autoboxing.

How this glossary is written

Each term is defined first in one sentence anyone can paste into a code review comment without needing a paragraph of context. The page then expands with a short code example, the common misunderstandings, and a link to the long-form guide when one exists. Definitions target modern Java (17 and 21 LTS); where a term means something different on older Java (pre-9, pre-modules), the legacy meaning is called out.

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