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Java Glossary and the Core Concepts

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Java Glossary and the Core Concepts

Whether you're new to Java or a seasoned veteran, a roundup of the core concepts in alphabetical order never hurts. Let's cover them from abstract to wrappers.

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In this article, we’ll explore the foundations and core concepts of the Java language and terminology.

The write-up is divided into sections, ordered alphabetically to enable fast and easy search for these definitions.


abstracta keyword used in a class or method definition, which specifies that the method/class is not going to be instantiated, but should be inherited by other methods or classes:

public abstract class Foo {
    abstract void runFoo();

API (Application Programming Interface) – is the way to expose a set of pre-defined classes and interfaces to external clients to interact with them, without sharing the implementation details

argument – an input specified in a method call; it can be a literal value, a variable, or an expression:

void fooMethod(int argument1);

array – a fixed-size collection of data of the same type, which can hold zero or more items:

int[] array = new int[16];

autoboxing – automatic conversion between the primitive types and their corresponding object wrapper classes:

Character a = 'a';


block – code between two matching open and close braces, which is the single logical unit of work inside the application:

{ some code }

boolean – a primitive type, holding only two values – true or false:

boolean condition = false;

break – a statement used to exit a loop/switch statement/labeled block; the application continues execution with the statement immediately following the containing block:

int hour = 8;
switch (hour) {
    case 8:
        //some code
        //some code

byte – a primitive type of the size of eight bits:

byte b = 100;

bytecode – the instruction set for Java Virtual Machine, created from source files into bytecode by the compiler


case – a keyword that defines a particular group of statements executed in a switch statement:

int hour = 8;
switch (hour) {
    case 8:
        //some code
        //some code

casting – conversion from one data type to another:

Object o = "test";
String str = (String) o;

catch – the block of code inside try/catch statement, responsible for handling exceptions:

try {
    // code
} catch (Exception e) {
   // exception handling

char – a keyword used to declare a variable of single type character:

char a = 'a';

checked exception – an Exception that is caught at the compilation time, usually in the block or thrown in the method header

class – the core type in Java that defines the implementation of a particular kind of object; it defines instance and class variables and methods, as well as specifies the interfaces it implements and the immediate superclass of the class, by default Object:

public class Foo {}

class method – a synonym of static class

class variable – a synonym of a static field or a static variable

classpath – an environment variable or a command-line argument indicating the path searched by the Java compiler and the runtime for class definitions

comment – a piece of explanatory text ignored by the compiler:

// first comment
/* comment block */
/** documentation */

compiler – a program used to translate source code into the code executed by a computer

concurrency – it is the ability of a program to run several tasks in parallel, a primary feature of multithreading

condition – a boolean expression controlling a conditional statement or loop:

if (condition) {}

constant – a final variable in Java, which means that the reference of it cannot be changed once initialized:

final int number = 20;

constructor – a method inside the class, which creates and initializes objects in it – needs to be public and names the same as the class:

public class Foo {
    public Foo(){}; // constructor

continue – a keyword used to resume application execution at the end of the current loop:

for (int i=1; i<10; i++){
    for (int j=1; j<10; j++){
        if (condition) continue;

curly brackets – please refer to block


declaration – officially, this is defined as a statement that establishes an identifier and associates attributes with it, without necessarily reserving its storage or providing the implementation

default – the optional destination used in a switch statement, when neither case statement matches the requested behavior:

int hour = 8;
switch (hour) {
    case 8:
        //some code
        //some code

definition – a declaration that reserves storage (for data) or provides an implementation (for methods)

deprecation – a class/method/interface that has been made obsolete by later versions of the library or of the language itself; it should not be used as there is no guarantee that it will exist in future versions

direct recursion – a recursion that starts in the stack of the calling method itself

do – a keyword used to declare a while loop ensuring execution of the first iteration of the loop before the check of the boolean condition:

do {
     // code
} while (expression);

DOM – Document Object Model, defined by the W3C, that allows applications to dynamically access and update the content, structure, and style of documents

downcast – a process of changing the data type from Object to the particular type, i.e., Integer:

Object o = 10;
Integer num = (Integer) o;

double – a Java primitive type of the type double:

double lat = 52.11


else – a keyword used in if/else condition statements, executed when the test condition is false:

if (condition) {
    // code
} else {
    // code

encapsulation – the process of protecting the state of objects by defining its attributes as private and channeling access to them through accessor and mutator methods

enum – a Java keyword used to declare the enumerated type (whose values are a fixed set of constants):

public enum Day {

exception – an exceptional circumstance preventing the program to continue working regularly (usually an error or bug); here are some best practices to deal with and further understand exceptions in Java

expression – a combination of operands and operators which causes particular behavior and produces results

extendsa keyword used to define the inheritance of classes or interfaces:

public class Foo extends FooMother {}


field – a variable defined outside of all defined methods, but inside of the class; in other words, a member of a class

final – a Java keyword indicating that an entity is immutable, thus, you can’t change its reference during the program execution:

final int number = 20;

finally – a block in a try/catch statement executed even Java exception or runtime error occurred:

try {
    // code
} catch (Exception e) {
    // exception handling
} finally {
    // code to be executed at the end

float – a Java keyword used to define a floating point number variable:

float cash = 24.5;

for – a Java control structure used for loop execution:

for (int i=0; i<10; i++){
    // code


garbage collection – the process by which the JVM automatically frees up unused memory; to go further into the GC process and potential problems in that process, you can read the deep-dive into memory leaks here

global variable – a variable that is visible to all methods in the class

GUI – graphical user interface


hash code – a value used to provide an efficient way to map object and its location, returned by a hash function

hash function – a method used to produce hash code from any data of arbitrary size to data of fixed size

hexadecimal – a number represented by the base of 16

HTML – HyperText Markup Language; a web content presentation language

HTTP(S) – HyperText Transfer Protocol (Secure); a protocol that defines all rules how the browser should communicate with a server


identifier – a name of a class, variable, method or interface defined in the code by the software developer:

String identifier = "Some string";

if – a Java control structure used to choose if execution of further actions should continue or not:

if (condition) {
    // code
} else {
    // code

immutable object – an object whose state or value is not changeable after creation

implements – a Java keyword used to indicate which interfaces are implemented by the current class:

public class Foo implements Foo {
    // implementation of all methods defined in the <em>Foo</em> interface

import – a statement used to enable the use of other classes or interfaces from different Java packages:

import java.util.*;

indirect recursion – a recursion that happens when method A calls method B while a call from the method B to the method A is still in progress

infinite recursion – a recursion that can technically continue indefinitely; it very often indicates a logic error and can lead to StackOverflow errors

inheritance – a feature of object-oriented programming, where classes contain all variables and methods defined in their supertypes

int – a Java primitive of the type integer:

int number = 10;

interface – a Java keyword used to define the collection of methods and constant values that can be furthermore implemented by other classes:

public interface IFoo {
    void start();
    void stop();
    int restart();

iteration – a single execution of a loop


JAR – Java Archive is the default Java packaging mechanism to aggregate multiple files into one (similar to .zip)

Java Core – provides the main features of Java, also named Java Standard Edition

Java EE – Java Enterprise Edition

JDK – Java Development Kit, the environment and core libraries used to write Java programs

JVM – Java Virtual Machine, the abstract machine where the compiled Java bytecode is executed


livelock – a situation when two separate threads are waiting for each other to check the condition of particular part of the program

local variable – a variable defined in the method body, visible only inside it

long – a Java primitive of the type long:

long bigNumber = 100L;


main method – a starting point for Java applications:

public static void main(String[] args){}

memory leak – a situation during the program execution where memory that is no longer being used cannot be removed by the garbage collector as there is still a reference to it; it eventually leads to OutOfMemoryException

method – a particular function implemented in a Java class:

public int doSthAndReturnInt();

module – a group of program components; in Java, the term that’s used for it is package

multithreaded – a program capable of concurrent execution on multiple threads

mutual recursion – this happens when two methods are calling each other recursively at the same time


namespace – an area of the program defined by packages, with established certain visibility rules (e.g. private access, public access, etc.)

native – a keyword indicating that particular method is not implemented in Java language itself, but in another programming language

nested class – a class, which is implemented inside the body of the other class

new – the operator used to create an instance of a class

null – a type indicating that object reference variable has no reference to any object existing in memory


object – an instance of a particular class; also the core concept of the OOP

OOP – Object Oriented Programming – a primary paradigm in modern software development, focused on objects as primitives, not the specific actions; each object is created/instantiated from a class

operator – a symbol used for arithmetic or boolean expressions, e.g. +,-,/,*,=

operator precedence – the order of processing conditions or equations with the multiple operators, similar to the mathematical concept of order of operations

overloading – using the same method name for various implementation, differentiated by parameters:

private int sum(int x, int y) {
    return (x + y);

private int sum(int x, int y, int z) { 
    return (x + y + z);

overriding – providing a different implementation of the original method in its subclass:

public class Foo {
    public void test(){
        // original implementation

public class BabyFoo extends Foo {
    public void test(){
        // overriden implementation


package – a name for a grouping of classes and interfaces in a namespace

primitive type – one of the following non-class variable type: boolean, byte, char, double, float, int, long or short

private – a Java modifier, used to specify the visibility of a method or a variable, so they can be accessed only within its class

protected – another modifier that makes variables or classes accessible to all other elements in the same package

public – a modifier allowing external access to a particular variable or method


recursion – a process where a method is invoked again from its existing call stack

reflection – the ability of the code to inspect and manipulate other code in the same runtime process

return– a Java keyword used to finish the execution of the method and return data back to the caller


scope – it determines the visibility of elements in the program, for example, local or global variables

serialization – the process of encoding and decoding the objects to the stream of bytes, and vice-versa

short – a keyword used to specify the variable of the type short:

short num = 2;

static – class member variable stored and accessed as a single instance for all objects of that class:

public static class Foo {
    public static int num = 10;
    public static void useMe(){
        // code

stream – a byte-stream of data sent from sender to receiver

String – an instance of an immutable String class, containing zero or more Unicode characters:

String myText = "Hello... It's me.";

super– a keyword allowing the access to members of a class inherited by the class in which it appears

switch – a control structure with multiple cases:
int hour = 8;
switch (hour) {
    case 8:
        //some code
        //some code

synchronized– a control statement that guarantees single-access semantics in a multithreaded environment


this – a statement that references the instance of the class where it appears

thread – a basic, single lightweight execution process supported natively by the JVM as well as by the OS

throwa statement used to throw an Exception:

void dontUseThisMethod(){
    throw Exception;

throws – a keyword in a method header indicating that one or more exceptions will be propagated from this method:

public void startEngine() throws IOException;

try – a block of code that allows exceptions to be caught using a catch block:

try {
    // code
} catch (Exception e) {
    // exception handling
} finally {
    // code to be executed at the end


unchecked exception – an error without handler defined in the program implementation, cannot be dealt with at compilation time

Unicode – a 16-bit character set defined by ISO 10646, designed to make exchange and display of information easier across various languages

URI, URL – Uniform Resource Identifier/Locator. You can read more about the difference between these two concepts here

upcast – a process of casting to super type – for example from String to Object:

String text = "Test";
Object o = (Object) o;


variable – an item of data associated with a specific type

variable declaration – the place in the application, where the specific variable is assigned to one of the existing Java types

virtual machine – see JVM

void – a keyword used to indicate that method does not return any value

volatile – a modifier specifying how the variable behaves in a multithreaded environment; the variable will never be cached in a thread-local – as it’s expected to be modified by different threads


while – a Java control structure used for looping:

while (condition) {
    // code

wrapper – an object that encapsulates primitive types into one of the classes from the java.lang package: Boolean, Byte, Character, Double, Float, Integer, Long or Short to provide additional methods

How do you break a Monolith into Microservices at Scale? This ebook shows strategies and techniques for building scalable and resilient microservices.

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