Java - Naming Conventions

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Java naming conventions are sort of guidelines that application programmers are expected to follow to produce a consistent and readable code throughout the application. We should follow these naming conventions in the day to day project work.

1. Packages naming conventions

A package should be named in lowercase characters. There should be only one English word after each dot.

The prefix of a unique package name is always written in all-lowercase ASCII letters and should be one of the top-level domain names, like com, edu, gov, mil, net, org.

Example:

package org.springframework.core.convert;
package org.hibernate.criterion;
package org.springframework.boot.actuate.audit;
package org.apache.tools.ant.dispatch;

Package naming convention used by Oracle for the Java core packages. The initial package name representing the domain name must be in lower case.

package java.lang;
package java.util;

2. Classes naming conventions

Class names should be nouns in UpperCamelCase (in mixed case with the first letter of each internal word capitalized). Try to keep your class names simple and descriptive.

Example:

class Employee
class Student
class EmployeeDao
class CompanyService

The class naming convention used by Oracle for the Java core packages.

class String
class Color
class Button
class System
class Thread
class Character
class Compiler
class Number

3. Interfaces naming conventions

In Java, interfaces names, generally, should be adjectives. Interfaces should be in titlecase with the first letter of each separate word capitalized. In some cases, interfaces can be nouns as well when they present a family of classes e.g. List and Map.

Example:

Runnable
Remote
ActionListener
Appendable
AutoCloseable
CharSequence
Cloneable
Comparable
Readable

4. Methods naming conventions

Methods always should be verbs. They represent action and the method name should clearly state the action they perform. The method name can be single or 2-3 words as needed to clearly represent the action. Words should be in camel case notation.
Examples:

public List <Customer> getCustomers();

public void saveCustomer(Customer theCustomer);

public Customer getCustomer(int theId);

public void deleteCustomer(int theId);

More Examples:

getName()
computeTotalWidth()
actionPerformed()
main()
print()
println()

5. Variables naming conventions

The variable name should start with a lowercase letter. Parameter names, member variable names, and local variable names should be written in lowerCamelCase.

Example:

firstName
orderNumber
lastName
phoneNo
id
counter
temp

6. Constants naming conventions

Constant variable names should be written in upper characters separated by underscores. These names should be semantically complete and clear.

Example:

RED
YELLOW
MAX_PRIORITY
MAX_STOCK_COUNT

7. Abstract classes naming conventions

I observed in many standard libraries, the naming conventions used for Abstract class is class name must start with Abstract or Base prefix. This naming convention can vary from organization to organization. 

Example:

AbstractHibernateDao
AbstractCommonDao
AbstractBase

Let's take an example from the Spring Framework:

AbstractBean
AbstractBeanDefinition
AbstractUrlBasedView
AbstractIdentifiable

8. Exception classes naming conventions

I observed in many standard libraries, the naming conventions used for custom Exception class is class name must end with Exception suffix. 

Example:

TransactionException
SQLDataException
ResourceNotFountException
ResourceAlreadyExistException

The exception class naming convention used by Oracle for the Java core packages.

ArithmeticException
ArrayIndexOutOfBoundsException
ArrayIndexOutOfBoundsException
ClassNotFoundException
CloneNotSupportedException
EnumConstantNotPresentException
Exception
IllegalAccessException
IllegalArgumentException
IllegalMonitorStateException
IllegalStateException
IllegalThreadStateException
IndexOutOfBoundsException

9. Enumeration naming conventions

Enum Class members should be spelled out in upper case words, separated by underlines.

Example:

public enum Day {
    SUNDAY,
    MONDAY,
    TUESDAY,
    WEDNESDAY,
    THURSDAY,
    FRIDAY,
    SATURDAY;
}

10. Generic types naming conventions

Generic type parameter names should be uppercase single letters. The letter 'T' for a type is typically recommended. In JDK classes, E is used for collection elements, S is used for service loaders, and K and V are used for map keys and values.

Example:

public interface Map <K,V> {}
public interface List<E> extends Collection<E> {}
Iterator<E> iterator() {}

11. Annotations naming conventions

Annotation names follow title case notation. They can be adjective, verb or noun based on the requirements.

Example:

public @interface FunctionalInterface {}
public @interface Deprecated {}
public @interface Documented {}
public @Async Documented {}
public @Test Documented {}

12. Other Specific Naming Conventions

Apart from the above java standard naming conventions, there are few more naming conventions that would be followed in many standard libraries such as Spring, Apache, Hibernate etc.

Note that these naming conventions can be varied as per different libraries or organizations.

is prefix can be used for boolean variables and methods.

This is the naming convention for boolean methods and variables used by Oracle for the Java core packages. 

Using the is prefix solves a common problem of choosing bad boolean names like status or flag. isStatus or isFlag simply doesn't fit, and the programmer is forced to choose more meaningful names.

Setter methods for boolean variables must have set prefix as in:

void setFound(boolean isFound);

There are a few alternatives to the is a prefix that fits better in some situations. These have, can and should prefixes:

boolean hasLicense();
boolean canEvaluate();
boolean shouldAbort = false;

The term compute can be used in methods where something is computed.

valueSet.computeAverage();
matrix.computeInverse();

Give the reader the immediate clue that this is a potentially time-consuming operation, and if used repeatedly, he might consider caching the result. Consistent use of the term enhances readability.

The term find can be used in methods where something is looked up.

vertex.findNearestVertex();
matrix.findSmallestElement();
node.findShortestPath(Node destinationNode);

Give the reader the immediate clue that this is a simple lookup method with a minimum of computations involved. Consistent use of the term enhances readability.

The term initialize can be used where an object or a concept is established.

printer.initializeFontSet();

The Plural form can be used on names representing a collection of objects.

Collection<Point>  points;
int[]              values;

Enhances readability since the name gives the user an immediate clue of the type of the variable and the operations that can be performed on its elements.

Generic variables should have the same name as their type.

void setTopic(Topic topic) // NOT: void setTopic(Topic value)
                           // NOT: void setTopic(Topic aTopic)
                           // NOT: void setTopic(Topic t)

void connect(Database database) // NOT: void connect(Database db)
                                // NOT: void connect(Database oracleDB)

The name of the object is implicit and should be avoided in a method name.

line.getLength();   // NOT: line.getLineLength();