Defining annotation types

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Annotations

In Java, an annotation is a form of syntactic metadata that can be added to Java source code. It provides data about a program that is not part of the program itself. Annotations have no direct effect on the operation of the code they annotate. Classes, methods, variables, parameters and packages are allowed to be annotated.

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The idea behind Annotations

The Java Language Specification describes Annotations as follows:

• An annotation is a marker which associates information with a program construct, but has no effect at run time.

Annotations may appear before types or declarations. It is possible for them to appear in a place where they could apply to both a type or a declaration.

What exactly an annotation applies to is governed by the "meta-annotation" @Target. See "Defining annotation types" for more information.

Annotations are used for a multitude of purposes. Frameworks like Spring and Spring-MVC make use of annotations to define where Dependencies should be injected or where requests should be routed.

Other frameworks use annotations for code-generation. Lombok and JPA are prime examples, that use annotations to generate Java (and SQL) code.

This topic aims to provide a comprehensive overview of:

• How to define your own Annotations?
• What Annotations does the Java Language provide?
• How are Annotations used in practice?

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Defining annotation types

Annotation types are defined with @interface. Parameters are defined similar to methods of a regular interface.

@interface MyAnnotation {
     String param1();
     boolean param2();
     int[] param3(); // array parameter
}

Default values

@interface MyAnnotation {
    String param1() default "someValue";
    boolean param2() default true;
    int[] param3() default {};
}

Meta-Annotations

Meta-annotations are annotations that can be applied to annotation types. Special predefined meta-annotation define how annotation types can be used.

@Target

The @Target meta-annotation restricts the types the annotation can be applied to.

@Target(ElementType.METHOD)
@interface MyAnnotation {
    // this annotation can only be applied to methods
}

Multiple values can be added using array notation, e.g. @Target({ElementType.FIELD, ElementType.TYPE})

Available Values

Version ≥ Java SE 8

@Retention

The @Retention meta-annotation defines the annotation visibility during the applications compilation process or execution. By default, annotations are included in .class files, but are not visible at runtime. To make an annotation accessible at runtime, RetentionPolicy.RUNTIME has to be set on that annotation.

@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation {
    // this annotation can be accessed with reflections at runtime
}

Available Values

@Documented

The @Documented meta-annotation is used to mark annotations whose usage should be documented by API documentation generators like javadoc. It has no values. With @Documented, all classes that use the annotation will list it on their generated documentation page. Without @Documented, it's not possible to see which classes use the annotation in the documentation.

@Inherited

The @Inherited meta-annotation is relevant to annotations that are applied to classes. It has no values. Marking an annotation as @Inherited alters the way that annotation querying works.

• For a non-inherited annotation, the query only examines the class being examined.
• For an inherited annotation, the query will also check the super-class chain (recursively) until an instance of the annotation is found.

Note that only the super-classes are queried: any annotations attached to interfaces in the classes hierarchy will be ignored.

@Repeatable

The @Repeatable meta-annotation was added in Java 8. It indicates that multiple instances of the annotation can be attached to the annotation's target. This meta-annotation has no values.

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Runtime annotation checks via reflection

Java's Reflection API allows the programmer to perform various checks and operations on class fields, methods and annotations during runtime. However, in order for an annotation to be at all visible at runtime, the RetentionPolicy must be changed to RUNTIME, as demonstrated in the example below:

@interface MyDefaultAnnotation {
 
}
 
@Retention(RetentionPolicy.RUNTIME)
@interface MyRuntimeVisibleAnnotation {
 
}
 
public class AnnotationAtRuntimeTest {
 
	@MyDefaultAnnotation
	static class RuntimeCheck1 {
 
	}
 
	@MyRuntimeVisibleAnnotation
	static class RuntimeCheck2 {
 
	}
 
	public static void main(String[] args) {
		Annotation[] annotationsByType = RuntimeCheck1.class.getAnnotations();
		Annotation[] annotationsByType2 = RuntimeCheck2.class.getAnnotations();
 
		System.out.println("default retention: " + Arrays.toString(annotationsByType));
		System.out.println("runtime retention: " + Arrays.toString(annotationsByType2));
	}
}

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Basic Programs

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• Creating Your First Java Program
• Type Conversion
• Getters and Setters
• Reference Data Types
• Java Compiler - javac
• Documenting Java Code
• Command line Argument Processing
• Java Command - java and javaw
• Literals
• Primitive Data Types

Strings in Java

• Comparing Strings
• Changing the case of characters within a String
• Finding a String Within Another String
• String pool and heap storage
• Splitting Strings
• Joining Strings with a delimiter and Reversing Strings
• String concatenation and StringBuilders
• Substrings and Platform Independent new line Separator
• Adding toString() method for custom objects
• Remove Whitespace from the Beginning and End of a String and Case insensitive switch
• Replacing parts of Strings and Getting the nth character in a String
• Counting occurrences of a substring or character in a string

StringBuffer and StringBuilder

• String Buffer
• String Builder
• String Tokenizer and Splitting a string into fixed length parts

Date Class

• Convert java.util.Date to java.sql.Date and Basic date
• LocalDate and LocalDateTime objects
• Creating a Specific Date and Converting Date to String format
• Convert formatted string representation of date to Date object
• Comparing Date objects
• Converting String into Date and Time Zones and java.util.Date

Dates and Time (java.time.*)

• Dates and Time
• Date Time Formatting and Date Manipulations
• LocalTime

BigDecimal

• Comparing and Using BigDecimals
• BigDecimal.valueOf()
• Mathematical operations with BigDecimal

BigInteger

• Initialization and Generating random BigIntegers
• BigInteger Mathematical Operations Examples
• NumberFormat

Bit Manipulation

• Checking, setting, clearing, and toggling individual bits. Using long as bit mask
• Checking if a number is a power of 2
• Signed vs unsigned shift and Expressing the power of 2

Arrays

• Creating and Initializing Arrays
• Creating a List from an Array
• Multidimensional and Jagged Arrays
• Array Covariance and Arrays to Stream
• Iterating over arrays
• Sorting arrays and Length of an Array
• Finding an element in an array nd change the size of an array
• Primitives and boxed types and Remove an element from an array
• Comparing arrays for equality and Copying arrays

Collections

• Removing items from a List within a loop
• Constructing collections from existing data
• ArrayList and adding objects and Iterating over Collections
• Immutable Empty , Sub and Unmodifiable Collection
• Removing matching items from Lists and Join lists
• Creating your own Iterable structure for use with Iterator or for-each loop

Lists

• Sorting a generic list
• Classes implementing List
• Replacement of a List element and Moving objects around in the list
• Creating, Adding and Removing element from an ArrayList
• Positional Access Operations and Iterating over elements in a list

Sets

• Initialization and Basics of Set
• Types and Usage of Sets

Maps

• Iterating Map Entries Effciently
• Using Default Methods of Map from Java 8
• Usage of HashMap and Iterating through the contents of a Map
• Merging, combine and composing Maps and Add multiple items
• Creating and Initializing Maps
• Check if key exists, Add an element and Clear the map
• LinkedHashMap, WeakHashMap and SortedMap

TreeMap and TreeSet

• TreeMap and TreeSet of a simple Java type
• TreeMap and TreeSet of a custom Java type
• TreeMap and TreeSet Thread Safety

Queues and Deques

• Usage of the Queue and Deque
• Stacks and BlockingQueue
• LinkedList as a FIFO Queue and Queue Interface
• Dequeue Interface

Enums

• Declaring and using a basic enum
• Enums with constructors and Abstract Methods
• Implements Interface and Singleton pattern
• Using methods and static blocks and Zero instance enum
• Enum constant specific body and Getting the values of an enum
• Compare and Contains for Enum values and Get enum constant by name
• Convert enum to String and Enums with static fields
• Enum Map , EnumSet class, Enum starting with number and Hashtable

Operators

• Increment/Decrement and Conditional Operator
• Bitwise, Logical and String Concatenation Operators
• Arithmetic Operators
• Shift and Instanceof Operators
• Assignment Operators
• Conditional-and ,Conditional-or and Relational Operators
• Equality and Lambda operator

Constructors

• Default and Call parent Constructor

Object Class Methods and Constructor

• hashCode() method
• toString() and finalize() method
• equals() method
• wait(), notify(), getClass(), clone() methods
• Object constructor

Annotations

• Defining annotation types
• Built-in annotations
• Compile time processing using annotation processor
• Repeating and Inherited Annotations
• Getting Annotation values at run-time and Annotations for 'this' and receiver parameters

Immutable Class

• What is the advantage of immutability and Rules and Example

Immutable Objects

• Creating an immutable version of a type using defensive copying
• Typical design flaws which prevent a class from being immutable

Visibility (controlling access to members of a class)

• Private ,Public ,Package and Protected Visibility

Generics

• Creating a Generic Class
• Diamond and Declaring a Generic Method
• Generic parameter and Benefits of Generic class and interface
• Creating a Bounded Generic Class
• Generics to auto-cast and Use of instanceof with Generics

Classes and Objects

• Method Overloading and Overriding
• Constructors and Initializing static final
• Basic Object Construction and Use

Nested and Inner Classes

• Static vs Non Static Nested Classes
• Anonymous Inner Classes and non-static inner class

The java.util.Objects Class

• Basic use for object null check and Objects.nonNull() method

Default Methods

• Why Use, Usage and Accessing overridden of default methods
• interface methods, multiple inheritance and Abstract class

Packages

• Using Packages to create classes and Protected Scope

Inheritance

• What is Inheritance
• Abstract Classes
• Abstract class and Interface usage
• Programming to an interface
• Overriding in Inheritance, Variable shadowing and Narrowing and Widening

Reference Types

• Different Reference Types

Console I/O

• Reading user input from the console
• Aligning strings in console and Implementing Basic Command-Line Behavior

Streams

• Using Streams
• Creating a Frequency Map, Infinite and Consuming Streams
• Collect Elements of a Stream into a Collection
• Flatten Streams, Parallel Stream and Implement Mathematical Functions
• Creating a Stream and Finding Statistics about Numerical Streams
• Concatenate Streams and Reduction with Streams
• IntStream to String and Finding the First Element that Matches a Predicate
• Create a Map based on a Stream and Joining a stream to a single String
• Streams of Primitives and Stream operations categories

InputStreams and OutputStreams

• Reading,Writing,Copying,Closing,Wrapping and InputStream

Readers and Writers

• BufferedReader and StringWriter Example

Preferences

• Using preferences, Adding event listeners and Getting sub-nodes of Preferences
• Exporting, Importing preferences and Getting, Setting preferences values

Collection Factory Methods

• List<E>, Set<E> and Map<K, V> Factory Method Examples

Alternative Collections

• Multimap in Guava, Apache and Eclipse Collections
• Apache HashBag, Guava HashMultiset and Eclipse HashBag
• Compare operation with collections - Create collections

Concurrent Collections

• Insertion into ConcurrentHashMap, Thread-safe and Concurrent Collections

super keyword

• Super keyword use with examples

Serialization

• Basic and Custom Serialization
• Serialization with Gson, Versioning and serialVersionUID
• Custom JSON Deserialization with Jackson

Optional

• Map,Return default value and Throw an exception
• Filter, FlatMap andUsing Optional containers for primitive number types

Object References

• Object References as method parameters

Exceptions and exception handling

• Catching an exception with try-catch
• The try-with-resources statement
• Custom Exceptions and Handling InterruptedException
• Introduction and Return statements in try catch block
• The Java Exception Hierarchy - Unchecked and Checked Exceptions
• Creating and reading stacktraces
• Throwing an exception and Advanced features of Exceptions
• try-catch-finally statements and throws clause in a method declaration

Calendar and its Subclasses

• Creating, Subtracting and Finding AM/PM Calendar

Using the static keyword

• Reference to non-static member from static context

Properties Class

• Reference to non-static member from static context

Lambda Expressions

• Introduction to Java lambdas
• Using Lambda Expressions to Sort a Collection
• Implementing multiple interfaces, Listener and Java Closures with lambda expressions
• Memory utilization, own functional interface and Traditional style to Lambda
• return only returns from the lambda and Execute-around Pattern

Basic Control Structures

• Switch, do while, For Each, Continue Statement in Java
• If, Else If, Else Control, For Loops and Ternary Operator
• Try-Catch-Finally, Break, While Loops, If Else and Nested break / continue

BufferedWriter

• Write a line of text to File

New File I/O

• Creating paths, Manipulating, Retrieving, Reading and Writing files

File I/O

• Migrating from java.io.File to Java 7 NIO
• Reading an image, File Read/Write, Reading all bytes and Copying a file
• Stream vs Writer/Reader API and Reading a file with a Scanner
• Copying a file using InputStream and OutputStream
• Blocking or redirecting standard output / error
• Reading a file using BufferedInputStream

Scanner

• General Pattern that does most commonly asked about tasks
• Custom delimiters, Reading system input and Reading file input using Scanner

Interfaces

• Implementing multiple interfaces, Declaring and Implementing an Interface
• Usefulness of interfaces and Default methods
• Using Interfaces with Generics and Strengthen bounded type parameters

Regular Expressions

• Using capture groups and Escape Characters
• Matching a backslash, Not matching string and Matching with a regex literal

Comparable and Comparator

• Sorting a List using Comparable<T> or a Comparator<T>
• compareTo, compare Methods and Natural vs explicit sorting

Java Floating Point Operations

• Comparing floating point values
• OverFlow and UnderFlow, Formatting the floating point values

Currency and Money

• Add custom currency

Object Cloning

• Cloning performing a deep copy and copy factory
• Cloning using a copy constructor and implementing Clonable interface

Recursion

• Basic idea of Recursion
• StackOverflowError & recursion to loop
• Computing the Nth Fibonacci Number and power of a number

Converting to and from Strings

• Converting String to other datatypes
• Converting other datatypes to String

Random Number Generation

• Pseudo Random Numbers in Specific Range
• Generating Random Numbers with a Specified Seed

Singletons

• Enum Singleton and Singleton without use of Enum, Thread safe Singleton
• Extending singleton (singleton inheritance)

Autoboxing

• Using int and Integer interchangeably and Auto-unboxing may lead to NullPointerException
• Different Cases When Integer and int can be used interchangeably

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