Fundamentals
#JDK consists of development tools required for developing a Java application.
It also consists of Java runtime environment for executing a Java application.
#.java files are in src folder of the project
.class files are in bin folder of the project
#A single .java file can have multiple classes, but they are compiled into their own separate .class files.
It is a good practice to have a single Java class in a .java file.
#JIT compiler - converts reusable byte code to machine language.
#There are 50 keywords in Java 8.
Apart from keywords, there are other words which are used as names to identify components in a program, e.g. HelloWorld, main. These words are called Identifiers.
#Identifiers should start with a letter, $ or _
It should not be a Java keyword or a literal
#Where there is functionality, there is data. And for implementing the functionality, we need to store the data.
#char datatype is of 2 bytes and the values range is \u0000 to \uffff
#String is one of the non-primitive types.
#Implicit conversion happens when a value of smaller data type needs to be assigned to a variable of larger compatible data type. Also called widening conversion, it is done automatically by Java.
#In narrowing conversions, char data type can be converted to byte or short
#Does the code below require type casting?
long rollNo = 88843L;
float rollNoFloat = rollNo;
Ans - No
Explanaton: there are two types of losses, loss of magnitude and loss of precision.
int to byte is loss of magnitude because you cant store 500 in a byte.
long to float is loss of precision because the value range of float is larger than that of long.
eg. - 23500000000 is stored as 2.35e10 in float
Rule is: loss of magnitude - explicit cast required
loss of precision - no cast required
#Control stucture:
1] Decision Making
2] Looping
3] Branching
# ARRAY can be used to store data of primitive as well as reference data type
#// Another way of creating a 2-D array
int[][] dayWiseTemperature = new int[][] {
{30, 35, 28},
{29, 32, 0},
.
.
.
};
#For-Each loop:
for (int score : marks) {
System.out.println(score);
}
#String args[] is an array of strings.
It is an array of command line arguments. Such arguments are passed as inputs to your program either through the command prompt of your system, or through the run configuration of your IDE.
#Valid Array Declaration:
1] int myArray2[];
2] int myArray3[]=new int[5];
3] int []myArray5=new int[5];
4] int myArray7[]=null;
5] int score[][] = new int[2][];
6] int score[][] = new int[2][2];
7] int score[][] = new int[2][];
score[0] = new int[2];
#Class:
Instance variables are automatically initialized to their default values during object creation.
Local variables (declared in methods) have to be initialized before they can be used.
#When a method has a local variable with the same name as an instance variable, the local variable gets higher preference inside that method. This is called shadowing of a field. In such a case, "this" keyword can be used to refer to the instance variables.
"this" can also be used to invoke one constructor or method from another constructor or method.
#local variables are not assigned to default variable.
#Memory:
-All local variables and method invocations are stored in the stack.
-Earlier languages like C/C++ made the programmer responsible for freeing the memory occupied by such resources. Java, on the other hand, has a garbage collector which automatically deallocates the memory used by these resources.
-When a reference is local to some method then it will be removed from the stack as soon as the method has finished executing.
#Encapsulation:
private members (model, color, mileage variables of Car class below) are hidden and are accessible only through public methods.
Abstraction:
Invoking methods (start or displayDetails method of Car class) without knowing their details.
#Relationships:
1] Association: When an object utilizes another object in order to perform its activities, a relationship of association is established between them. This is termed as 'uses-a' relationship.
Example: A driver uses a car to travel. (Uses objects as parameters)
Association (uses-a) is denoted by a line.
2] Aggregation: When an object contains another object as its attribute, we have an aggregation between them. It is termed as "has-a" relationship.
Example: A car has an engine (here, Car is an aggregate class with full ownership on the Engine object)
Aggregation (has-a) is denoted by a link with a diamond head.
3] Inheritance:
#Types of Inheritance:
1] Single Inheritance
2] Hierarchical Inheritance
3] Multi level Inheritance
4] Multiple Inheritance (Not Supported in JAVA due to diamond problem, but by Interfaces)
#The ability of an object to behave differently to a behavior invocation is known as Polymorphism.
#Method Overriding:
-When we override a method in the child class, it should have the same signature as that of the parent class.
-It should not have a weaker access privilege.
-Private methods are not overridden.
#super keyword:
- to call parent call methods
- to invoke the parent class constructor
- to access the instance variable of parent class
#static blocks:
static blocks are used to initialize static variables when it cannot be done in a single line. They can be used to add preprocessing if required. Static blocks get executed when a class gets loaded.
e.g.-
static {
int randomNumber = 30;
counter = (int) Math.pow(randomNumber, 3);
}
#NOTE: A static context, i.e. static blocks and static methods, cannot access non-static (instance) members directly.
However, non-static methods can access static members.
#Final keyword:
-Final field can not be changed
-Final class can not be subclassed
-Final method can not be overridden
#Abstract Methods:
A class can be abstract even without any abstract methods.
e.g.-
abstract class Automobile {
public abstract void start(); // Notice the semicolons in the method declarations
public abstract void stop();
}
#Abstract classes enforce inheritance
and
Abstract methods enforce overriding
Hence,we achieve dynamic binding
#Interfaces:
An interface is used to define a generic template which can be implemented by various classes.
It contains method signatures and constant declarations. Since Java 8, it can also have default and static methods.
The methods declared in an interface are implicitly public and abstract, and the data fields are implicitly public, static and final, i.e. constants.
The classes implementing an interface must implement all the specified methods. Otherwise, they should be made abstract.
#default and static in Interfaces:
"Default" methods can be used to introduce new functionalities into interfaces without breaking the existing classes which implement those interfaces. The default methods provide their own definitions, which the implementing classes need not override.
"Static" methods in interfaces allow us to easily organize the related helper and utility methods in our libraries.
Note: If a class implements multiple interfaces having the same default methods, it has to override them.
From the overridden methods, the default methods of a specified interface can be called using the super keyword.
E.g. PassengerCar.super.
#"SavingsAccount extends Account" is correct if both Account and SavingsAccount are both either interfaces or classes.
#Access specifiers:
1] Public: Accessible everywhere.
2] Private: Accessible only inside the same class.
3] Protected: Accessible in the same packages and to sub classes of the same class.
4] Default: Accessible inside the same packages (Members without any access specifiers)
Note: A class can have only public or default access.
Note: Protected methods of a class can be inherited by subclasses, and can become public in the subclasses.
#PMD is a source code analyzer, which checks code against a predefined set of rules.
It detects inefficient code like unused variables, empty switch/if/while, unnecessary object creation, etc.
The rules are defined in XML format (.exsd)
#PMD rules we are going to follow are:
1] Unused imports should not be present
2] System.out.println statements should not be present
3] Variable naming convention should be followed
variable names should be in camel case
4] Method naming convention should be followed
method names should be in camel case
5] Class naming convention should be followed
class names should be in pascal case
6] Package naming convention should be followed
package names should be in lowercase
7] printStackTrace() should not be present
#In Java, the Object class is the implicit super class of all classes.
A method that takes an Object type parameter can accept any object as the parameter.
#equals() compares objects to check for equality.
By default, it uses memory address to compare objects for equality (just like ==).
To make it work for different requirements, it needs to be overridden.
#hashCode() uses an object's data to generate a hash value, which should be a 32 bit integer.
By default, it derives the hash value based on the memory address of the object being used.
In Java, hash tables and other such data structures make use of hash values for better performance.
To make it work for different requirements, it needs to be overridden.
#Equal objects must have equal hash codes
In other words, if two objects are equal according to the equals() method, then the hashCode() method on each of the two objects must produce the same integer result.
To maintain this contract both the methods should be overridden in tandem
It is important to understand that if the hash codes of two objects are same, it doesn't prove that the objects are equal, i.e. it is possible for two unequal objects to have the same hash codes.
#toString() returns a textual representation of the object.
The returned text should be concise, easy to read, and informative.
By default, it returns a string consisting of the name of the object's class, the '@' character, and the unsigned hexadecimal representation of the hash code of the object.
It should be overridden to provide a meaningful textual representation.
#The Comparable interface imposes an ordering among the objects of a class. This is called the class' natural ordering.
It has only one method, compareTo()
When overridden, it should return:
--a negative integer if this object is less than the specified object
--zero if this object is equal to the specified object
--a positive integer if this object is greater than the specified object
The wrapper classes and the String class implement the Comparable interface.
#While overriding the equals() method, hashCode() method "should" be overridden to maintain the hashCode-equals contract.
#Strings are immutable (final), i.e. they are constants, and their values cannot change after creation. This improves memory usage and makes them thread-safe*.
#the String class maintains a literal pool, which is like a cache in the heap.
--The pool keeps a reference for every string literal object.
--If the same string literal value is used again, the reference to the existing object is used from the pool.
--The intern() method is implicitly used to add and use string object references from the pool. This method can also be used explicitly.
e.g. -
String name1 = "Emily";
Initially the string pool is empty. When a string literal value is used, intern() is invoked implicitly.
A String object is created, and a reference to it is added to the pool. The same reference is then returned to 'name1' for use.
NOTE: If the String object is explicitly created using the "new" keyword, the literal pool is not used.
While creating String objects using the new keyword, the intern() method can be explicitly invoked to use the literal pool. This works the same way as directly creating strings using literals.
String name4 = new String("Emily").intern();
#the equal() and hashCode() methods are already overridden in the String class.
#The StringBuffer class represents thread-safe and mutable strings, whose capacity can be dynamically increased.
#StringBuilder is the non-synchronized version of StringBuffer. It is used when thread safety is not required.
It is faster than String and StringBuffer.
Note: The equals() and hashCode() methods in the StringBuffer and StringBuilder classes are not overridden.
#Integer wrappedInt = 2; --autoboxing.
int primitiveInt = wrappedInt; --unboxing
#Note: Similar to the String pool, Byte, Short, Integer and Long classes cache values in the range -128 to 127.
Wrapper classes are commonly used when we need to convert numeric strings into other data types. For this purpose, we have methods like parseDouble(), parseInt(), etc.
Also, type casting cannot be used to convert any wrapper type to another. We can make use of methods like intValue(), byteValue(), floatValue(), etc. to perform such conversions. Consider the example below
e.g. -
Integer phoneNo = 44281234;
Long phoneNo = phoneNo.longValue();
// Converts Integer into a Long value
#string pool + (compile time reference) creates a new object.
#An annotation is a meta-data that provides data about the program and is not part of the program itself.
--Its purpose is to give additional information to the compiler, or for some processing during compile-time or runtime.
--It can be added to classes, methods, variables, parameters and packages.
e.g.-
@SuppressWarnings(value="
Here, SuppressWarnings is the annotation name, value is the element and 'unused' is its value.
This annotation prevents warnings related to unused local variables and unused private methods.
A few other built-in annotations:
@Override is used on a method. It ensures that the method is actually overriding a parent's method. Otherwise, a compilation error occurs.
@Deprecated denotes a method as obsolete. A warning occurs when such a method is used.
#Java provides varargs to make things easier. It is a construct that allows methods to accept zero or more arguments.
--It internally uses arrays to process the variable number of arguments.
Limitations:
--There can be only one varargs argument in a method.
--If present, varargs must be the last parameter.
#Regular Expression (Regex) :
String regex = "Hello.*";
here, Hello = literals
. = Meta Character (Reserved character)
* = Quantifiers (frequency of occurance of characters)
This regex will match with any string that starts with "Hello"
The regex API (java.util.regex) provides classes and interfaces to work with regular expressions.
The String class uses this API to support regex in four methods:
matches(), split(), replaceFirst(), replaceAll()
Note:
-->If meta characters are needed to be used as literals, they have to be escaped with a double backslash.
-->To refer to captured groups in the replacement methods, $<group no.> is used. E.g. str.replaceAll(regex, "$1");
-->"\w" is a meta character. But in Java, '\' is an escape character. Hence, we have to escape it using another backslash. E.g. "[\\w]+"
#<<2 Uppercase Characters>>-<<9 digits>>:<<2 Uppercase Characters>>
[A-Z]{2}-[\\d]{9}:[A-Z]{2}
password regex - password.matches(".{6,}") && password.matches("[\\w]*[\\d][
For "johnny99@infy.com"
\\w+@infy\\.(com|in)
\\w+\\d+@\\w+\\.com
[\\w&&[^\\s]]+@infy\\.com
[a-zA-Z0-9@.]+
#An enum is a data type which contains a fixed set of constants. For example,
days of a week (Sunday, Monday, Tuesday, Wednesday, Thursday, Friday and Saturday)
directions (North, South, East and West)
The enum fields are static and final implicitly, making them compile time constants.
Enums can be defined inside a interface, a class and a static context.
A few things about enums:
-->Enums in Java are considered to be reference types like classes and interfaces, and thus, a programmer can define constructors, methods and variables, inside an enum.
-->The Java compiler automatically generates a static method called values() for each enum. This method returns an array of all the constants defined inside the enum.
-->The constructor for an enum type must have default or private access. And it cannot be invoked using the new operator.
-->Values can be provided to enum constants at the time of their creation. These values are passed to the constructor when the constant is created.
We know the instance Direction.SOUTH gets initialized automatically when used for the first time. And since it is static and final by default, only one copy of SOUTH is created.
This feature of enums can be used as a solution in situations where a single copy of an object is required for the whole application lifecycle.
Such recurring situations are called "design problems", and their solutions are called "design patterns".
The above pattern is called the "Singleton Design Pattern".
Some benefits of using design patterns are:
They provide a standard approach to solve a recurring problem and save time.
They promote reusability, which leads to more robust and highly maintainable code.
They make our code easy to understand and debug as they are already defined.
They lead to faster development.
A design pattern is a proven and time tested solution to a recurring design problem.
Types of design patterns:
1] Creational : Offers flexibility to decide who(how,which,when) is responsible for the creation of the object. e.g.- Builder, factory, singleton
2] Structural : focus on how related classes or objects are composed together to form a larger structure. e.g. - adaptor, composite, decorator, bridge
3] Behavioral : defines communicaation of the objects and controls the flow within the participating objects. e.g.- Command, Iterator, Observer
#Singleton pattern is a design solution where an application needs to have one and only one instance of any class, in all possible scenarios without any exceptional condition.
There are many ways of implementing singleton design pattern, such as using private constructors and static methods, enum, etc.
The simplest approach in Java for implementing singleton is through enum.
e.g.-
class Registrar {
private static Registrar reg;
private Registrar() {
System.out.println("Registrar created!");
}
public static Registrar getRegistrar() {
if(reg == null) reg = new Registrar();
return reg;
}
}
// Using enum
public enum Registrar {
REGISTRAR("R1001");
String value;
private Registrar(String value) {
this.value = value;
}
}
public enum Day {
SUNDAY(1), MONDAY(2), TUESDAY(3), WEDNESDAY(4), THURSDAY(5), FRIDAY(6), SATURDAY(
7);
private int value;
private Day(int value) { this.value = value; }
public int getValue() { return this.value; }
public static void main(String[] args) {
//Printing all constants of an enum.
for(Day day: Day.values())
System.out.println("Day:"+day.
}
}
enum enumDemo {
INSTANCE;
public void display() {
System.out.println("Display called");
}
}
public class UserInterface {
public static void main(String[] args) {
enumDemo.INSTANCE.display();
}
}
#Builder pattern separates the construction logic for an object from its representation so that the same construction process can create different representations.
It provides a step-by-step way to build the object and provide a method that will return the final object.
For example,
StringBuilder.append()
It is an overloaded method which appends string representation of the argument of any data type to the current object. E.g. append(boolean b), append(Object obj), etc.
#DATE :
Date class allows us to represent date in terms of year, month, day, hour, minute, second and millisecond values.
Date today = new Date();
SimpleDateFormat sdf = new SimpleDateFormat("dd-MMM-yyyy"
String todayString = sdf.format(today);
NOTE: Some methods of the Date class are now deprecated, and it is recommended to use the Calendar class instead.
java.util.Calendar is an abstract class.
Its object is created using the getInstance() method.
e.g. - Calendar today = Calendar.getInstance();
int currentYear = today.get(Calendar.YEAR); // Here Calendar.YEAR is a field constant
System.out.println("Current Year: " + currentYear);
This method follows the Factory Design Pattern, and actually returns a GregorianCalendar (a child class of Calendar) object with the current system date, time, time-zone and locale.
Note:
While checking Calendar objects for equality, all the field values are compared.
In the Date and Calendar classes, the month index starts from zero.
The Date and Calendar objects are not thread-safe.
java.time.LocalDate is an immutable date-time object that represents a date. It does not store time and time-zone.
e.g. - LocalDate today = LocalDate.now();
java.time.LocalTime is an immutable date-time object that represents time up to nanoseconds of precision. It does not store the time-zone.
e.g. - LocalTime rightNow = LocalTime.now();
java.time.LocalDateTime is an immutable date-time object that represents date and time. It does not store the time-zone.
e.g. - LocalDateTime rightNow = LocalDateTime.now();
The ChronoField enum is used to provide field-based access to date and time components.
And the ChronoUnit enum can be used to specify the desired unit of a field.
e.g. -
LocalDate today = LocalDate.now();
LocalDate anotherDay = LocalDate.of(2010, 2, 16);
int thisYear = today.get(ChronoField.YEAR);
long durationInYears = anotherDay.until(today, ChronoUnit.YEARS);
String date = anotherDay.format(
// Output: Tuesday, 16 February, 2010
Note:
In the LocalDate and LocalDateTime classes, the month index starts from 1.
The LocalDate, LocalTime and LocalDateTime objects are thread-safe.
the Calendar class solves a design problem… the problem of creating its objects based on parameters.
The Factory design pattern is used when there is a super class with multiple sub-classes, and we need an object of one of the sub-classes based on the input.
This pattern uses a factory class to create objects of different sub-classes based on the user requirement.
e.g. -
1] Calendar.getInstance() implements factory pattern as it returns a different instance of Calendar based on the parameters.
2] Calendar.getInstance() returns an object of GregorianCalendar with the default time zone and locale.
3] Calendar.getInstance(Locale locale) returns an object of BuddhistCalendar if the locale is Thai. It returns an object of JapaneseImperialCalendar if the locale is Japanese. Otherwise it returns an object of GregorianCalendar.
#enterprise applications have non-functional requirements (NFRs) like:
1] Usability
2] Performance
3] Availability
4] Security
5] Scalability
6] Maintainability
7] Reliability
#n-tier structure:
client tier
presentation tier
business tier
persistence tier
#client-server architecture:
if the client request for a static resource through URL, it is retrieved from a set of static content.
Otherwise, the web container component of the server generates the dynamic content
#The Application servers (App Servers) should be able to take care of the load, availability, security, performance monitoring, etc.
Examples:
WebSphere Application Server (WAS) by IBM
WildFly (JBoss) Application Server by Red Hat
Geronimo by Apache
WebLogic Application Server by Oracle
GlassFish by Oracle
#Use Case Diagrams are used in UML (Unified Modeling Language to visualize a system design) to depict Use Cases. They are used to identify, clarify and organize the system requirements as well as to capture the functional requirements during the development stage.
A Use Case is a list of steps which define the interactions between an entity and a system.
#Note: DAO stands for Data Access Object
#sequence diagrams. They are used to represent the dynamic behaviour of the objects in a system by using interactions. They describe interactions by focusing on the sequence of messages that are exchanged, without giving information about the object structure.
#The details representing a real world user can be encapsulated into an object. Such objects are called beans.
#A factory class provides the objects of various classes as and when required. This ensures that all object-creation code remains in a single place. A factory contains static methods to create and return objects of other classes.
e.g. -
public static UserService createPremiumUserService() {
return new PremiumUserService();
}
public static UserService createRegularUserService() {
return new RegularUserService();
}
// Similarly for other such classes
using the interface reference and getting the required object from the factory has made the code completely independent of the implementation while following the business standards.
This introduces more abstraction and flexibility into our code, and also makes it resilient to changes.
Such a convention is called the Factory Design Pattern.
#"Generics" are used to create classes, interfaces and methods in which the type of the object on which they operate is specified as a parameter.
--Type checking is done at compile time
--Eliminates the need for casting
--Generic algorithms can be implemented
class class-name<type-parameter-
Here, the type-parameter-list specifies the type parameters. By convention, the type parameter is denoted by a single capital letter and is commonly one among E (element), T (type), K (key), N (number) and V (value)
e.g. -
class Stack<E> {
private E[] elements;
public E push(E element) {
// Code to push element
}
public E pop() {
// Code to pop element
}
}
Stack<String> strStack = new Stack<String>();
Stack<Integer> integerStack = new Stack<>(); // Java 7 or higher versions
//Generic types can also be used without type parameters. Such a type is called raw type.
Stack stack = new Stack(); // Raw type
//There can be more than one type parameter for a class or interface.
class MyClass<K,V> { }
Among all the flexibility and security, Generics however, possess some restrictions:
A type parameter cannot be instantiated
E e1 = new E(); // Compilation error
A generic class cannot extend the java.lang.Throwable * class.
A generic type cannot be used with a primitive type. It must always be a reference type.
public class UserInterface {
public static <E> void display(E[] list) { // A generic method to display an array of objects
for (int i = 0; i < list.length; i++)
System.out.println(list[i] + ", ");
}
}
UserInterface.<String>display(
NOTE: A generic method can return a generic type value.
Also, a generic method can be declared inside a non-generic class.
#Arrays
Advantages:
1] Compile time type checking
2] Can hold primitive type data as well as objects
Disadvantages:
1] It cannot grow and shrink dynamically
2] It does not have any built-in algorithm for searching or sorting
The "collections framework" provides a set of interfaces and classes for representing and manipulating collections. Introduced as part of J2SE 1.2, it standardizes the way we store and access data from collections. It is a part of the java.util package.
Accessing the elements of a collection:
1] The for loop can be used for ordered collections
2] The enhanced for loop (for-each) can be used for ordered and unordered collections
3] The Iterator interface can be used for collections implementing the Collection interface
The Iterator interface provides the following methods:
1] hasNext()
2] next()
3] remove()
Iterator interface follows the Iterator Design Pattern.
#ArrayList and LinkedList classes implements List interface. The equals() method works across all the implementations of the List interface and returns true if and only if they contain the same elements in the same order. We can traverse the elements of a list using the for-loop, the enhanced for-loop, the iterator or the list iterator.
ArrayList is used for more frequent access.
LinkedList is used for more frequent updating and deletion.
class ListTester {
public static void main(String args[]) {
LinkedList obj = new LinkedList();
obj.add("A");
obj.add(10);
obj.add("C");
obj.addFirst("D");
System.out.println(obj); //[D, A, 10, C]
}
}
#Set uses the methods from the Collection interface and does not declare any new method of its own.
Being unordered, sets cannot be accessed using indexes. The enhanced for-loop and the iterator are two ways of traversing and accessing the elements of a set.
Implementation of Set Interface:
1] HashSet (uses hash table)
2] LinkedHashSet (uses hash table and linked list) (ordered by insertion)
3] TreeSet (use tree based data structure) (natural ordered)
public class Test {
public static void main(String args[]) {
Set s = new TreeSet();
s.add("4");
s.add(8); //Runtime Exception coz of compareTo()
Iterator itr = s.iterator();
while (itr.hasNext())
System.out.print(itr.next() + " ");
}
}
#Map does not extend the Collection interface. So there is no iterator for maps. Moreover, map values cannot be retrieved without knowing the keys. Hence, there is no direct way of traversing a map.
Working with the set of keys:
Set setOfKeys = map.keySet();
Working with the Collection of values:
Collection valueCollection = map.values();
Working with the set of entries:
Set<Entry> setOfEntries = map.entrySet();
Implementation of Map Interface:
1] Hash Map (hash table implementation)(unordered)
2] TreeMap (tree based implementation)(natural ordering)
3] LinkedHashMAp (order of insertion)
4] ConcurrentHashMap serialized(thread safe) version of hash map
Note: TreeMap and ConcurrentHashMap do not allow null keys.
Map<String, Double> map = new HashMap<String, Double>();
map.add("pi", 3.14159); //wrong it has put() method
map.add("log(1)", new Double(0.0)); //wrong
map.add("e", 2.71828D); //wrong
map.add('x', new Double(123.4)); //wrong
public static Iterator getIterator(List list) {
Collections.reverse(list);
return list.iterator();
}
public static void main(String[] args) {
List list = new ArrayList();
list.add("1"); list.add("2"); list.add("3");
Iterator iter = getIterator(list);
while(iter.hasNext())
System.out.print(iter.next() + ", "); //3,2,1
}
public class Sorter {
public static void main(String[] args) {
ArrayList<Integer> collection = new ArrayList<Integer>();
collection.add(1); collection.add(5); collection.add(3);
Collections.sort(collection);
collection.add(2);
Collections.reverse(
System.out.println(collection)
}
}
Exception
#In Java, all exceptions are objects of the java.lang.Exception class. These objects carry the information related to the exception, including the stack trace.
#Whenever an exceptional event occurs, the runtime environment (JRE) generates the exception object and throws it.
#Two kinds of Exception:
1] Checked Exception
2] Unchecked Exception
#A try block is immediately followed by one or more catch blocks or a finally block. When an exception is caught and handled by a catch block, the execution continues from immediately after the try-catch block.
e.g. - catch(Exception1 | Exception2 | Exception3 e) {
// Code for handling exceptions
}
#The finally block will be disrupted if an exception occurs inside it, or if System.exit() is invoked before it.
#All the exceptions belong to the Exception class, which is a child of the Throwable class.
Note: Exceptions are either handled or declared to be thrown.
If there is a checked exception which the method doesn't handle, it has to be declared using the throws clause.
#We can create a user-defined exception class by extending the Exception class:
We can also set messages by using the parameterized constructor:
public class DivisionException extends Exception {
public DivisionException(String message) {
super(message);
}
}
Note: Extending RuntimeException instead of Exception will make the custom exception unchecked.
XML
#Enterprise applications consist of modules built on different platforms. These modules may have to interact with each other and share data. So, we need a proper structure for representing data, irrespective of the platform.
XML is one such language used to represent data in our applications in a structured manner. (It is extensible as it allows users to define tags)
#XML is said to be a well-formed XML as it follows the below "syntax rules".
--It has a root element that contains all the other elements
--Tags are case sensitive and are properly nested
--The attribute values are enclosed within quotes
#This can be prevented by setting rules for our XML files. These rules can specify elements and attributes that are allowed. We can also add rules for constraints on data types , for validating data, etc.
All such rules are defined in a document called Document Type Definition (DTD) or Schema file.
DTD is an old specification with its own syntax and has certain limitations, e.g. DTD cannot specify the data type of the contents in a tag. Schema, on the other hand, is a newer specification which uses the XML syntax itself. It also overcomes the limitations of DTD.
Logging
#In large applications, it becomes essential to keep track of all the events which are generated during the execution of an Enterprise Application. And keeping a log of the associated information helps in quick problem diagnosis, debugging, and maintenance.
Logging is the process of tracking the execution of a program where
Any event that is of interest to the programmer or the administrator can be logged
Relevant messages can be recorded in the event of exceptions and errors
The logs can be analyzed by the administrators later
Some of the popular logging APIs are
1] JDK Logging API
2] Apache Log4j
3] Commons Logging API
Log4j is written in Java and is open source. It provides mechanisms to directly log information to a variety of destinations such as database, file, console, etc. It also logs information in various formatting styles such as HTML, XML, etc.
Components:
1] The Logger : Accepts all the information to be logged during the execution of an application and redirects them to a specified Destination i.e., Appender
2] Appender : Specifies the output destination for logging. (FileAppender, ConsoleAppender, WriteAppender, JDBCAppender)
3] Layout : Specifies the format of the logged information (SimpleLayout, PatternLayout, HTMLLayout, XMLLayout)
Log4j can be configured programmatically or by using a configuration file.
log4j.xml and log4j.properties are the log4j configuration files.
A project needs the log4j-1.2.15.jar file in the build path to perform logging.
Levels in logger specify the severity of an event to be logged.
The org.apache.log4j.Level class defines all the levels. Each level has a unique integer associated with it.
The levels provided by Log4j are:
1] ALL
2] TRACE
3] DEBUG
4] INFO
5] WARN
6] ERROR
7] FATAL
8] OFF
The logging level can be decided based on the requirement. For example, trace during development and error during deployment.
public static Logger getRootLogger() // to get the root logger object.
public static Logger getLogger(String name) // to get the logger with a specific name.
JAR is a file containing packaged class files and associated resource file, usually used to distribute application and library.
DOMConfigurator.configure("
Logger logger = Logger.getLogger(this.
logger.error(e.getMessage(), e);
throw e;
PROPERTIES
#As enterprise applications contain several modules, it is important to ensure consistency in the communication with the user. Generally, communication happens in the form of messages. These messages are used across modules and may require to be modified from time-to-time.
Whenever the messages are modified, the application needs to be rebuilt.
It would be better to have an external file for all such needs, and read them at runtime.
A "properties" file can help us with this.
It is a text file used to store any kind of textual information in the form of key-value pairs.
It can be easily understood by a Java application with the help of the java.util.Properties class.
#Properties files are usually used for
--Standardization of messages (user friendly error messages)
--Configuration of enterprise applications
--Internationalization or localization
#Some methods of properties class:
getProperty(key)
getProperty(key,defaultValue)
load(inputStream)
#A utility class with name 'AppConfig.java' will be available in the 'resources' package of the demos. This class has the code necessary for reading & loading key-value pairs from a properties file.
In AppConfig.java, the following line opens the file for reading
inputStream = new FileInputStream("
And the following loads the content into the static properties object
PROPERTIES.load(inputStream);
Now we can get a property by specifying its key
AppConfig.PROPERTIES.
AppConfig.PROPERTIES.
MULTITHREADING
#In enterprise application environments, multithreading is used by App servers for maintaining thread pools to serve multiple users simultaneously. A thread pool is a group of ready-made threads with no overhead of creating new ones.
#ways to create thread:
1] extending thread class (overridding run() method)
MyThread myThread = new MyThread();
2] implementing Runnable interface (implementing run() method)
Thread threadObj = new Thread(myRunnable);
In the previous example, observe how the thread object (threadObj) knows how to invoke the operation, the run() method, without knowing how the operation is performed. This behavior follows a design pattern called the Command Design Pattern.
This pattern is used to encapsulate a request as an object and pass to an invoker, wherein the invoker does not know how to service the request but uses the encapsulated command to perform an action.
In our example, the Runnable object (myRunnable) is the request object which is invoked by threadObj. Eventually, the encapsulated run() method is executed.
Sometimes there are objects which need to be accessed by multiple threads. Since threads share the memory of their parent process, they can access those "shared resource" together.
Every object has a built-in mutually exclusive lock mechanism called monitor. It can be used to achieve synchronization among threads. At a time, only one thread can acquire the monitor on an object.
Methods or code blocks that should be accessed by only one thread at a time should be marked as "synchronized" keyword.
#synchronized method:
public synchronized void withdraw(Double amount) {
// Check balance
// Withdraw amount
}
#synchronized block:
public void withdraw(Double amount) {
synchronized(this) {
// Check balance
// Withdraw amount
}
}
Such methods or blocks thus become thread-safe.
Inter-thread Communication methods:
wait()
notify()
notifyAll()
NOTE: These methods can be called from within a synchronized context only.
--If a thread needs to wait for an object's state to change while executing a synchronized method, it may call wait()
--The thread calling wait() will release the lock on the particular object and will wait for a notification
--It will wait till it is notified by another thread holding the lock on the same object
--If there are multiple threads waiting on the same object, the notify() method will notify any one among them. All the waiting threads can be notified using notifyAll()
The methods notify(), notifyAll(), wait() belong to Object class.
#While working with multi-threaded applications, a developer needs to take care of several aspects of threads like
--creating and supervising a number of threads
--synchronizing them
--handling their communication
--organizing their individual results
All of it takes a great deal of effort. Moreover, they do not contribute to the business functionality.
Some important packages of the concurrency API are as follows:
java.util.concurrent
java.util.concurrent.locks
java.util.concurrent.atomic
Also, new concurrent data structures have been added in the Java Collections Framework.
An Executor initiates and controls the execution of threads, thereby improving the maintainability of programs.
The java.util.concurrent.Executor interface provides the execute() method, which executes a specified thread.
void execute(Runnable thread)
The java.util.concurrent.
It defines the shutdown() method for an orderly shutdown. During the shutdown, the previously submitted tasks are executed, but new tasks will not be accepted.
Since Java 5, we can make use of java.util.concurrent.Callable<
The Callable interface declares only one method - call()
This method represents a task that needs to be completed by a thread before returning a value. The callable thread class has to implement the Callable interface and override the call() method.
ExecutorService provides the submit() method to submit callable objects for execution and return their Future objects.
A Future object represents the value that will be returned by a thread in the future. This value can be retrieved using the get() method of the Future object.
If the result is ready, it will be returned
If not, the calling thread will be blocked
The java.util.concurrent.locks package provides support for locks as an alternative for using synchronized keyword to control access to a shared resource.
The Lock interface defines a lock, whose implementations provide more extensive locking operations compared to synchronized methods and blocks.
The important methods declared in Lock are:
lock()
tryLock()
unlock()
One of the implementation of the Lock interface is the Reentrant Lock.
A reentrant lock is the one in which if a thread tries to acquire a lock that it already holds for a different block, the request succeeds, i.e. a thread can acquire the lock multiple times without blocking on itself.
It means that locks are acquired on a per thread basis rather than per invocation basis. It is implemented by associating with each lock a counter and an owning thread.
When the counter is zero, the lock is unheld
When a thread acquires a previously unheld lock, the counter is set to one
If the same thread acquires the lock again, the counter is incremented by one
When the owning thread exits the synchronized block, the counter is decremented by one
When the counter reaches zero, the lock is released
The need for Fork/Join becomes clear in case of divide-and-conquer or recursive task-processing problems, where a work can be broken into smaller tasks, and their results can be combined to get a final result.
Fork means splitting a task into smaller subtasks which can be executed concurrently. Each subtask can be executed in parallel by different CPUs, or different threads on the same CPU
Join means merging the results of all the subtasks into a single result
The goal is to use all the available processing power to enhance the performance of an application. The core Fork/Join framework class is java.util.concurrent.
In Java SE 8, java.util.Arrays implements Fork/Join framework in its parallelSort() method for sorting large arrays in a multiprocessor system.
While going through the concurrency API, it will be useful to know that the java.util.concurrent package provides a new set of collections for concurrent programming.
Some interfaces and classes in the java.util.concurrent package are:
BlockingQueue
ConcurrentHashMap
TESTING
#Testing of single small units of code such as a method or a class and asserting certain behavior is called unit testing. It is done by developers.
JUnit is an open-source unit testing framework for Java. It provides classes to write and run automated tests. It provides
--annotations to create and customize tests
--test fixtures (to fix state) for setting up each test
--assertions to test for expected results
--test suites for grouping tests
The org.junit.Test annotation turns a public method into a test method.
@Test
public void isValidUsernameValidTest() { }
Once a class has a test method, it is called a test class or a test case, and can be run as a JUnit test:
Right click on the class ---> Run As ---> JUnit Test
The JUnit Runner is responsible for constructing the instances of the test classes before running the tests. It is also responsible for making them available for garbage collection after the tests.
@Test
public void isValidUsernameValidTest() {
Validator val = new Validator();
Assert.assertTrue(val.
}
The above test asserts that the isValidUsername() method will return true for the given input. If the method has been properly implemented, it will return true, and the test will pass.
The Assert class provides static methods for testing a variety of conditions.
Here are some useful methods in the Assert class:
assertEqual()
assertTrue()
assertFalse()
assertNull()
assertNotNull()
NOTE: if an input is expected to cause an exception, assertions cannot be made.
JUnit allows us to create rules for adding a new behavior or redefining the behavior of each test method in a test class. The @Rule annotation is used for this purpose. It marks the public fields of the type TestRule which is an abstract class, and allows developers to create custom rules.
TestRule has a lot of implementation classes, out of which ExpectedException is an important one. It is used to verify expected exception types and messages.
Here is an example of using ExpectedException class along with @Rule annotation:
public class MyTest {
@Rule
public ExpectedException ee = ExpectedException.none();
@Test
public void isValidUsernameInvalidTest() throws Exception {
ee.expect(Exception.class);
ee.expectMessage("VALIDATOR.
Validator val = new Validator();
val.isValidUsername("EMAIL"))
}
}
Useful methods of ExpectedException:
none() - return object of ExpectedException that expect no exception i.e., empty rule
expect() - verifies that a code throws the specified exception
expectMessage() - verifies the exception message of that exception
JUnit provides test suites to create groups of test classes which can be executed together.
The following annotations are used for creating test suites:
@RunWith(Suite.class)
@Suite.SuiteClasses
e.g.-
import org.junit.runner.RunWith;
import org.junit.runners.Suite;
@RunWith(Suite.class)
@Suite.SuiteClasses({UserTest.
public class TestSuiteDemo { }
Code coverage aims at determining the extent to which code is tested during unit testing.
Some commonly used coverage tools are as follows:
--JCov
--JaCoCo
--Cobertura
--EclEmma
--Emma
We will be using EclEmma in the course.
EXTRAS:
Lambda function in java
Comsumer Interface in java
EXTRAS:
Lambda function in java
Comsumer Interface in java