Different ways to run Spring boot App

 What are the different ways to run Spring boot app ?

Spring Boot New Project

 How to create Spring boot new project ?

   

Spring Boot Introduction

 What is Spring boot ?

What are Spring Boot features ?

What is the difference between Spring and Spring Boot 

To know, Please watch below video.

Switch Enhancements in Java

• In this post, we are going to learn about recent enhancements to switch statement in Java.
• There are 4 enhancements to switch statement.

1. List of case constants
2. Switch expression
3. yield statement
4. case statement with arrow operator    

List of case constants

• If we want execute same block of code for some switch case constants , we write like below.

Output:

• The above example can be re-written using list of constants like below.

Switch expression, yield statement:

• Switch expression is simply switch statement which can return a value.
• We can return a value using yield keyword. We can write above using switch expression and yield keyword as below.

• We must add semicolon at the end of switch expression. Because it is treated a statement.
• We no need to write break statement because yield returns value and exit from switch expression. If we write break statement, we get compiler error.
• Normally default is optional for switch. But default is mandatory for switch expression.

Case statement  with arrow operator :

• We can use arrow operator after case statement in place of semicolon.

    case "JAN" :

    can be written as below.

    case "JAN" ->

• Arrow operator case statement can be in below three forms.

        case "JAN" -> expression;

        case "JAN" -> { block of statements}

        case "JAN" -> throw exception;

• Let's look at few examples using arrow operator .

• We can directly return value from case statement. We don't need to use yield keyword. 
• We don't need to use break because we exit from switch after returning value from switch.
• We must write default statement.

• When we use block , we must yield keyword to return value from switch expression.
• We don't need to use break statement. default statement is mandatory.

Conclusion:

These enhancements are really good addition to Java programming language. It optimizes the code. Thanks for reading . Happy Coding 😀.

var keyword in Java

• We declare local variables in a method with its data type.

        

• From Java 10 or higher version onwards , we can use var keyword to declare local variables in a method. Same variables can be declared with var keyword like below.

• Java compiler takes the variables type from it's initializer. 20 is integer value, so age data type is int. 
• You can use var keyword to declare reference variables.

• You can use var to declare variable inside for loop, forEach loop.

 

• You can use var keyword as variable name also.

Restrictions :

• You can't use var to declare a variable without initializing it. You can't use with null variables.

    

• You can't use var to declare multiple variables in single declaration.

    

• You can't use var to declare array initializer .

    

• You can't use var to declare class level instance , static variables.

• You can't use var keyword to declare method parameters.

• You can't use var as method return type.

Conclusion:

var keyword is good addition to Java programming language. It shortens the variables declaration. This is all for today. Happy Coding 😀

Sealed classes and Interfaces in Java

 

   

• The above Test class is final . It means no other class can extend it. 

    

• When we write a class in Java, any other class  can extend. We can't add a condition to allow only some classes can extend our class. 
• We can now leverage the sealed classes feature to define list of classes which can extend our class.
• We can declare sealed class with sealed keyword and  We need specify list sub classes using permits keyword.

• Classes in permit clause must extend the sealed class otherwise we get compile time error.
• Classes which are not in permit clause try to extend Test class, we get compile time error.
• Sealed class's subclasses must be final or sealed or non-sealed.

• Test classes defined Test1, Test 2 as its direct sub classes. 
• We have declared Test1 as non-sealed . It means any number of classes can extend Test1. We no need to use final, sealed , non-sealed keywords in Test1 subclasses declarations. These are not subclasses for Test class.
• We have declared Test2 as sealed. All sealed class rules are applicable here.  
• We can't use non-sealed keyword for a class which doesn't extend any sealed class. 

• Sealed classes and its sub classes must be in same module. if the sealed class is in unnamed module, its sub classes also should  be in same unnamed module. Sub classes can be in different package.

Sealed Interfaces:

• Sealed interface rules are same like sealed classes.
• We use sealed keyword to declare sealed interface.
• We can specify list of classes which can implement sealed interface and list of interfaces which can extend sealed interface.

 

• Classes and interfaces which are in permits clause only must implement or extend sealed interface . Other classes and interfaces should not implement or extend the sealed interface.
• Implemented classes must be final or sealed or non-sealed. 
• Sub Interfaces must be either sealed or non-sealed.
• A class can extend a sealed class and implement a sealed interface.

Conclusion:

This feature is useful to define definite level of inheritance for our classes and interfaces. When our developing rest services , We can use this feature for interface to specify what all classes can implement it. It is mainly useful when we are developing some library or framework.

This is all about sealed classes and interfaces . If you have any questions, please comment below. Happy Coding 😀.

Java Record

You have a Student data in the database . To return student data, you may write modal class as below. 

public class Student {
private int rollNumber;
private String name;
private String address;
public Student(int rollNumber, String name, String address) {
super();
this.rollNumber = rollNumber;
this.name = name;
this.address = address;
}
public int getRollNumber() {
return rollNumber;
}
public String getName() {
return name;
}
public String getAddress() {
return address;
}
@Override
public int hashCode() {
return Objects.hash(address, name, rollNumber);
}
@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (!(obj instanceof Student)) {
return false;
}
Student other = (Student) obj;
return Objects.equals(address, other.address) && Objects.equals(name, other.name)
&& rollNumber == other.rollNumber;
}

}

• Here We have created Student modal with three fields . 
• It has a constructor to create Student object.
• It has getter methods to read student data. 
• It also has overriden hashcode and equals methods to avoid duplicates.
• If you have multiple tables, You need to all this boiler plate code for all the modal classes.
• To avoid boiler plate code , We can use lombok library. 
• We can rewrite the above Student modal class as below using lombok library.

 import lombok.Data;

@Data

public class Student {

private int rollNumber;

private String name;

private String address;

}

• Instead of using lombok library, We can use Java record.
• Record is special class which is used to store read only data. 
• We can create a record using record keyword.  

Syntax

record recordName(variables declaration) {

//optional body

}

  

 

• We can write the above Student modal class using record as below

public record Student(int rollNumber, String name, String address) {

}

• We must declare variables within record parenthesis. These are private and final by default. 
• Java compiler automatically creates constructor , getter methods, toString, equals , hashCode methods for these variables. 
• Java will not generate setter methods. Because record is immutable. Once we have created record object, we can't modify it's data. 
• We can use record keyword as variable name also. 
• We can create record object using new keyword. 

public record Student(int rollNumber, String name, String address) {

}

public class Test {

public static void main(String[] args) {

List<Student> studentList = new ArrayList<>();

var student1 = new Student(1, "Hari", "Nellore");

var student2 = new Student(2, "krishna", "Chennai");

studentList.add(student1);

studentList.add(student2);

studentList.forEach(studentObj->{

System.out.println(studentObj.rollNumber()+" "+studentObj.name()+" "+studentObj.address());

});

} }

Output

1 Hari Nellore
2 krishna Chennai

• Here getter methods are not prefixed by get word. They are same as variable name. 
• To test equals, hashcode methods implementation , let's add Student objects to HashSet collection object. 

• We can use generics with record. 
• Static variables are allowed within record body. Instance variables are not allowed within record body. 
• Both Static and instance methods are allowed to write inside record.
• Abstract method is not allowed. 

 

 

How to write our own constructor in record:

• Normally Java compiler generated constructor is sufficient. 
• We may need to implement our own constructor to write validation conditions. For example if the name is empty or null throw an IllegalArgumentException exception.
• We can implement constructor in two ways.

1. Canonical constructor
2. Non Canonical constructor

Canonical constructor

• We can write canonical constructor in two forms . one is full form, other one is compact form.
• Let's write canonical constructor in full form

• Here constructor parameters should match exactly as the variables defined in record definition . Variables type , name and order must be same. 
• You can write same constructor using compact  form as follows.

• We don't need to declare constructor parameters. 
• We don't need to initialize the record variables. 
• At end of constructor , constructor parameters are automatically assigned to corresponding record variable. 

Non Canonical constructor

• This constructor is useful when we want to add default values for some fields while creating record object.
• We must call canonical constructor as first statement inside this constructor. 

• Here we are using this keyword to call canonical constructor. Address field data is default Nellore for all record objects. 

I hope you have learned something new today. Thanks for reading . Please share it with your friends. Happy Coding 😀. Jai Hind !!