Lombok constructor

Lombok can generate a constructor for a class. We can choose to generate no argument constructor or constructor with all fields as arguments or constructor for some specific fields.

In this article, we will take a look at different constructor annotations such as @NoArgsConstructor, @AllArgsConstructor, @RequiredArgsConstructor to generate class constructors.

Lombok @NoArgsConstructor
Use this annotation to generate a no argument constructor for a java class. This annotation is applied at class level. Example,

import lombok.NoArgsConstructor;

@NoArgsConstructor
public class User {
  
}

The generated constructor will not accept any arguments and will look as below

public User() {
}

Lombok @AllArgsConstructor
This annotation will generate a constructor that accepts all the fields of the class as arguments. Example,

import lombok.AllArgsConstructor; 

@AllArgsConstructor 
public class User { 

  private String name;

  private int age;
}

This will generate a constructor of below form

public User(String name, int age) {

}

You can use @NoArgsConstructor and @AllArgsConstructor together. In this case, two constructors will be generated.

If any field is marked with @NonNull annotation, then the generated argument constructor will also have a null check for that parameter.
If the value of that field is null while calling the constructor, a NullPointerException will be thrown. Example,

import lombok.AllArgsConstructor; 
import lombok.NonNull;

@AllArgsConstructor 
public class User { 
  @NonNull
  private String name; 
 
  private int age; 
}

Now, when an object of User is created with null value for name as

User user = new User(null, 22);

Below will be the result

Note that it is not possible to generate a constructor for some specific fields with @AllArgsConstructor.

@RequiredArgsConstructor
This annotation generates a constructor for all those fields which are marked as @NonNull or final.
For all @NonNull and final fields, a single constructor will be generated, which accepts these fields.

So, this annotation can be used to generate constructor for some specific fields. Example,

import lombok.RequiredArgsConstructor

@RequiredArgsConstructor
public class User {
  @NonNull
  private String name;

  private final int age;

  private String gender;

}

This will generate a constructor of below form

public User(String name, int age){

}

Important:
If there are no @NonNull and final fields in a class, then @RequiredArgsConstructor will generate a constructor without any arguments.
In this case, if both @RequiredArgsConstructor and @NoArgsConstructor are applied over a class, then the compiler will raise an error as.

Reason is obvious.
Making private constructor
There may be a case where you need to write a private constructor so that the class can not be directly instantiated.
Lombok constructor annotations provide staticName attribute which takes a string value and generates a public static method with this name.
This public static method returns an object of the class. Example,

import lombok.NoArgsConstructor

@NoArgsConstructor(staticName="getUser")
public class User {

}

Now, you can not create an object of this class as

User user = new User();

For creating an object, below syntax needs to be used

User user = User.getUser();

Note that this has nothing to do with singleton pattern and each time getUser() is called, a new object will be created.

Hope the article was useful.