Java entity - why do I need an empty constructor?

Question

This might sound stupid to you, but why do I need to define an empty constructor in my @Entitys?

Every tutorial I saw said : every entity needs an empty

Answer 1

All the answers are fine.

But let's talk with code. Following snippets of code will give you more clarity.

PersonWithImplicitConstructor.java

public class PersonWithImplicitConstructor {
    
    private int id;
    
    private String name;

}

First we have to compile the .java file

javac PersonWithImplicitConstructor.java

Then class file will be generated.

Running the javap on top this class file will give you the following information.

javap PersonWithImplicitConstructor.class

Compiled from "PersonWithImplicitConstructor.java"
public class PersonWithImplicitConstructor {
  public PersonWithImplicitConstructor();
}

NOTE: If you want more information, you can use -p flag on javap.

The next java file will have parameterised constructor only.

PersonWithExplicitConstructor.java

public class PersonWithExplicitConstructor {
    
    private int id;
    
    private String name;

    public PersonWithExplicitConstructor(int id, String name) {
        this.id = id;
        this.name = name;
    }
}

javac PersonWithExplicitConstructor.java

javap PersonWithExplicitConstructor.class

Compiled from "PersonWithExplicitConstructor.java"
public class PersonWithExplicitConstructor {
  public PersonWithExplicitConstructor(int, java.lang.String);
}

PersonWithBothConstructors.java

public class PersonWithBothConstructors {

    private int id;
    
    private String name;

    public PersonWithBothConstructors() {

    }

    public PersonWithBothConstructors(int id, String name) {
        this.id = id;
        this.name = name;
    }
    
}

javac PersonWithBothConstructors.java

javap PersonWithBothConstructors.class

Compiled from "PersonWithBothConstructors.java"
public class PersonWithBothConstructors {
  public PersonWithBothConstructors();
  public PersonWithBothConstructors(int, java.lang.String);
}