Purpose of this() in following code snippet?

user_10095353 0
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package test;

public class Employee {
    String name;
    int age;
    
    Employee() {}
    
    Employee(String newName, int newAge) {
        this();
        name = newName;
        age = newAge;
    }
    
    public static void main(String args[]) {
        Employee e = new Employee("N", 43);
        System.out.println();
    }    
}

In the above code, what is the actual point of "this()" in the overloaded constructor from a usefulness perspective besides being an example of calling the no-argument constructor from the overloaded constructor?

java
3 Answers

This looks like it's the wrong way round to me.

A typical use of this() would be to provide a default value for the constructor when not supplied in the argument as you cannot call the constructor directly. Adapting your code for example:

package test;

public class Employee {
    String name;
    int age;

Employee() {
    this("Default", 42);
}

Employee(String newName, int newAge) {
    name = newName;
    age = newAge;
}

public static void main(String args[]) {
    Employee e = new Employee();
    System.out.println();
}

}

user_76978460

In that exact snippet?

Literally nothing.

Any constructor MUST neccessarily have, at the top of it, either a this() call, or a super() call. You can't not. If you fail to write it, javac will inject: super(); for you, and if that isn't valid (for example, your superclass does not have a protected+ no-args constructor), then your code won't compile.

So that's the difference between the snippet as pasted and a hypothetical one where the this(); is removed. Desugaring, you get either:

Desugared, WITHOUT the this():

package test;

public class Employee {
    String name;
    int age;
    
    Employee() {
        super(); // invokes java.lang.Object's no-args, which does nothing.
    }
    
    Employee(String newName, int newAge) {
        super(); // invokes java.lang.Object's no-args, which does nothing.
        name = newName;
        age = newAge;
    }
}

Desugared, WITH the this():

package test;

public class Employee {
    String name;
    int age;
    
    Employee() {
        super(); // invokes java.lang.Object's no-args, which does nothing.
    }
    
    Employee(String newName, int newAge) {
        this();
        name = newName;
        age = newAge;
    }
}

Now, inject, say, a System.out.println("Hello!"); in the no-args constructor and now there is a small difference: With the this(), you'd see Hello! printed, and without it, you won't. Either way, though, you end up calling your superclass's constructor, as that is something that has to happen, one way or another. (Only java.lang.Object doesn't have to, hardcoded in the VM; Object has no superclass).

user_7686440

To call the default constructor, this() is used.

user_140267880
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