Java Constructor invocations

In Java, a constructor is a special method that is called when an object of a class is created. Constructors are used to initialize the state of an object, and they have the same name as the class.

There are several ways to invoke a constructor in Java:

1. Using the new operator: To create a new object using a constructor, you can use the new operator followed by the class name and the arguments for the constructor. For example:

Person p = new Person("John", 30);

This will create a new Person object with the name "John" and the age 30.

2. Using the this keyword: Within a class, you can use the this keyword to refer to the current object. You can use the this keyword to call another constructor in the same class. For example:

public class Person {
  private String name;
  private int age;

  public Person() {
    this("John", 30);
  }

  public Person(String name, int age) {
    this.name = name;
    this.age = age;
  }

  // ...
}

In this example, the Person class has two constructors: a default constructor that takes no arguments, and a constructor that takes a String for the name and an int for the age. The default constructor calls the second constructor using the this keyword and passes the default values for the name and age as arguments.

3. Using the super keyword: Within a subclass, you can use the super keyword to call a constructor in the superclass. For example:

public class Student extends Person {
  private String major;

  public Student(String name, int age, String major) {
    super(name, age);
    this.major = major;
  }

  // ...
}

In this example, the Student class is a subclass of the Person class. It has a constructor that takes three arguments: a String for the name, an int for the age, and a String for the major. The constructor calls the constructor of the superclass using the super keyword and passes the name and age arguments to it.