A class is a blueprint; an object is an instance created from it. A constructor is a special method that initializes a new object. Together they're the foundation of Java's object-oriented model.
{
String name;
age;
{
.name = name;
.age = age;
}
String {
+ name;
}
}
The constructor runs when you create an object, setting up its initial state. this refers to the current object, disambiguating fields from parameters of the same name.
Person p = new Person("Ann", 30); // `new` allocates the object + calls the constructor
p.greet(); // "Hi, I'm Ann"
The new keyword allocates memory on the heap and invokes the constructor; p holds a reference to the new object.
public class Person {
private String name;
private int age;
public Person(String name, int age) { // primary constructor
this.name = name;
this.age = age;
}
public Person(String name) { // overloaded — different parameters
this(name, 0); // calls the other constructor (chaining)
}
}
You can have multiple constructors with different parameter lists (overloading), and one can call another with this(...) to avoid duplicating initialization.
public class Empty {
// no constructor defined → Java provides an implicit no-arg constructor
}
Empty e = new Empty(); // works
// BUT: once you define ANY constructor, the implicit one is gone
If you write no constructor, Java supplies a no-argument default. As soon as you define any constructor, that default disappears (you must add it explicitly if needed).
Classes, objects, and constructors are the absolute foundation of Java programming — everything you build is composed of objects created from classes.
Understanding how constructors initialize state (this, the new keyword's role, constructor overloading and chaining for flexible creation, and the default-constructor rule) is essential for correctly creating and initializing objects.
These concepts underpin all OOP work in Java, and constructor behavior (especially the disappearing default and chaining) is a common source of both interview questions and real bugs.