Constructors

A constructor initializes an object's fields at the moment it's created with new — it shares the class's name and has no return type.

class Dog {
    String name;
    int age;

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

Dog rex = new Dog("Rex", 3);

this.name refers to the field on the object being constructed, distinguishing it from the constructor parameter of the same name.

Common mistakes

• Omitting this. when a parameter shadows a field of the same name — name = name; just assigns the parameter to itself and never touches the field.
• Forgetting that once you define any constructor, Java stops generating the free no-argument constructor automatically.