The primary object-oriented construct in Java is the class, which is a combination of data (fields) and behavior (methods). The fields and methods of a class are known as its members.

Access to members can be controlled through access modifiers, the two most common ones being:

• public: the member can be accessed by any code (no restrictions).
• private: the member can only be accessed by code in the same class.

You can think of a class as a template for creating instances of that class. To create an instance of a class (also known as an object), the new keyword is used:

class Car {
}

// Create two car instances
Car myCar = new Car();
Car yourCar = new Car();

Fields have a type and a name (defined in camelCase) and can be defined anywhere in a class (by convention cased in PascalCase):

class Car {
    // Accessible by anyone
    public int weight;

    // Only accessible by code in this class
    private String color;
}

One can optionally assign an initial value to a field. If a field does not specify an initial value, it wll be set to its type's default value. An instance's field values can be accessed and updated using dot-notation.

class Car {
    // Will be set to specified value
    public int weight = 2500;

    // Will be set to default value (0)
    public int year;
}

Car newCar = new Car();
newCar.weight; // => 2500
newCar.year;   // => 0

// Update value of the field
newCar.year = 2018;

Private fields are usually updated as a side effect of calling a method. Such methods usually don't return any value, in which case the return type should be void:

class CarImporter {
    private int carsImported;

    public void importCars(int numberOfCars)
    {
        // Update private field from public method
        carsImported = carsImported + numberOfCars;
    }
}