How can constructors initialise object states?

Constructors initialise object states by setting the initial values for the object's attributes when it is created.

In object-oriented programming, a constructor is a special method that is used to initialise a newly created object. It is called automatically when an object is created. The name of the constructor is the same as the class name and it doesn't have a return type. The purpose of a constructor is to initialise the state of an object, which means assigning values to its attributes.

For example, consider a class 'Car' with attributes 'colour' and 'model'. When a new 'Car' object is created, the constructor can be used to set the initial values for 'colour' and 'model'. This is done by passing the values as arguments to the constructor when the object is created. The constructor then assigns these values to the object's attributes.

Here is an example in Java:

```
public class Car {
String colour;
String model;

// This is the constructor
public Car(String colour, String model) {
this.colour = colour;
this.model = model;
}
}

// Creating a new Car object
Car myCar = new Car("Red", "Toyota");
```

In this example, the constructor `Car(String colour, String model)` is used to initialise the 'colour' and 'model' attributes of the 'Car' object. When the 'Car' object 'myCar' is created with the new keyword, the constructor is called with the arguments "Red" and "Toyota". These values are then assigned to the 'colour' and 'model' attributes of the 'myCar' object.

In some programming languages like Python, the constructor is named `__init__`. Despite the different name, it serves the same purpose of initialising the state of an object.

In conclusion, constructors play a crucial role in object-oriented programming by allowing programmers to set the initial state of an object. This ensures that every object starts its life in a well-defined state.