Instance variables in Smalltalk Vs java

Question

In Pharo by Example Book, I read

Instance variables in Smalltalk are private to the instance itself. This is in contrast to Java and C++, which allow instance variables (also known as “fields” or “member variables”) to be accessed by any other instance that happens to be of the same class.

I think it is common to other languages like Java or c# also . An instance of class cannot access instance variable of another instance of same class. How it is specific to smalltalk only ?

In Smalltalk, two instances of the same class cannot access each other’s instance variables unless the class defines “accessor methods”

How can an instance access another instance's instance variable using accessor method ?

using namespace std;
#include<iostream>
class car {
  private:
    int mileage;
  public:
    car(int); // Constructor
    int odometer();
};

car::car(int m) {
  mileage = m;
}

int car::odometer() {
  return mileage;
}

int main(void) {
  car ford(10000);
  car honda(20000);

  cout<<ford.odometer(); 
  cout<<honda.odometer();
}

In this example ford and honda are two instances of the same class Car. How ford can use the mileage of honda object or vice versa ? This is my actual question. Sorry if this is so naive

Answer 1

I think it is common to other languages like Java or c# also . An instance of class cannot access instance variable of another instance of same class. How it is specific to smalltalk only ?

Sorry, but this thinking is incorrect.

In Java, an instance of a class can access the private fields of another instance of the same class.

For a class Point with two fields x and y, a common implementation of the equals method (in fact generated by Intellij) is:

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;

    Point point = (Point) o;

    if (x != point.x) return false;
    if (y != point.y) return false;

    return true;
}

In this, the current object is accessing the fields of the other object point directly, as would not be allowed in Smalltalk.

This is also allowed in C# and many other languages. Whether Smalltalk is really the only language disallowing it I'm not sure.

By convention, the accessors of a Smalltalk class frequently are the same as the instance variable, but you need to actually write the accessors, and there is no syntax for accessing fields on a different instance directly.

Answer 2

Have a look at this Java code:

public class Person {
    Integer wallet = new Integer(100);
}

and run the Thief class:

public class Thief {
    Person victim = new Person();

    public static void main (String[] args) {
        Thief thief = new Thief();
        System.out.println("I take $"+ thief.victim.wallet + " from your wallet!");
    }
}

The result is: I take $100 from your wallet!

It does not even have to be an instance of the same class.

In case you declare a Java variable as private, instances of the same class could still access each other's instance variables. If you adapt the above example, a thief could still steal from another thief:

public class Thief {
    private Integer wallet = new Integer(100);

    public Thief victim() {
        Thief victim = new Thief();
        return victim;
    }

    public static void main (String[] args) {
        Thief thief = new Thief();
        System.out.println("I take $"+ thief.victim().wallet + " from your wallet!");
    }
}

This is not possible in Smalltalk.

Answer 3

This is in contrast to Java and C++, which allow instance variables to be accessed by any other instance that happens to be of the same class.

It's true.