Tell, Don't Ask Summary

Accessors and Mutators

• Accessor (getter): Methods to retrieve field values
• Mutator (setter): Methods to modify field values
• Example:

  class Circle {
    private double r;
  
    public double getR() {     // accessor
      return this.r;
    }
  
    public void setR(double r) { // mutator
      this.r = r;
    }
  }
  

Accessor/Mutator vs Public Fields

Advantages over public fields:

• Adds layer of abstraction
• Can rename fields without affecting clients
• Can add validation in mutators
• Can control access to fields
• Example with validation:

  public void setR(double r) {
    if (r > 0) {
      this.r = r;
    }
  }
  

Problems with Accessors/Mutators

• Can leak implementation details
• Creates coupling between client and class
• Makes code harder to maintain
• Example of coupling:

  // Client code depends on internal representation
  double cX = c.getX();
  double cY = c.getY();
  double r = c.getR();
  boolean isInCircle = ((x - cX) * (x - cX) + 
                       (y - cY) * (y - cY)) <= r * r;
  

Tell, Don't Ask Principle

• Tell objects what to do
• Don't ask for internal state
• Let objects perform their own operations
• Better example:

  boolean isInCircle = c.contains(x, y);
  

Benefits of Tell, Don't Ask

• Maintains encapsulation
• Reduces coupling between classes
• Makes code more maintainable
• Keeps related computations together
• Allows implementation changes without affecting clients

Guidelines

• Tasks performed only on class fields should be in that class
• Avoid getters/setters unless absolutely necessary
• Think in terms of object responsibilities
• Focus on object behavior rather than state
• Design methods around what objects should do