equation.java

abstract class Equation {
  
  abstract double get(double x);
  
  public Equation compose(Equation that) {
    return new Equation {
      double get(double x) {
        this.get(that.get(x));
      }
    }
  }
  
}

class LinearEquation extends Equation {
  
  private final double constant;
  private final double slope;
  
  public LinearEquation(double constant, double slope) {
    this.constant = constant;
    this.slope = slope;
  }
  
  double get(double x) {
    return slope * x + constant;
  }
  
}