Inheritance problems in Java

I have some problems with getting inheritance to work. In the parent class, the array Coefficients is private. I have some access methods but I still can't get it to work.

import java.util.ArrayList;
public class Poly {

private float[] coefficients;
public static void main (String[] args){
    float[] fa = {3, 2, 4};
    Poly test = new Poly(fa);

}

public Poly() {
    coefficients = new float[1];
    coefficients[0] = 0;
}

public Poly(int degree) {
    coefficients = new float[degree+1];
    for (int i = 0; i <= degree; i++)
        coefficients[i] = 0;
}


public Poly(float[] a) {
    coefficients = new float[a.length]开发者_Python百科;
    for (int i = 0; i < a.length; i++)
        coefficients[i] = a[i];
}

public int getDegree() {
    return coefficients.length-1;
}

public float getCoefficient(int i) {
    return coefficients[i];
}

public void setCoefficient(int i, float value) {
    coefficients[i] = value;
}

public Poly add(Poly p) {
    int n = getDegree();
    int m = p.getDegree();
    Poly result = new Poly(Poly.max(n, m));
    int i;

        for (i = 0; i <= Poly.min(n, m); i++) 
            result.setCoefficient(i, coefficients[i] + p.getCoefficient(i));
        if (i <= n) {
            //we have to copy the remaining coefficients from this object
            for ( ; i <= n; i++) 
                result.setCoefficient(i, coefficients[i]);
        } else {
            // we have to copy the remaining coefficients from p
            for ( ; i <= m; i++) 
                result.setCoefficient(i, p.getCoefficient(i));
        }
    return result;
}

public void displayPoly () {
    for (int i=0; i < coefficients.length; i++)
        System.out.print(" "+coefficients[i]);
    System.out.println();
}

private static int max (int n, int m) {
    if (n > m)
        return n;
    return m;
}

private static int min (int n, int m) {
    if (n > m)
        return m;
    return n;
}

public Poly multiplyCon (double c){
    int n = getDegree();
    Poly results = new Poly(n);
    for (int i =0; i <= n; i++){ // can work when multiplying only 1 coefficient
        results.setCoefficient(i, (float)(coefficients[i] * c)); // errors   ArrayIndexOutOfBounds for setCoefficient
       }

    return results;
   }

  public Poly multiplyPoly (Poly p){
    int n = getDegree();
    int m = p.getDegree();
    Poly result = null;
    for (int i = 0; i <= n; i++){
        Poly tmpResult = p.multiByConstantWithDegree(coefficients[i], i); //Calls new method
        if (result == null){
            result = tmpResult;
        } else {
            result = result.add(tmpResult);
        }
    }
    return result;
}
  public void leadingZero() {
    int degree = getDegree();
    if ( degree == 0 ) return;
    if ( coefficients[degree] != 0 ) return;
    // find the last highest degree with non-zero cofficient 
    int highestDegree = degree;
    for ( int i = degree; i <= 0; i--) {
         if ( coefficients[i] == 0 ) {
              highestDegree = i -1;
         } else {
              // if the value is non-zero
              break;
         }
    }
    float[] newCoefficients = new float[highestDegree + 1];
    for ( int i=0; i<= highestDegree; i++ ) {
           newCoefficients[i] = coefficients[i];
    }
    coefficients =   newCoefficients;
}

  public Poly differentiate(){
    int n = getDegree();
    Poly newResult = new Poly(n);
    if (n>0){   //checking if it has a degree
        for (int i = 1; i<= n; i++){
             newResult.coefficients[i-1]= coefficients[i] * (i); // shift degree by 1 and multiplies
     }
     return newResult;

     } else {
    return new Poly(); //empty
     }
   }


    public Poly multiByConstantWithDegree(double c, int degree){ //used specifically for multiply poly
    int oldPolyDegree = this.getDegree();
    int newPolyDegree = oldPolyDegree + degree;
    Poly newResult = new Poly(newPolyDegree);
    //set all coeff to zero
    for (int i = 0; i<= newPolyDegree; i++){
        newResult.coefficients[i] = 0;
    }
    //shift by n degree
    for (int j = 0; j <= oldPolyDegree; j++){
        newResult.coefficients[j+degree] = coefficients[j] * (float)c;
    }

    return newResult;
}
}

Can anyone help me fix my Second class that inherits from the one above? I cant seem to get my multiply and add methods for the second class to work properly.

public class QuadPoly extends Poly
{
 private float [] quadcoefficients;
 public QuadPoly() {
   super(2);

}
public QuadPoly(int degree) {
    super(2);
}
public QuadPoly(float [] f) {
    super(f);
    if (getDegree() > 2){
        throw new IllegalArgumentException ("Must be Quadratic");
    }

}
public QuadPoly(Poly p){
    super(p.coefficients);
    for (int i = 0; i < coefficients.length; i++){
        if (coefficients[i] < 0){
            throw new Exception("Expecting positive coefficients!");
        }
       }
 }
 //  public QuadPoly(Poly p){
  //  super(p.coefficients);
 //}
 public QuadPoly addQuad (QuadPoly p){
     return new QuadPoly(super.add(p));
}

 public QuadPoly multiplyQuadPoly (QuadPoly f){
    if (quadcoefficients.length > 2){
        throw new IllegalArgumentException ("Must be Quadratic");
    }

    return new QuadPoly(super.multiplyPoly(f));
}

I would make the coefficients protected or use an accessor method.

I wouldn't throw a plain checked Exception. An IllegalArgumentException would be a better choice.

What is quadcoefficients? They don't appear to be set anywhere.

You put coefficients private. I wouldn't change this but I would add a getter method into Poly class:

public class Poly {
    //somecode here
    public float[] getCoefficients(){
        return this.coefficients;
    }
}

Then I would use it by the getter method in other code;

public QuadPoly(Poly p){
    super(p.getCoefficients);
    //some more code here
}

Even if you make coefficient protected, you are trying to reach coefficients field of another Object, which is a parameter. So it is not related to inheritance and the problem.