Why am i keep getting an arrayIndexOutOfBound exception error in my code

Question

My code is used to implement a polynomial and turn it into its derivative counterpart for instance 3x^2 + 2x derivative is 6x + 2 but for some reason every time I run my code in the main method it says I'm getting an arrayIndexOutOfBoundException sometimes even type mismatch, is the logic to my code wrong?

public class Der {

private static Der[] polyVal;
private String ope;
private int coe;
private String var;
private int exp;
private String ope2;

private static int deg;

Der(String ope, int coe, String var, int exp, String ope2) {
    this.ope = ope;
    this.coe = coe;
    this.var = var;
    this.exp = exp;
    this.ope2 = ope2;
}

//Accessor methods 
public String getOpe() {return this.ope;}
public int getCoe() {return coe;}
public String getVar() {return var;}
public int getExp() {return exp;}
public String getOpe2() {return this.ope2;}
// mutator methods 
public void setOpe(String newOpe) {this.ope = newOpe;}
public void setCoe(int newCoe) {this.coe = newCoe;}
public void setVar(String newVar) {this.var = newVar;}
public void setExp(int newExp) {this.exp = newExp;}
public void setOpe2(String newOpe2) {this.ope2 = newOpe2;}


public static void setEle(int degree) {
    deg = degree;
    polyVal = new Der[deg+1];
}
/*
public void setValues(Der p) {
    int i = 0;
    for(i; i < deg; i++) {

    }
}
*/

public void addObj(Der newDer) {
    int size = polyVal.length-1;
    int k = 0;
    while(polyVal[k] == null && k < size) {
        polyVal[k] = newDer;
        k++;
    }
}

public static Der[] differentiate(Der[] polynomial) throws IllegalArgumentException {
    Der[] diffPoly = polynomial.clone();
    for(int i = 0; i < deg+1; i++) {
        Der newP = diffPoly[i];
        int dCoe = newP.getCoe();
        if(dCoe == 0) {dCoe = 1;}
        String dVar = newP.getVar();
        if(dVar.indexOf(1) != '^') {throw new IllegalArgumentException("Invalid format");}
        int dExp = newP.getExp();
        if(dExp != 0 || dExp != 1) {
            dCoe = dCoe * dExp;
            dExp = dExp-1;
            newP.setCoe(dCoe);
            newP.setExp(dExp);
            //newP.setVar(this.var);
            //newP.setOpe(this.ope);
            //newP.setOpe2(this.ope2);
        }else if(dExp == 1) {
            dExp = 0;
            newP.setExp(dExp);
            //newP.setVar(this.var);
            //newP.setOpe(this.ope);
            //newP.setOpe2(this.ope2);
        }else if(dExp == 0 || dVar == "") {
            dCoe = 0;
            newP.setCoe(dCoe);
            //newP.setVar(this.var);
            //newP.setOpe(this.ope);
            //newP.setOpe2(this.ope2);
        }
        diffPoly[i] = newP;
        if(diffPoly[0].getOpe() == "+") {throw new IllegalArgumentException("Invalid format");}
    }
    return diffPoly;
 }


public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    System.out.println("pleas enter the degree of the polynomial");
    int degreeVal = sc.nextInt();
    //setEle(degreeVal);
    System.out.println("Now start adding in the expression in the format" + "- or noting" + "some int" + "some letter with ^" + "some int" + "+ or -");
    Der[] polynomial = new Der[degreeVal];
    int j = 0;
    while(j < degreeVal+1){
        Der obj = new Der(sc.next(), sc.nextInt(), sc.next(), sc.nextInt(), sc.next());
        polynomial[j] = obj;
        j++;
    }
    differentiate(polynomial);
    for(int u = 0; u < degreeVal; u++) {
        System.out.println(polynomial[u]);
    }

}

}