I can't seem to figure out what's wrong with my code, I keep getting these errors. java.lang.IllegalArgumentException Exception in thread "main" java.lang.NullPointerException at FractionTesterHG.main(FractionTesterHG.java:135) 

public class Fraction {

int numerator;

int denominator;

public Fraction () {

numerator=0;

denominator=1;

}

public Fraction(int x) {

numerator=x;

denominator=1;

}

public Fraction(int x,int y) {

numerator=x;

denominator=1;

if(this.denominator==0)

throw new IllegalArgumentException("Does not work");

}

public String toString() {

return this.numerator+"/"+this.denominator;

}

public double evaluate() {

return (double)numerator/denominator;

}

public boolean isImproper() {

return false;

}

public Fraction multiplyBy(Fraction another) {

Fraction f=new Fraction(this.numerator*another.numerator, this.denominator*another.denominator);

return f;

}

public void invert() {

if(this.denominator==0) {

throw new IllegalStateException("Will not work");

}else {

int temp=this.numerator;

this.numerator=this.denominator;

this.denominator=temp;

}

}

}