How to fix bad operand types for binary operator?

[aunu prio]

I have several class for my CourseGrades class. I'm trying to find out average of them and the lowest & the highest number among them.
Can anyone please help me with this? I'm getting following error msg:

CourseGrades.java:34: error: inconvertible types
total += (double)(grades[i]);
^
required: double
found: GradedActivity
CourseGrades.java:45: error: bad operand types for binary operator '>'
if ((grades[0]>grades[1])&& (grades[0]>grades[2]) && (grades[0]>grades[3]))
^
first type: GradedActivity
second type: GradedActivity
CourseGrades.java:45: error: bad operand types for binary operator '>'
if ((grades[0]>grades[1])&& (grades[0]>grades[2]) && (grades[0]>grades[3]))
^
first type: GradedActivity
second type: GradedActivity
CourseGrades.java:45: error: bad operand types for binary operator '>'
if ((grades[0]>grades[1])&& (grades[0]>grades[2]) && (grades[0]>grades[3]))
^
first type: GradedActivity
second type: GradedActivity
CourseGrades.java:47: error: bad operand types for binary operator '>'
else if ((grades[1]>grades[0])&& (grades[1]>grades[2]) && (grades[1]>grades[3]))
^
first type: GradedActivity
second type: GradedActivity
CourseGrades.java:47: error: bad operand types for binary operator '>'
else if ((grades[1]>grades[0])&& (grades[1]>grades[2]) && (grades[1]>grades[3]))




//***********************************************************************

//This is gradded Activity class
public class GradedActivity
{
    private double score;
    public GradedActivity()
    {
        score = 0;
    }
    public GradedActivity(double s)
    {
        score = s;
    }
    public void setScore(double s)
    {
        score = s;
    }
    public double getScore()
    {
        return score;
    }
    public char getGrade()
    {
        char letterGrade;
        
        if (score>= 90)
            letterGrade = 'A';
        else if (score>= 80)
            letterGrade = 'B';
        else if (score>= 70)
            letterGrade = 'C';
        else if (score>= 60)
            letterGrade = 'D';
        else
            letterGrade = 'F';
        return letterGrade;
    }
      
    public String toString()
    {
        String str = "" + score;
        str += "\nGrade: " + getGrade();
        return str;
    }
}

//********************************************
//And following is my CourseGrades class
//********************************************
public class CourseGrades implements Analyzable
{
    final int  MAX_NUM = 4;   
    private GradedActivity[] grades;
    public CourseGrades()
    {
        grades = new GradedActivity[MAX_NUM];
    }
    public void setLab(GradedActivity a)
    {
        grades[0]= a;
    }
	
    public void setPassFailExam(PassFailExam b)
    {
        grades[1]= b;
    }
    public void setEssay(Essay c)
    {
        grades[2]= c;
    }
    public void setFinalExam(FinalExam d)
    {
        grades[3]= d;
    }
   
    public double getAverage()
    {
        double total = 0.0;
        for(int i = 0; i<MAX_NUM; i++)
        {
            total += (double)(grades[i]);  //Error
        }
        //double total = (double)(grades[0] + grades[1]); ////Error
        //total += grades[2]; 
        //+ grades[3];
        return total/4.0 ;
    }
    public GradedActivity getHighest()
    {
        GradedActivity highest = new GradedActivity();
   
        if ((grades[0]>grades[1])&& (grades[0]>grades[2]) && (grades[0]>grades[3]))  //Error
            highest = grades[0];
        else if ((grades[1]>grades[0])&& (grades[1]>grades[2]) && (grades[1]>grades[3])) //Error
            highest = grades[1];
        else if ((grades[2]>grades[0])&& (grades[2]>grades[1]) && (grades[2]>grades[3])) //Error
            highest = grades[2];
        else if ((grades[3]>grades[0])&& (grades[3]>grades[1]) && (grades[3]>grades[2])) //Error
            highest = grades[3];
        return highest;
    }
    public GradedActivity getLowest()
    {
    }
}

[Fixed code tags and formatted - see UseCodeTags for details]

[Matthew Brown]

Hi Aunu. Welcome to the Ranch!

It looks like you're trying to cast a GradedActivity object (which is what grades[i] is) to a double. Why would you expect that to work? Presumably you intend that to be the score of the GradedActivity?

The same elsewhere - you're trying to use an object as if it was a number.

[Campbell Ritchie]

Welcome again

I think you need a new way to iterate the array and find which is the largest; it is a standard beginner's exercise and quite simple to implement. Those multiple ifs are inelegant, I am afraid, and potentially error‑prone.