What is the code for this?

public interface ArraysLabInt{
//maxElement returns the max value in the array arr
public int maxElement(int [] arr);
//averageVal returns the average of all the values public double averageVal(double [] arr);
/*everyFourth returns an array arr but...
each multiple of 4 is replaced by 1111*/
public int [] everyFourth(int [] arr);
//populateGrades returns an array of doubles that represent 8 grades.
//The user should be inputting these grades from the scanner
//The user input should be done within the structure of the method, not within Main
public double [] populateGrades();
}