Tried to compile the following code and got these errors:

Course.java:43: incompatible types
found   : java.lang.String
required: double
                double cStart = df.format(cStart);
                                          ^
Course.java:44: incompatible types
found   : java.lang.String
required: double
                double cEnd = df.format(cEnd);
                                        ^

Here are my two classes:

import java.text.DecimalFormat;

public class Course implements Comparable<Course>{
	
	private String name;
	private double cStart;
	private double cEnd;
	
	public Course(String cName, double cStart, double cEnd) {
		this.name = cName;
		this.cStart = cStart;
		this.cEnd = cEnd;
	}
		
	public String getName() {
		return name;
	}
	
	public double getSTime() {
		return cStart;
	}

	public double getETime() {
		return cEnd;
	}

	public int compareTo(Course c) {
		if(cEnd > c.getETime())
			return 1;
		else if(cEnd < c.getETime())
			return -1;
		return 0;
	}

		public String toString() {
		//Decimal to two places
		DecimalFormat df = new DecimalFormat("#.##");
		cStart = df.format(cStart);
		cEnd = df.format(cEnd);
		
		//Change back to strings with colons for time
		String cStartA = Double.toString(cStart);
		String cEndA = Double.toString(cEnd);
		cStartA = cStartA.replace('.',':');
		cEndA = cEndA.replace('.', ':');
		
		return name + "\n" + cStartA + "\n" + cEndA + "\n\n";
	}
	
}
import java.io.File;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Scanner;

public class Fitter {
	
	private ArrayList<Course> courses;
	private ArrayList<Course> fittedCourses;
	private double cEndTime;
	private double cStartTime;
	private double lastEnd = 8;
	
		public Fitter(File courseList) throws FileNotFoundException, NumberFormatException{
		//initialize variables
		courses = new ArrayList<Course>();
		fittedCourses = new ArrayList<Course>();
		Scanner courseScanner = new Scanner(courseList);
		//scan lines in File
		while(courseScanner.hasNextLine()) {
			//get name and start/ end times
			String cName = courseScanner.nextLine();
			String cStartS = courseScanner.nextLine();
			String cEndS = courseScanner.nextLine();
		
				//*********************************************
				System.out.println("cName: " + cName);
				System.out.println("cStartS: " + cStartS);
				System.out.println("cEndS: " + cEndS);

		
			//convert times into decimals, first checking number formatting
			String[] cStartSplit = cStartS.split(":");
		
			double cStartH = Double.parseDouble(cStartSplit[0]);
			double cStartM = Double.parseDouble(cStartSplit[1]);
			
				
			if ((cStartH < 8) || (cStartH > 17))
				throw new NumberFormatException();
			if (cStartM > 60)
				throw new NumberFormatException();

			cStartS = cStartSplit[0] + "." + cStartSplit[1];
			double cStart = Double.valueOf(cStartS.trim()).doubleValue();
			
			
			String[] cEndSplit = cEndS.split(":");
			
			double cEndH = Double.parseDouble(cEndSplit[0]);
			double cEndM = Double.parseDouble(cEndSplit[1]);
		
			
			if ((cEndH < 8) || (cEndH > 17))
				throw new NumberFormatException();
			if (cEndM > 60)
				throw new NumberFormatException();

			cEndS = cEndSplit[0] + "." + cEndSplit[1];
			double cEnd = Double.valueOf(cEndS.trim()).doubleValue();
			
			
			Course newCourse = new Course(cName, cStart, cEnd);
			courses.add(newCourse);
			//skip blank line
			if(courseScanner.hasNextLine())
				courseScanner.nextLine();
		}
	}
	
	public void fit() throws NumberFormatException{
		//sort by size
		Collections.sort(courses);
		//for each course (starting with earliest end time) add
		//if no conflict exists
		for(Course course : courses) {
			if(lastEnd < course.getSTime()) {
				fittedCourses.add(course);
				lastEnd = course.getETime();
			}	
		}
	}

	
	public String toString() {
		String info = "We made the following schedule: \n\n";
		
		
		for(Course course : fittedCourses) {
			info += course.toString() + "\n";
		}
		return info;
	}

}

Now, I'm new to using DecimalFormat, but the problem that I seem to be encountering is the type that cStart and cEnd wind up as. Before I added all the code aside from the last line to the Course class's toString method
(code read as:

return name + "\n" + cStart + "\n" + cEnd + "\n\n";

), the code was compiling and running as it should (obviously printing as 9.2 rather than 9:20), and it seemed as if both cStart and cEnd were being treated as doubles.

If I try to do something to manage cStart and cEnd as anything other than a double, I'm told that the variables are in fact a double... What's going on?

Thanks.

Yes, they are doubles, and nothing can change that.
When you use df.format that takes your double value, formats it according to the specified format, and returns that value to you as a String. So you need to declare a String variable to hold the formatted value.

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