Java - Formatting Double data types, getting error

Question

Here is the code for my homework assignment. It's long overdue and I didn't do so well on it, but wanted to try to work on my mistakes and problems to actually do better in school; but anyways:

  double x1, y1, x2, y2, x3, y3; // Three points for three vertices. 
                                 // (x1, y1), (x2, y2), (x3, y3)

  Scanner input = new Scanner(System.in);

  // The program asks the user what the name of the triangle is. Then the program
  // asks the user for the cordinates of each point.
  System.out.print("Enter a three letter name for the triangle: ");
  String triangleName = input.next();
  System.out.print("Coordinates of vertex " + triangleName.substring(0,1).toUpperCase() + ":");
  x1 = input.nextDouble();
  y1 = input.nextDouble();
  System.out.print("Coordinates of vertex " + triangleName.substring(1,2).toUpperCase() + ":");
  x2 = input.nextDouble();
  y2 = input.nextDouble();
  System.out.print("Coordinates of vertex " + triangleName.substring(2,3).toUpperCase() + ":");
  x3 = input.nextDouble();
  y3 = input.nextDouble();

  double a, b, c; // These values will serve as the three side
                  // lengths between the points.

  // The program displays side lengths in this block.
  System.out.println("--- Side lengths ---");
  a = Math.sqrt(Math.pow(x2-x1, 2) + Math.pow(y2-y1, 2));
  System.out.printf(triangleName.substring(0,1).toUpperCase()
                      + triangleName.substring(1,2).toUpperCase() + ":"
                      + "%.3f", a);
  System.out.println();
  b = Math.sqrt(Math.pow(x3-x2, 2) + Math.pow(y3-y2, 2));
  System.out.printf(triangleName.substring(1,2).toUpperCase()
                       + triangleName.substring(2,3).toUpperCase() + ":"
                       + "%.3f", b);
  System.out.println();
  c = Math.sqrt(Math.pow(x3-x1, 2) + Math.pow(y3-y1, 2));
  System.out.printf(triangleName.substring(2,3).toUpperCase()
                       + triangleName.substring(0,1).toUpperCase() + ":"
                       + "%.3f", c);
  System.out.println();

  double perimeter, area, xCentroid, yCentroid, inRadius, inArea, semiPerimeter, triangleHeight, triangleBase;

  // The program displays other measures in this block.
  System.out.println("--- Other measures ---");
  perimeter = a + b + c;
  System.out.printf("Perimeter         = " + "%.3f", perimeter);
  System.out.println();
  triangleHeight = y3 - 0.1;
  triangleBase = x2 - x1;
  area = (triangleHeight * triangleBase) / 2;
  System.out.printf("Area              = " + "%.3f", area);
  System.out.println();
  xCentroid = (x1 + x2 + x3) / 3;
  yCentroid = (y1 + y2 + y3) / 3;
  System.out.printf("Centroid          = ( " + "%.3f", xCentroid + ", " + yCentroid + ")");
  System.out.println();
  semiPerimeter = 0.5 * perimeter;
  inRadius = area / semiPerimeter;
  System.out.format("Incircle radius   = " + "%.3f", inRadius);
  System.out.println();
  inArea = Math.PI * Math.pow(inRadius, 2);
  System.out.format("Incircle area     = " + "%.3f", inArea);
  System.out.println();

The problem in the code resides on the line for formatting the xCentroid and yCentroid values respectively. I am getting an error that says "java.util.IllegalFormatConversionException: f != java.lang.String" . The math in the program runs fine and works too. I've removed the "%.3f"'s in the statement to see if I get the correct answers, which I do. It's just this format is pissing me off. Please respond, and help. I'll be up for awhile and will be patient.

NEW CODE (FIXED) :

  System.out.print("Centroid          = ");
  System.out.print("(");
  System.out.printf("%.3f", xCentroid);
  System.out.print(", ");
  System.out.printf("%.3f", yCentroid);
  System.out.print(")");
  System.out.println();

Answer 1

In this code

System.out.printf("Centroid          = ( " + "%.3f", xCentroid + ", " 
                                                        + yCentroid + ")");

especially this

xCentroid + ", " + yCentroid + ")"

is not going to be a number, but a String

I guess what you want is

System.out.printf("Centroid          = (%.3f)", xCentroid + yCentroid);