Shifting non static Coordinates in java

Question

I have a method shift that needs to construct a new object of class Coordinate with coordinates shifted dx to the right and dy down. For example, calling pos.shift(-1,1) with pos representing the position (1,1) would result in a new position (0,2). Eclipse gives me this error message:

Cannot make a static reference to the non static method shift(int, int) from type Coordinate

This is my piece of code, which should do this shifting:

/** move the position by dx to the right and by dy down */
public Coordinate shift(int dx, int dy) {
    pos = pos.shift(this.x + dx, this.y + dy);
}

Maybe I am just way too stupid but I have no good idea to solve this.

This is the whole code:

 /** represents a position on a board */ 
 public class Coordinate {

  /** variables specifying horizontal position on the board */
  private int x;

  /** variable specifying vertical positoin on the board */
  private int y;

 /** constructor creating a Coordinate from x and y values */
 public Coordinate(int x, int y) {
     this.x = x;
     this.y = y;
 }

 /** getter for the x value */
 public int getX() {
     return this.x;
 }

 /** getter for the y value */
 public int getY() {
     return this.y;
 }

 /** check whether this position is valid for the given (quadratic) board size */
 public boolean checkBoundaries(int xSize, int ySize) {
     boolean check;
     if (xSize == (0|1|2) && ySize == (0|1|2)) {
         check = true;
     } else {
         check = false;
     }
 return check;
 }

 /** move the position by dx to the right and by dy down */
 public Coordinate shift(int dx, int dy) {
     pos = pos.shift(this.x + dx, this.y + dy);
 }
}

Anyone with a good idea?

Answer 1

Within your method shift there are several problems

 public Coordinate shift(int dx, int dy) {
     pos = pos.shift(this.x + dx, this.y + dy);
 }

• This method refers to a variable pos, pos has not been declared and so does not exist. It also seems to serve no purpose as all the co-ordinate shifting can be done without it.
• Method shift has no return statement, meaning that no new Coordinate is returned
• If pos was a variable of type Coordinate then shift would call pos.shift would call pos.shift would call pos.shift etc forever until a StackOverflowException occured

A corrected version of this method is

 public Coordinate shift(int dx, int dy) {
     return new Coordinate(this.x + dx, this.y + dy);
 }

Note how:

• A new Coordinate is created using the constructor
• This new Coordinate is passed out of the method

Method checkBoundaries(int xSize, int ySize)

This method does not impact the specific problem mentioned. It doesn't however behave how you think. 0|1 is a bitwise operator, it does not give multiple options for a condition. What you wanted was several conditions seperated by the logical operator ||

 public boolean checkBoundaries(int xSize, int ySize) {
     boolean check;
     if ((xSize==0 || xSize ==1 || xSize ==2) && (ySize==0 || ySize ==1 || ySize ==2)) {
         check = true;
     } else {
         check = false;
     }
 return check;
 }