non-static method getCostPerSquareKilometer() cannot be referenced from a static context

Question

I've read through all the duplicates that are around. And my brain somehow doesn't understand this. Basically, I'm trying to call a method from a different class and then using the method's value in a calculation. However this problem seems to keep arising for me. I really need help, I've been at it for 2+ hours now.

Here's the Code I'm trying to import the method into.

/* Alex Pedersen V 0.1 13 - 02 - 2015 Forest Aflevering */
  public class Forest {

  /* Strings */
  private String name;
  private String location;

  /* Integers */
  private int squareMeters;
  private int price;
  private int totalcost;

  /* Reference types */
  private Owner forestOwner;
  private Tree newtree;
  private Tree sumcost;
  private Tree costPerSquareKilometer;

    /* Constructor med parametere */
    public Forest(String nameInput, String locationInput, int sizeOfForest, int pricePerSquare) {
      /* Initialisering af instans variabler */
      name = nameInput;
      location = locationInput;
      squareMeters = sizeOfForest;
      price = pricePerSquare;
      totalcost = 0;
    }

    /* Constructor uden parametere */
    public Forest() {
      /* Initialisering af instans variabler */
      name = "Rold Skov";
      location = "Nordjylland";
      squareMeters = 4000;
      price = 100;
      totalcost = 0;
    }

      /* ## ACCESORS ## */

      /* String Returns */
      public String getName() {
        return name;
      }

      public String getLocation() {
        return location;
      }

      public Owner getForestOwner() {
        return forestOwner;
      }

      /* Int Returns */
      public int getSquareMeters() {
        return squareMeters;
      }

      public int getPrice() {
        return price;
      }

      public Tree getCostPerSquareKilometer() {
        return costPerSquareKilometer;
      }

      /* ## MUTATORS ## */

      /* String Mutators */
      public void setName(String newNameInput) {
        name = newNameInput;
      }

      public void setLocation(String newLocation) {
        location = newLocation;
      }

      /* Int Mutators */
      public void setSquareMeters(int newSquareMeters) {
        squareMeters = newSquareMeters;
      }

      public void setPrice(int newPrice) {
        price = newPrice;
      }

      /* Reference String Mutators */
      public void setForestOwner(Owner newForestOwner) {
        forestOwner = newForestOwner;
      }

      public void setTree(Tree newTree) {
        newtree = newTree;
      }

      public void setCostPerSquareKilometer() {
        Tree sumcost = new Tree();

        costPerSquareKilometer = Tree.getCostPerSquareKilometer();

      }

      /* ## PRINTS ## */
      public void printForest() {
        System.out.println("Forest Owner: " + forestOwner + "\nName: " + name + "\nLocation: " + location + "\n Square Meters: " + squareMeters + "\n Price: " + price);
      }

      public void printForest2() {
        if (newtree != null) {
          System.out.println("Tree: " + newtree + "\nForest Owner: " + forestOwner + "\nName: " + name + "\nLocation: " + location + "\n Square Meters: " + squareMeters + "\n Price:" + price);
        }
        else {
          System.out.println("No trees in this program :(!");
        }

      }
  }

And here's the code that I'm trying to import the getCostPerSquareKilometer from.

public class Tree {

/* Strings */
private String sort;

/* Integers */
private int lifespan;
p int costPerSquareKilometer;
private int revenuePerSquareKilometer;

    public Tree() {
        /* Initialisering af variabler */
        sort = "Aske";
        lifespan = 100;
        costPerSquareKilometer = 1000;
        revenuePerSquareKilometer = 2000;
    }

    public Tree(String sortInput, int lifepanInput, int costPerSquareKilometerInput, int revenuePerSquareKilometerInput, int squareMetersInput) {
        sort = sortInput;
        lifespan = lifepanInput;
        costPerSquareKilometer = costPerSquareKilometerInput;
        revenuePerSquareKilometer = revenuePerSquareKilometerInput;
    }

    /* ## ACCESORS ## */

    /* String Returns */
    public String getSort() {
        return sort;
    }

    /* Int Returns */
    public int getLifespan() {
        return lifespan;
    }

    public int getCostPerSquareKilometer() {
        return costPerSquareKilometer;
    }

    public int getRevenuePerSquareKilometer() {
        return revenuePerSquareKilometer;
    }

    /* ## MUTATORS ## */

    /* String Mutators */
    public void setSort(String newSort) {
        sort = newSort;
    }

    /* Int Mutators */
    public void setLifespan(int newLifeSpan) {
        lifespan = newLifeSpan;
    }

    /* Double Mutators */
    public void setCostPerSquareKilometer(int newCostPerSquareKilometer) {
        costPerSquareKilometer = newCostPerSquareKilometer;
    }

    public void setRevenuePerSquareKilometer(int newRevenuePerSquareKilometer) {
        revenuePerSquareKilometer = newRevenuePerSquareKilometer;
    }

    /* ## PRINT ## */
    public void printTree() {
        System.out.println("Sort: " + sort + "\nLifespan: " + lifespan + "\nCost per KM^2: " + costPerSquareKilometer + "\nRevenue per KM^2" + revenuePerSquareKilometer);
    }
}

Thank you very much in advance :>

Answer 1

You should change your line:

costPerSquareKilometer = Tree.getCostPerSquareKilometer();

To

 costPerSquareKilometer = sumcost.getCostPerSquareKilometer();

Currently you are calling it like . which means method has to be static while you defined method as non static i.e. no static keyword when you defined the method. So that means you can access method only on object instance of Tree in your case, hence use sumcost instance to access cost.

Answer 2

The problem is as described by the error. Static calls can only be made to static methods. The getCostPerSquareKilometer is not static, so cannot be referenced as a static.

  public void setCostPerSquareKilometer() {
    Tree sumcost = new Tree();

    costPerSquareKilometer = Tree.getCostPerSquareKilometer();

  }

Updated:

  public void setCostPerSquareKilometer() {
    Tree sumcost = new Tree();

    costPerSquareKilometer = sumcost.getCostPerSquareKilometer();

  }

This change, means that you would be calling the method on the object and not on the class