为什么此字符串值为null？

Question

So I'm finishing up my code, and encountered an error and don't know why!

The String "Size" is null when it gets to the "baseArea()" Method.

No errors are displayed.

EDIT: Added the Main method of the class, where everything is referenced, please note this is not the full code in the class !

here is the relevant code:

public class OrderingSystem 
{
private Canvas canvas;
private double Price;
private String Topping1 = setTopping1();
private String Topping2 = setTopping2();
private String Sauce = setSauce();
private String Size;
private String Crust;
private double BaseArea;

/**
 * Constructor for the ordering system.
 */
public OrderingSystem()
{
    canvas = new Canvas("Pizza Ordering", 900, 650);         
}

/**
 * Method to draw the outline of the order screen.
 */
public void drawOrderScreen()
{
    canvas.setForegroundColor(Color.BLACK);
    // vertical dividers
    canvas.drawLine(300, 0, 300, 600);
    canvas.drawLine(600, 0, 600, 600);

    // halfway divider
    canvas.drawLine(0, 300, 900, 300);
    setSauce();
    startToppings();
    startOrdering();
    setSize();
    baseArea(BaseArea, Size);
    Crust();
}

public String setSize(){
    System.out.print("What size would you like: Large, Medium or Small? :   ");
    Scanner sizescanner = new Scanner(System.in);
    String Size = sizescanner.nextLine();

    if (Size.equals("Large")){
        System.out.print( "Large selected !  ");
    }
    else if (Sauce.equals("Medium")){
        System.out.print( "Medium selected !"  );
    }

    else if (Sauce.equals("Small")){
        System.out.print( "Small selected !"  );
    }

    else {
        sizescanner.reset();
        System.out.print("Invalid Size! ");
        setSize();
    }
    return Size;
}

public double baseArea(double baseArea,String Size){

    if (Size.equals("Large")){
        baseArea = 176.7150;
    }
    else if (Size.equals("Medium")){
        baseArea = 113.0976;
    }

    else if (Size.equals("Small")){
        baseArea = 78.54;
    }
    return baseArea;
}

Answer 1

Your setSize method gets the size from the Scanner but never actually sets the value of the Size field, only returns it.

Change the return type of your setSize method to void , and replace the return Size; call in that method with this.Size = Size; .

Another note, your recursion is also slightly broken. After you call setSize in the else block, you'll need to return from the method to prevent the field being updated with the invalid Size.

Answer 2

In your setSize() method, unless you enter 'Large' everything else will become null as you check if the Sauce value is equal to what was entered:

if(Size.equals("large"){
  doSomething();
} else if (Size.equals("medium"){
  doSomething();
} else if(Size.equals("small"){
  doSimething();
}