Why does this get a run-time error?

[Dennis Noren]

The error is NullPointerException, at the line in the Shape constructor:

description.insert(0, sb);

The full code:

import java.awt.Color;
   import java.util.*;
   import java.text.*;
 
    class Shape implements Cloneable{
      Color color;
      StringBuffer description;
   
       Shape() {
         color = Color.green;
         description.append("It is green");
      }
   
       Shape(Color col, StringBuffer sb) {
         System.out.println("In shape constructor with col-sb");
         color = col;
         description.insert(0, sb);
      }
   
       public Color getColor() {
         return color;
      }
   
       public StringBuffer getDesc() {
         return description;
      }
   
       public Object clone () throws CloneNotSupportedException{
         return super.clone();
      }
   }
 
    class Rectangle extends Shape {
      double height;
      double width;
      String name = "A rectangle";
   }
 
    class TestOverride {
       public static void main(String args[]) {
         StringBuffer shape1Name = new StringBuffer("A green shape");
         System.out.println(shape1Name);
      Shape shape1 = new Shape(Color.green, shape1Name);
         System.out.println(shape1.getDesc());
      }
   }

Before the run error, the program displays

A green shape
In shape constructor with col-sb

so it does get into the constructor

[Ernest Friedman-Hill]

Remember that in Java, every variable of non-primitive type is a pointer. Declaring the member

StringBuffer description;

creates a pointer to a StringBuffer that is initialized to null; when you try to use it, you get a NullPointerException. At some point, you have to actually create a StringBuffer object; i.e, replace that line with

StringBuffer description = new StringBuffer();

and you'll be all set.

[Dennis Noren]

Thanks much. Slowly, I learn. ;-)

[Ernest Friedman-Hill]

I'm sorry that I forgot to mention this before, but you may not have read our naming policy on your way in. It requires that you use a full, real (sounding) first and last name for your display name. An initial isn't enough for a last name. You can change your display name here. Thanks!