Problems with toString()

Question

This is probably a weird question, but how do I get this program to return the result using toString()? It's for a homework assignment...
import java.util.Scanner;
public class LED_Digit {
     private int myTop, myTopBothSides, myTopLeft, myTopRight, myMiddle, myBottomBothSides, myBottomLeft, myBottomRight, myBottom;
     public void drawLED_Digit(int top, int topBothSides, int topLeft, int topRight, int middle, int bottomBothSides, int bottomLeft,
               int bottomRight, int bottom) {
     for (int t = 1; t <= top; t++) {
          System.out.print(" - "); }
          System.out.println();
     for (int tbth = 1; tbth <= topBothSides; tbth++) {
          System.out.println("|   |");}
     for (int tl =1; tl <= topLeft; tl++) {
               System.out.println("|   "); }
     for (int tr = 1; tr <= topRight; tr++) {
               System.out.println("    |"); }
     for (int m = 1; m <= middle; m++) {
               System.out.print(" - "); }
     for (int bbth = 1; bbth <= bottomBothSides; bbth++) {
               System.out.println();
               System.out.println("|   |");}
     for (int bl = 1; bl <= bottomLeft; bl++) {
               System.out.println();
               System.out.println("|   "); }
     for (int br = 1; br <= bottomRight; br++) {
               System.out.println();
               System.out.println("    |"); }
     for (int b = 1; b <= bottom; b++) {
               System.out.print(" - "); }
     myTop = top;
     myTopBothSides = topBothSides;
     myTopLeft = topLeft;
     myTopRight = topRight;
     myMiddle = middle;
     myBottomBothSides = bottomBothSides;
     myBottomLeft = bottomLeft;
     myBottomRight = bottomRight;
     myBottom = bottom;
     public static void main(String[] args) {
          Scanner keyboard = new Scanner(System.in);
          System.out.print("What is the number? " );
          int num = keyboard.nextInt();
          findDig(num);
     public static void findDig(int num) {
     if (num == 0) {
          LED_Digit zero = new LED_Digit();
          zero.drawLED_Digit(2, 3, 0, 0, 0, 0, 0, 0, 2);
          System.out.println(); }
     else if (num == 1){
          LED_Digit one = new LED_Digit();
          one.drawLED_Digit(0, 0, 0, 4, 0, 0, 0, 0, 0);
          System.out.println(); }
     else if (num == 2) {
          LED_Digit two = new LED_Digit();
          two.drawLED_Digit(2, 0, 0, 1, 2, 0, 1, 0, 2);
          System.out.println(); }
     else if (num == 3) {
          LED_Digit three = new LED_Digit();
          three.drawLED_Digit(2, 0, 0, 1, 2, 0, 0, 1, 2);
          System.out.println(); }
     else if (num == 4) {
          LED_Digit four = new LED_Digit();
          four.drawLED_Digit(0, 1, 0, 0, 2, 0, 0, 1, 0);
          System.out.println(); }
     else if (num == 5) {
          LED_Digit five = new LED_Digit();
          five.drawLED_Digit(2, 0, 1, 0, 2, 0, 0, 1, 2);
          System.out.println(); }
     else if (num == 6) {
          LED_Digit six = new LED_Digit();
          six.drawLED_Digit(2, 0, 1, 0, 2, 1, 0, 0, 2);
          System.out.println(); }
     else if (num == 7) {
          LED_Digit seven = new LED_Digit();
          seven.drawLED_Digit(2, 0, 0, 1, 0, 0, 0, 1, 0);
          System.out.println(); }
     else if (num == 8) {
          LED_Digit eight = new LED_Digit();
          eight.drawLED_Digit(2, 1, 0, 0, 2, 1, 0, 0, 2);
          System.out.println(); }
     else if (num == 9) {
          LED_Digit nine = new LED_Digit();
          nine.drawLED_Digit(2, 1, 0, 0, 2, 0, 0, 1, 2); }
     else {
          System.err.println("Not a valid digit"); }
}

Answer 1

That seems like it puts me more on the right track, but I must be doing it wrong because now it says I have a "NullPointerException". And I'm still not sure how to incorporate toString() (it is at the bottom of the code).
import java.util.Scanner;
public class LED_Digit {
     static StringBuilder sb;
     public void drawLED_Digit(int top, int topBothSides, int topLeft, int topRight, int middle, int bottomBothSides, int bottomLeft,
               int bottomRight, int bottom) {
     for (int t = 1; t <= top; t++) {
               sb.append(" - "); }
               sb.append('\n');
     for (int tbth = 1; tbth <= topBothSides; tbth++) {
               sb.append("|   |" + '\n');}
     for (int tl =1; tl <= topLeft; tl++) {
               sb.append("|   " + '\n'); }
     for (int tr = 1; tr <= topRight; tr++) {
               sb.append("    |" + '\n'); }
     for (int m = 1; m <= middle; m++) {
               sb.append(" - "); }
     for (int bbth = 1; bbth <= bottomBothSides; bbth++) {
               sb.append('\n');
               sb.append("|   |" + '\n');}
     for (int bl = 1; bl <= bottomLeft; bl++) {
               sb.append('\n');
               sb.append("|   " + '\n'); }
     for (int br = 1; br <= bottomRight; br++) {
               sb.append('\n');
               sb.append("    |" + '\n'); }
     for (int b = 1; b <= bottom; b++) {
               sb.append(" - "); }
     public static void main(String[] args) {
          Scanner keyboard = new Scanner(System.in);
          System.out.print("What is the number? " );
          int num = keyboard.nextInt();
          findDig(num);
          StringBuilder();
     public static void StringBuilder() {
          // TODO Auto-generated method stub
          System.out.print(sb);
     public static void findDig(int num) {
     if (num == 0) {
          LED_Digit zero = new LED_Digit();
          zero.drawLED_Digit(2, 3, 0, 0, 0, 0, 0, 0, 2);}
     else if (num == 1){
          LED_Digit one = new LED_Digit();
          one.drawLED_Digit(0, 0, 0, 4, 0, 0, 0, 0, 0);}
     else if (num == 2) {
          LED_Digit two = new LED_Digit();
          two.drawLED_Digit(2, 0, 0, 1, 2, 0, 1, 0, 2); }
     else if (num == 3) {
          LED_Digit three = new LED_Digit();
          three.drawLED_Digit(2, 0, 0, 1, 2, 0, 0, 1, 2);}
     else if (num == 4) {
          LED_Digit four = new LED_Digit();
          four.drawLED_Digit(0, 1, 0, 0, 2, 0, 0, 1, 0);}
     else if (num == 5) {
          LED_Digit five = new LED_Digit();
          five.drawLED_Digit(2, 0, 1, 0, 2, 0, 0, 1, 2);}
     else if (num == 6) {
          LED_Digit six = new LED_Digit();
          six.drawLED_Digit(2, 0, 1, 0, 2, 1, 0, 0, 2); }
     else if (num == 7) {
          LED_Digit seven = new LED_Digit();
          seven.drawLED_Digit(2, 0, 0, 1, 0, 0, 0, 1, 0);}
     else if (num == 8) {
          LED_Digit eight = new LED_Digit();
          eight.drawLED_Digit(2, 1, 0, 0, 2, 1, 0, 0, 2); }
     else if (num == 9) {
          LED_Digit nine = new LED_Digit();
          nine.drawLED_Digit(2, 1, 0, 0, 2, 0, 0, 1, 2); }
     else {
          System.err.println("Not a valid digit"); }
     public String toString() { return ; }
}