Need HELP with java applet--- throws NullPointerException

[mjunit]

Hello im a beginner in java programming.

When running this program i always receive an error saying "nullpointerexception String is null" any ideas how to fix it?

The error is on line 68 (look for an arrow)


import java.applet.Applet;
import java.awt.*;
public class Numbers extends Applet {
TextField tf_x1, tf_x2, tf_y1, tf_y2;
int drawX1, drawY1, drawX2, drawY2;
public String x1_str, y1_str, x2_str, y2_str, slope_str, distanceOutput, slopeOutput, eqnofLineOutput, midpointOfLineOutput, yInterceptOutput, xInterceptOutput;
double x1, x2, y1, y2, slope, FinalYIntercept, midPointX, midPointY;
public String str_default = "0";
public void init() {
setSize(1000,600);
Label label_1 = new Label("(x1, y1)");
Label label_2 = new Label("(x2, y2)");
tf_x1 = new TextField(str_default, 4);
tf_y1 = new TextField(str_default, 4);
tf_x2 = new TextField(str_default, 4);
tf_y2 = new TextField(str_default, 4);
add(label_1);
add(tf_x1);
add(tf_y1);
add(label_2);
add(tf_x2);
add(tf_y2);
add(new Button("Draw and Calculate"));
add(new Button("Clear"));
}
private void getNewInput(){
x1_str = tf_x1.getText();
drawX1 = Integer.parseInt(x1_str);
y1_str = tf_y1.getText();
drawY1 = Integer.parseInt(y1_str);
x2_str = tf_x2.getText();
drawX2 = Integer.parseInt(x2_str);
y2_str = tf_y2.getText();
drawY2 = Integer.parseInt(y2_str);
x1_str = tf_x1.getText();
x1 = Double.parseDouble(x1_str);
y1_str = tf_y1.getText();
y1 = Double.parseDouble(y1_str);
x2_str = tf_x2.getText();
x2 = Double.parseDouble(x2_str);
y2_str = tf_y2.getText();
y2 = Double.parseDouble(y2_str);
}
public double FindingSlope(){
return (y2 - y1) / (x2 - x1);
}

private void resetInputFields(){
tf_x1.setText(str_default);
tf_y1.setText(str_default);
tf_x2.setText(str_default);
tf_y2.setText(str_default);
}
private void resetCoordinates(){
drawX1 = 0; drawY1 = 0; drawX2 = 0; drawY2 = 0;
x1 = 0; y1 = 0; x2 = 0; y2 = 0;
}
public void paint(Graphics g){

g.drawLine(350, 50, 350, 500);
g.drawLine(600, 250, 100, 250);
g.drawString("Slope:", 675, 175);
g.drawString("Distance:", 675, 200);
g.drawString("Midpoint Of Line:", 675, 225);
g.drawString("Equation Of Line:", 675, 250);
g.drawString("X Intercept:", 675, 275);

g.drawString(slopeOutput, 750, 175); <=======HERE

g.setColor(Color.blue);
g.drawLine(drawX1 + 350, -drawY1 + 250, drawX2 + 350, -drawY2 + 250);
}


public boolean action(Event ev, Object arg){
if(arg.equals("Draw and Calculate")){
getNewInput();
slope = FindingSlope();
if (slope >= 0 || slope == 0 || slope <= 0){
slopeOutput = (Double.toString (slope));
}
else{
slopeOutput = ("Undefined");
}



repaint();
}
else if(arg.equals("Clear")){
resetInputFields();
resetCoordinates();
repaint();
}
return true;
}
}

[hardwired]

This probably had some parameters in an html file that initialized one or more of the fields.
Try giving a value to the variable slopeOutput:

    public String x1_str, y1_str, x2_str, y2_str,
                  slope_str, distanceOutput, [b][COLOR="Red"]slopeOutput = "1.0"[/COLOR][/b],
                  eqnofLineOutput, midpointOfLineOutput,
                  yInterceptOutput, xInterceptOutput;