Java initialization question

问题

JPanel p2 = new JPanel(new GridLayout(6,1));
ButtonGroup tubtype = new ButtonGroup();
JRadioButton roundrButton = new JRadioButton("Round", true);
tubtype.add(roundrButton);
JRadioButton ovalrButton = new JRadioButton("Oval", false);
tubtype.add(ovalrButton);
calcButton = new JButton("Calculate Volume");
exitButton = new JButton("Exit");
hlength = new JTextField(5);
hwidth = new JTextField(5);
hdepth = new JTextField(5);
hvolume = new JTextField(5);
lengthLabel = new JLabel("Enter the tub's length (ft):");
widthLabel = new JLabel("Enter the tub's width (ft):");
depthLabel = new JLabel("Enter the tub's depth (ft):");
volumeLabel = new JLabel("The tub's volume (ft^3):");
p2.add(roundrButton);
p2.add(ovalrButton);
p2.add(lengthLabel);
p2.add(hlength);
p2.add(widthLabel);
p2.add(hwidth);
p2.add(depthLabel);
p2.add(hdepth);
p2.add(volumeLabel);
p2.add(hvolume);
p2.add(calcButton);
p2.add(exitButton);
tab.addTab( "Hot Tubs", null, p2, " Panel #1" );

calcButtonHandler2 ihandler =new calcButtonHandler2();
calcButton.addActionListener(ihandler);
exitButtonHandler ghandler =new exitButtonHandler();
exitButton.addActionListener(ghandler);
FocusHandler hhandler =new FocusHandler();
hlength.addFocusListener(hhandler);
hwidth.addFocusListener(hhandler);
hdepth.addFocusListener(hhandler);
hvolume.addFocusListener(hhandler);

// add JTabbedPane to container
getContentPane().add( tab );
setSize( 550, 500 );
setVisible( true );
} 
public class calcButtonHandler implements ActionListener {

public void actionPerformed(ActionEvent e) {
DecimalFormat num =new DecimalFormat(",###.##");
double sLength, sWidth, sdepth, Total;

sLength = Double.valueOf(plength.getText());

sWidth =Double.valueOf(pwidth.getText());

sdepth =Double.valueOf(pdepth.getText());

if(e.getSource() == pcalcButton) {
Total = sLength * sWidth * sdepth;
pvolume.setText(num.format(Total));
try{
String value=pvolume.getText();
File file = new File("output.txt");
FileWriter fstream = new FileWriter(file,true);
BufferedWriter out = new BufferedWriter(fstream);
out.write("Length= "+sLength+", Width= "+sWidth+", Depth= "+sdepth+" so the volume of Swimming  Pool is "+value);
out.newLine();
out.close();
}
catch(Exception ex){}
}
}
}

public class calcButtonHandler2 implements ActionListener {

public void actionPerformed(ActionEvent g) {
DecimalFormat num =new DecimalFormat(",###.##");
double cLength, cWidth, cdepth, Total;

cLength = Double.valueOf(hlength.getText());

cWidth = Double.valueOf(hwidth.getText());

cdepth = Double.valueOf(hdepth.getText());

try
{
AbstractButton roundrButton;
if(roundrButton.isSelected())
{

Total = Math.PI * Math.pow(cLength / 2.0, 2) * cdepth;
}
else 
{
Total = Math.PI * Math.pow(cLength * cWidth, 2) * cdepth;
}
hvolume.setText(""+num.format(Total));
}

catch(Exception ex){}
}
}
}



class exitButtonHandler implements ActionListener {
    public void actionPerformed(ActionEvent g){
        System.exit(0);
    }
}
class FocusHandler implements FocusListener {
    public void focusGained(FocusEvent e) {
    }
    public void focusLost(FocusEvent e) {
    }


public static void main( String args[] )
{
    Final tabs = new Final();
    tabs.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
}

I am getting an error that says my roundrButton may not have been initialized. Please help.

回答1:

The problem is these lines here:

try
{
    AbstractButton roundrButton;
    if(roundrButton.isSelected())
    {

You're declaring a variable called roundrButton, and then attempting to call .isSelected() on it. You never assign anything to roundButton, though, so it is going to be null.

It seems like you might want to be referring to the roundrButton that you declared earlier. Have you considered making it a field of your class?

回答2:

The error happens here:

try {
    AbstractButton roundrButton;
    if (roundrButton.isSelected()) {

You declared a variable called roundrButton but did not assign any object to it.

Then you called a method on, well, who knows? There is no object on which to call the method, because the variable roundrButton never received a value.

回答3:

I guess your problem is in method actionPerformed of class calcButtonHandler2 :

    try { 
AbstractButton roundrButton;
if(roundrButton.isSelected())

roundrButton is indeed not initialized and it will lead to a NullPointerException at runtime.

来源：https://stackoverflow.com/questions/7130300/java-initialization-question