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Thread: Nullpointerexception

  1. #1
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    Default Nullpointerexception

    I can't figure out how to fix this null pointer exception and i've been going at it for a while now. Any help would be appreciated. I'm still a noob at this so go easy on some of the ways I did stuff, however I would love any tips that you guys have to offer.

    class 1
    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;
     
    public class VGrepGUI extends JFrame{
     
    	static JTextField inputTextField;
    	static JTextField searchField;
    	static JTextField currentFileField;
    	static JTextField outputSectionField;
    	JTextArea textArea;
    	JScrollPane scrollPane;
    	static LayoutManager MyLayout;
    	VGrep functions = new VGrep();
     
    	public static void main(String[] args) {
    		VGrepGUI grepTool = new VGrepGUI();
     
    		JFrame frame = new JFrame();
    		JPanel mainPanel = new JPanel();
    		//mainPanel.setLayout();
     
    		frame.setTitle("VGrep Tool");//set frame properties
    		frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
    		frame.setLocationRelativeTo(null);					
    		frame.setVisible(true);				
    		mainPanel.add(grepTool.inputFileSection());//call on createGUI to retrieve jpanel to create GUI and add it to the JFrame
    		mainPanel.add(grepTool.searchCriteriaSection());
    		mainPanel.add(grepTool.currentFileDisplay());
    		mainPanel.add(grepTool.scrollWindow());
    		mainPanel.add(grepTool.outputButtonSection());
    		frame.add(mainPanel);
    		frame.pack();
    		frame.setSize(750, 750);
     
     
    	}
     
    	public JPanel inputFileSection(){
     
    		JPanel inputPanel = new JPanel();
    		JLabel inputFileLabel = new JLabel("Input File:");
    		inputTextField = new JTextField("Select a file to search...", 35);
    		inputTextField.addActionListener(new inputTextFieldListener());
    		inputTextField.addFocusListener(new inputTextFieldFocusListener());
     
    		JButton browseForInputFile = new JButton("Browse");
    		browseForInputFile.addActionListener(new browseInputListener());
     
    		inputPanel.add(inputFileLabel);
    		inputPanel.add(inputTextField);
    		inputPanel.add(browseForInputFile);
     
    		return inputPanel;
    	}// end inputFileSection method
     
    	public JPanel searchCriteriaSection(){
    		JPanel searchPanel = new JPanel();
    		JLabel searchLabel = new JLabel("Search For:");
    		searchField = new JTextField("Enter search criteria here...", 35);
    		searchField.addActionListener(new searchFieldListener());
    		searchField.addFocusListener(new searchFieldFocusListener());
    		JButton searchButton = new JButton("Start Search");
    		searchButton.addActionListener(new searchButtonListener());
     
    		searchPanel.add(searchLabel);
    		searchPanel.add(searchField);
    		searchPanel.add(searchButton);
    		return searchPanel;
    	}
     
    	public JPanel currentFileDisplay(){
    		JPanel fileDisplayPanel = new JPanel();
    		JLabel currentFileLabel = new JLabel("Searching through file:");
    		currentFileField = new JTextField(40);
    		currentFileField.setEditable(false);
     
    		fileDisplayPanel.add(currentFileLabel);
    		fileDisplayPanel.add(currentFileField);
    		return fileDisplayPanel;
    	}
    	/*public JTextArea getArea(){
    		textArea = new JTextArea(600,550);
    		return textArea;
    	}*/
     
    	public JPanel scrollWindow(){
    		JPanel scrollPanel = new JPanel();
     
     
    		textArea = new JTextArea(30,60);
     
    		scrollPane = new JScrollPane(textArea, scrollPane.VERTICAL_SCROLLBAR_ALWAYS,
    				scrollPane.HORIZONTAL_SCROLLBAR_NEVER);		
     
    		scrollPanel.add(scrollPane);
    		return scrollPanel;
     
    	}
    	public JPanel outputButtonSection(){
    		JPanel outputPanel = new JPanel();
    		JLabel outputLabel = new JLabel("Output File:");
    		outputSectionField = new JTextField(35);
    		JButton browse = new JButton("Browse");
    		browse.addActionListener(new browseOutputListener());
    		JButton save = new JButton("Save");
    		save.addActionListener(new saveButtonListener());
     
    		outputPanel.add(outputLabel);
    		outputPanel.add(outputSectionField);
    		outputPanel.add(browse);
    		outputPanel.add(save);	
     
    		return outputPanel;
    	}//end outputButtonSection
     
    	//action listener classes
    	class inputTextFieldListener implements ActionListener{
    		public void actionPerformed(ActionEvent e){
     
    		}
     
    	}//end inputTextFieldListener
    	class browseInputListener implements ActionListener{
    		public void actionPerformed(ActionEvent e){
    			functions.browseFiles();
    		}
    	}//end browseInputListener
    	class searchFieldListener implements ActionListener{
    		public void actionPerformed(ActionEvent e){
     
    		}
    	}//end searchFieldListener
    	class searchButtonListener implements ActionListener{
    		public void actionPerformed(ActionEvent e){
     
    			functions.searchThroughFile();
    		}
    	}//end searchButtonListener
    	class browseOutputListener implements ActionListener{
    		public void actionPerformed(ActionEvent e){
    			functions.browseSaveFile();
    		}
    	}//end browseOutputListener
    	class saveButtonListener implements ActionListener{
    		public void actionPerformed(ActionEvent e){
     
    		}
    	}//end saveButtonListener
    	//focus listeners
    	class inputTextFieldFocusListener implements FocusListener{
    		public void focusLost(FocusEvent a){
    			String str2 = inputTextField.getText();
    			if(str2.equals("")){
    				inputTextField.setText("Select a file to search...");
    			}
    		}
    		public void focusGained(FocusEvent b){
    			String str = inputTextField.getText();
    			if(str.equals("Select a file to search...")){
    				inputTextField.setText("");
    			}
    		}
    	}//end focuslistener for inputtextfield
    	class searchFieldFocusListener implements FocusListener{
    		public void focusLost(FocusEvent a){
    			String str2 = searchField.getText();
    			if(str2.equals("")){
    				searchField.setText("Enter search criteria here...");
    			}
    		}
    		public void focusGained(FocusEvent b){
    			String str = searchField.getText();
    			if(str.equals("Enter search criteria here...")){
    				searchField.setText("");
    			}
    		}
    	}
    }//end VGrepGUI class

    class 2:
    import javax.swing.*;
    import java.awt.*;
    import java.io.*;
    import java.util.*;
     
    public class VGrep {
    	static boolean searching;
    	static VGrepGUI gui = new VGrepGUI();
    	JFileChooser fc;
    	static File inputFile;
    	StringBuffer sbuffer = new StringBuffer();
    	BufferedReader reader = null;
    	private final static String newline = "\n";
     
    	public static void main(String[] args){
    		VGrep grepFunctions = new VGrep();	
     
    	}
     
    	public void browseFiles(){
    		fc = new JFileChooser();
    		if(fc.showOpenDialog(null) == JFileChooser.APPROVE_OPTION) {
    			inputFile = fc.getSelectedFile();
    			gui.inputTextField.setText("" + inputFile.getAbsolutePath());
    		}
    	}//end browse files
    	public void searchingFileBox(){
    		while(searching == true){
    			gui.currentFileField.setText("" + inputFile.getName());
    		}
    	}//end searchingFileBox
    	public String getSearchCriteria(){
    		String str = gui.searchField.getText();
    		return str;
    	}
    	public void searchThroughFile(){
    		if(inputFile != null){
    		try{
    			//open scanner to read file and eventually write to textarea
     
    			Scanner in = new Scanner(inputFile);
    			while(in.hasNextLine()){
    				String line = in.nextLine();
    				processString(line);
    			}
    			in.close();
    		}catch(Exception e){
    			System.out.println(e);
    		}
    		}else browseFiles();
     
    	}//end searchThroughFile
     
    	public void processString(String str1){
    		String str = getSearchCriteria();
    		if(str1.contains(str)){
    			printToTextArea(str1);
    		}
    	}//end processString
    	public void printToTextArea(String str){
    		gui.textArea.append("" + str + newline);
    	}//end printToTextArea
     
    	public void browseSaveFile(){
    		if(fc.showSaveDialog(null) == JFileChooser.APPROVE_OPTION);
    	}//end browseSaveFile
    	public void saveFile(){
     
    	}
    }


  2. #2
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    Default Re: Nullpointerexception

    By the way, I forgot to mention that the Nullpointerexception occurs after I select a file via browse button, type in what i'm searching for, and hit start search button.

  3. #3
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    Default Re: Nullpointerexception

    So this is where you are getting the problem?
    JButton browse = new JButton("Browse");
            browse.addActionListener(new browseOutputListener());
    And since browseOutputListener calls:
    class browseOutputListener implements ActionListener{
            public void actionPerformed(ActionEvent e){
                functions.browseSaveFile();
            }
        }//end browseOutputListener
    which calls:
    public void browseSaveFile(){
            if(fc.showSaveDialog(null) == JFileChooser.APPROVE_OPTION);
        }//end browseSaveFile
    Which is a broken method effectively. You have an if statement that does nothing. I would guess that is part of your problem.
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  4. #4
    Super Moderator copeg's Avatar
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    Default Re: Nullpointerexception

    Quote Originally Posted by kbwalker87 View Post
    By the way, I forgot to mention that the Nullpointerexception occurs after I select a file via browse button, type in what i'm searching for, and hit start search button.
    Please post the full NullPointerException error message. An exception message will point you to the exact point that the exception is thrown, which makes a world of difference in determining what is causing the error.

  5. #5
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    Default Re: Nullpointerexception

    wow, i guess I have overlooked that. I can't thank you enough. I'll let you know how it works out.

  6. #6
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    Default Re: Nullpointerexception

    Ok, that actually did not fix it, and I forgot that I haven't made it to that point in the program which is why I left it like that. And thanks copeg, but unfortunately it doesn't tell me where this is going on at which I thought was really strange. It shows up in the console window at the bottom and just says "java.lang.NullPointerException".

  7. #7
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    Default Re: Nullpointerexception

    If anyone is still following this thread at all, the problem is occurring in the "printToTextArea" method. I guess it has something to do with my reference to the JTextArea?

  8. #8
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    Default Re: Nullpointerexception

    Take a look at this Common Java Mistakes - Forgetting to initialize a variable.

    Most NullPointExceptions happen because of this reason (I'd say maybe ~80% of the time, the rest is from output that is suppose to/can be null and the code doesn't take that into consideration).

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