I'm making a vending machine .. and everything's pretty much together, I just have a few problems that I don't have the faintest clue how to fix

Firstly, when I choose a snack, the price of the thing comes up fine, no bother. But if I start to pay, then it doesn't count down how much left I have to pay, although the program itself still operates (quite badly, it doesn't calculate how much has been put into the machine and how much change to give etc.. )

Also when I get the box telling me how much change I have .. the price comes in -£0.00 instead of £0.00 .. I have no idea why this is either.

and finally .. I want the money buttons disabled when I first run the program, then when I make a selection I want them enabled, and the selection buttons disabled.. as far as I can see, the code should do that for me, but it just won't compile.

Any help would be much appreciated, thanks ... here is my code:

import static javax.swing.JOptionPane.*;
import java.awt.*;
import java.awt.event.*;
import java.io.File;
import javax.swing.*;
import java.text.DecimalFormat;

public class VendingMachine extends JFrame {
	
	JButton oneP, twoP, fiveP, tenP, twentyP, fiftyP, hundredP, twohundredP;
	JButton gingbisc, toffdel, jumcrisp, cashnut, omgchoc;
	JButton resetButton;
	
	JLabel messLabel = new JLabel("Amount to pay: ");
	JTextField message = new JTextField(4);
	double amount = 0;
	DecimalFormat pounds = new DecimalFormat("£0.00");

	
	public static void main(String[] args) {
		VendingMachine c = new VendingMachine();
		c.setTitle("Vending Machine");
		c.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		c.setSize(800, 600);
		c.setVisible(true);
	}
	
	
	public VendingMachine() {
		
		message.setText(pounds.format(amount));
		//Configuring the left side button icons
		setLayout(new BorderLayout());
		oneP = new JButton("1p");
		oneP.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				insertMoney(0.01);
			}});
	
		twoP = new JButton("2p");
		twoP.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				insertMoney(0.02);
			}});
		
		fiveP = new JButton("5p");
		fiveP.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				insertMoney(0.05);
			}});		

		tenP = new JButton("10p");
		tenP.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				insertMoney(0.1);
			}});

		twentyP = new JButton("20p");
		twentyP.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				insertMoney(0.2);
			}});
		
		fiftyP = new JButton("50p");
		fiftyP.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				insertMoney(0.5);
			}});
		
		hundredP = new JButton("£1");
		hundredP.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				insertMoney(1);
			}});
		
		twohundredP = new JButton("£2");
		twohundredP.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				insertMoney(2);
			}});
				
		// Making the icons
		ClassLoader cldr = this.getClass().getClassLoader();
		
	    gingbisc = new JButton("Ginger Biscuits", new ImageIcon(cldr.getResource("ginger.jpg")));
		toffdel  = new JButton("Toffee Delight", new ImageIcon(cldr.getResource("toffee.jpg")));
		jumcrisp = new JButton("Jumbo Crisps", new ImageIcon(cldr.getResource("crisps.jpg")));
		cashnut  = new JButton("Cashew Nuts",  new ImageIcon(cldr.getResource("cashews.jpg")));
		omgchoc  = new JButton("Omega Chocolate", new ImageIcon(cldr.getResource("chocolate.jpg")));
		
	    // Configuring the right side button icons
		gingbisc.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent arg0) {
	        	debitAmount(0.5);				
			}});
		toffdel.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent arg1) {
	        	debitAmount(0.7);				
			}});
		jumcrisp.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent arg2) {
	        	debitAmount(1.0);			
			}});
		cashnut.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent arg3) {
	        	debitAmount(1.2);			
			}});
		omgchoc.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent arg4) {
	        	debitAmount(1.5);			
			}});
        
	    // Left side panel components
		JPanel leftSide = new JPanel();
		leftSide.setLayout(new GridLayout(8, 1));
		leftSide.add(oneP);
		leftSide.add(twoP);
		leftSide.add(fiveP);
		leftSide.add(tenP);
		leftSide.add(twentyP);
		leftSide.add(fiftyP);
		leftSide.add(hundredP);
		leftSide.add(twohundredP);
		add("West", leftSide);

		
		// Right side panel components
		JPanel rightSide = new JPanel();
		rightSide.setLayout(new GridLayout(5, 1));
		rightSide.add(gingbisc);
		rightSide.add(toffdel);
		rightSide.add(jumcrisp);
		rightSide.add(cashnut);
		rightSide.add(omgchoc);
		add("East", rightSide);

		JPanel middle = new JPanel();
		middle.setLayout(new FlowLayout());
		middle.add(messLabel);
		message.setEditable(false);
		middle.add(message);
		add("Center", middle);
	}

	public void debitAmount(double pence) {
		amount = amount + pence;
		message.setText(pounds.format(amount));
	}
	public void insertMoney(double pence) {
		amount = amount - pence;
		message.setText(pounds.format(amount));
		
        if (amount > 0) message.setText(pounds.format(amount / 100.0));
        else {
            message.setText("");
            if (amount < 0) {
                showMessageDialog(this, "Your change is "
                    + pounds.format(amount),
                     "Change", JOptionPane.INFORMATION_MESSAGE);

            } else {
                showMessageDialog(this, "Thank you",
                    "Exact amount", JOptionPane.INFORMATION_MESSAGE);
			}
			 // Enabling thhe left buttons, disabling the right buttons
    void leftSide() {
		oneP.setEnabled(true);
		twoP.setEnabled(true);
		fiveP.setEnabled(true);
        tenP.setEnabled(true);
        twentyP.setEnabled(true);
        fiftyP.setEnabled(true);
        hundredP.setEnabled(true);
        twohundredP.setEnabled(true);
        gingbisc.setEnabled(false);
        toffdel.setEnabled(false);
        jumcrisp.setEnabled(false);
        cashnut.setEnabled(false);
        omgchoc.setEnabled(false);
    }

    // Enabling the right buttons, disabling the left buttons
    void rightSide() {
		oneP.setEnabled(false);
		twoP.setEnabled(false);
		fiveP.setEnabled(false);
        tenP.setEnabled(false);
        twentyP.setEnabled(false);
        fiftyP.setEnabled(false);
        hundredP.setEnabled(false);
        twohundredP.setEnabled(false);
        gingbisc.setEnabled(true);
        toffdel.setEnabled(true);
        jumcrisp.setEnabled(true);
        cashnut.setEnabled(true);
        omgchoc.setEnabled(true);
    }
		}
		

   }  
 }

You have leftSide() and rightSide() methods declared within the body of insertMoney(). Check your brackets.

bloody brackets .. anytime I try and change or add brackets, the whole program falls apart

and I can't shake the illegal start of expression error on 'void'

In all likelihood it's still due to misplaced brackets or you still have functions defined within functions. This is why proper indentation is important. It makes such things much more apparent.

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