Copy files in java gives me out of memory error

Hi,
I'm trying to create a class which only jobs is to copy any text files
before its get opened by text editor. I build a constructor that will
read from file then store its content in textContent variable. From
there, copy method would read from textContent and write its content in
a new file. But instead of doing its job, I get "Exception in thread
"main" java.lang.OutOfMemoryError: Java heap space
    at java.util.Arrays.copyOf(Arrays.java:2882)
    at
java.lang.AbstractStringBuilder.expandCapacity(AbstractStringBuilder.java:100)
    at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:390)
    at java.lang.StringBuilder.append(StringBuilder.java:119)
    at v1.CreateBackup.<init>(CreateBackup.java:16)
    at v1.EditorSWT.openFileDialog(EditorSWT.java:150)
    at v1.EditorSWT.access$0(EditorSWT.java:141)
    at v1.EditorSWT$1.widgetSelected(EditorSWT.java:107)
    at org.eclipse.swt.widgets.TypedListener.handleEvent(Unknown Source)
    at org.eclipse.swt.widgets.EventTable.sendEvent(Unknown Source)
    at org.eclipse.swt.widgets.Widget.sendEvent(Unknown Source)
    at org.eclipse.swt.widgets.Display.runDeferredEvents(Unknown Source)
    at org.eclipse.swt.widgets.Display.readAndDispatch(Unknown Source)
    at v1.EditorSWT.main(EditorSWT.java:34)
"
Can someone please tell me what I'm doing wrong? I'm not very
experienced in Java

**********************code**************************
package v1;

import java.io.*;

public class CreateBackup {
    private String textContent;

    public CreateBackup(String fromFile) {
        try {
            File input = new File(fromFile);
            BufferedReader br = new BufferedReader(new FileReader(input));
            StringBuilder sb = new StringBuilder();
            String line = br.readLine();

            while (line != null) {
                sb.append(line + "\n");
            }
            br.close();
            textContent = sb.toString();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public void copy(String toFile) {
        try {
            File output = new File(toFile);
            PrintWriter out = new PrintWriter(new BufferedWriter(new
FileWriter(output)));
            out.println(textContent);
            out.flush();
            out.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

*******this is how I call CreateBackup in main class*******************
private void openFileDialog() {
        FileDialog dialog = new FileDialog(shell, SWT.OPEN);
        dialog.setText(getResourceString("dialog.File.Open.text"));
        dialog.setFilterPath(System.getProperty("user.dir"));
        String[] filterExt = {"*.txt", "*.rtf", "*.doc", "*.*"};
        dialog.setFilterExtensions(filterExt);
        String fileName = dialog.open();
        // TODO Create backup file first before open the file
        String outFile = fileName + extension;
        CreateBackup backup = new CreateBackup(fileName);
        backup.copy(outFile);
        // Open the file
        try {
            File selectedFile = new File(fileName);
            if (selectedFile.isFile()) {
                try {
                    BufferedReader br = new BufferedReader(new FileReader(selectedFile));
                    StringBuilder sb = new StringBuilder();
                    String line = null;
                    while((line = br.readLine()) != null) {
                        sb.append(line + "\n");
                    }
                    br.close();
                    text.setText(sb.toString());
                } catch (FileNotFoundException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        } catch (NullPointerException e) {
            e.printStackTrace();
        }
    }