How do I create a Junit test case using Java code that testlogging functionality to the Log.txt

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Question:

How do I create a Junit test case using Java code that testlogging functionality to the Log.txt file? Please explain the testcase implemented.

Here is the code used:

//this method inputs username and password inlogin application
private voidLogsubmitActionPerformed(java.awt.event.ActionEvent evt){

try {
String password = jPassword.getText();
String username = jtxtUsername.getText();

File texting = new File("Log.txt");
//open file in append mode
BufferedWriter writer;
writer = new BufferedWriter(new FileWriter(texting,true));
//date and time
Date currentTime =Calendar.getInstance().getTime();
boolean valid = false;
//used a hard-coded username and password
if(password.equals("password")&&(username.equals("anna")))
{
jtxtUsername.setText(null);
jPassword.setText(null);

JOptionPane.showMessageDialog(null, "success");
writer.write(username + currentTime);
writer.write("success");
valid = true;

}
else if(!valid)

JOptionPane.showMessageDialog(null, "invalid password");
writer.write(username + currentTime);
writer.write("failed");
writer.close();

} catch (IOException ex){
Logger.getLogger(LoginApp.class.getName()).log(Level.SEVERE, null,ex);
}

}