3.1.6 Debugging Classes

hint

If needed, here are some Tips for Accessing Replit in your Sophia course.

1. Implementing toString() to Check an Object's State

In previous lessons about debugging arrays and collections, you have seen how the toString() methods provided in Java's standard libraries can facilitate tracking down and fixing problems. Using the toString() method with objects can be helpful in debugging, by allowing us to see the values currently stored in the object (its state).

All Java objects have a default toString() method, but this default version may not be very helpful with classes that you create. Let's take a look at the UserAccount class that you have worked with previously.

Here is the code (which needs to be in a file in Replit called UserAccount.java):

import java.time.LocalDate;
public class UserAccount {
  private String userName;
  private String password;
  private LocalDate dateJoined;
  private boolean activeUser;
  
  public UserAccount(String userName, String password) {
    this.userName = userName;
    this.password = password;
    this.dateJoined = LocalDate.now();
    this.activeUser = true;
  }
 
  // Allow read-only access to user name
  public String getUserName() {
    return userName;
  }
 
  // Allow read-only access to date joined
  public LocalDate getDateJoined() {
    return dateJoined;
  }
 
  // Allow activeUser to be read & set (can change)
  public boolean isActiveUser() {
    return activeUser;
  }  
 
  public void setActiveUser(boolean activeUser) {
    this.activeUser = activeUser;
  }
}

Here is a simple driver class that makes use of the UserAccount class. Type this code into a file named EmployeeExample.java:

public class UserAccountExample {
  public static void main(String[] args) {
    UserAccount account = new UserAccount("Sophia2", "TestTest123");
    System.out.println("Result from calling account.toString():");
    System.out.println(account.toString());    
  }
}

Compile the UserAccount class and then run the UserAccount example program. The results should look like this:

~/.../main/java$ javac UserAccount.java 
~/.../main/java$ java UserAccountExample.java 
Result from calling account.toString(): 
UserAccount@b3d7190 
~/.../main/java$ 

The exact hexadecimal (base 16) number in the last line of the output will vary from computer to computer, but that doesn't matter because it doesn't tell a human reader anything useful.

Here is the code for a toString() method to be added to the UserAccount class. This code produces a String with the names of the attributes and their current values:

public String toString() {
  String state = "UserName: " + userName + "\n";
  state += "password: " + password + "\n";
  state += "dateJoined: " + dateJoined + "\n";
  state += "activeUser: " + activeUser + "\n";
  return state;
} 

After adding this code to the UserAccount.java file, the code will need to be recompiled:

javac UserAccount.java

The results of running the UserAccountExample program with this version of the UserAccount class produces this output:

~/.../main/java$ javac UserAccount.java 
~/.../main/java$ java UserAccountExample.java 
Result from calling account.toString(): 
UserName: Sophia2 
password: TestTest123 
dateJoined: 2024-04-29 
activeUser: true 

~/.../main/java$ 

This version of toString() provides a much more useful representation of the data in the object. The format of the date (year - month - day) may be a bit unexpected, but it is the default format. You can use another Java format class to put the date into a more familiar format. First, the code (in UserAccount.java) needs to add an import near the top of the file (with the other import for java.time.LocalDate):

import java.time.format.DateTimeFormatter;

Then, in the body of the toString() method, add this statement to create the desired format:

DateTimeFormatter dateFormat = DateTimeFormatter.ofPattern("MM/dd/YYYY");

The method used to create a DateTimeFormatter is a bit different from how the constructor for DecimalFormat is used, but as you can hopefully make out, the code above formats the date as month/day/year. Calling dateFormat's format() method outputs the date in the specified format. The toString() method should now look like this:

public String toString() {
  DateTimeFormatter dateFormat = DateTimeFormatter.ofPattern("MM/dd/YYYY");
  String state = "UserName: " + userName + "\n";
  state += "password: " + password + "\n";
  state += "dateJoined: " + dateFormat.format(dateJoined) + "\n";
  state += "activeUser: " + activeUser + "\n";
  return state;
}

The output from the program should now look like this:

~/.../main/java$ javac UserAccount.java 
~/.../main/java$ java UserAccountExample.java 
Result from calling account.toString(): 
UserName: Sophia2 
password: TestTest123 
dateJoined: 04/29/2024 
activeUser: true 

~/.../main/java$  

Let's now look at an example of how this method might be useful. The constructor for the UserAccount class includes a line that sets the activeUser attribute to true, but let's imagine the programmer has made an error and the constructor looks like this instead:

public UserAccount(String userName, String password) {
  this.userName = userName;
  this.password = password;
  this.dateJoined = LocalDate.now();
}

Since activeUser is a boolean, the default value is false, if it is not initialized to true. If the code in the application's main() were like this, the result would not be what you expected for a newly created account:

public class UserAccountExample {
  public static void main(String[] args) {
    UserAccount account = new UserAccount("Sophia2", "TestTest123");
    if(account.isActiveUser()) {
      System.out.println(account.getUserName() + " is active.");
    }
    else {
      System.out.println(account.getUserName() + " is not active.");
    }
  }
}

The results from running this code look like this:

~/.../main/java$ javac UserAccount.java 
~/.../main/java$ java UserAccountExample.java 
Sophia2 is active. 
~/.../main/java$ 

This is not what is expected, so adding a call to the UserAccount object's toString() method can help us figure out the cause of the trouble:

public class UserAccountExample {
  public static void main(String[] args) {
    UserAccount account = new UserAccount("Sophia2", "TestTest123");
    if(account.isActiveUser()) {
      System.out.println(account.getUserName() + " is active.");
    }
    else {
      System.out.println(account.getUserName() + " is not active.");
    }
    // Added for debugging
    System.out.println("\nDebugging info:\n" + account.toString());
  }
}

Running the program now produces the following output:

~/.../main/java$ java UserAccountExample.java 
Sophia2 is active. 

Debugging info: 
UserName: Sophia2 
password: TestTest123 
dateJoined: 04/29/2024 
activeUser: true 

~/.../main/java$ 

The data displayed by the toString() method shows that the activeUser is set to false (the default value for a boolean variable in Java) since it is not assigned the value true by the constructor.

Putting back the line:

this.activeUser = true;

This will get the program working as intended.

Hexadecimal
A value expressed in a base 16 number system (rather than base 10).