Cannot invoke "String.equals(Object)" because "this.subject" is null

This error occurs when you try to call the equals() method on a null string object. The equals() method is used to compare two strings for equality. However, if one of the strings is null, then trying to call the equals() method will result in a NullPointerException.

To fix this error, you need to check if the string object is null before calling the equals() method. You can use the following code to perform the null check:

if (this.subject != null && this.subject.equals(other.subject)) {
// perform some action
}

This code checks if the subject string object is not null before calling the equals() method. If the subject object is null, then the equals() method will not be called and the NullPointerException will be avoided.