Hackerrank questions - Error and Exception class both are subclass to Throwable - Interfaces cannot have constructors and instance variables unlike abstract classes (but they can have default/static methods depending on the Java version) - Multiple threads running non-thread safe for incrementing loop code, results will always be different. Coding: - I returned false for one class method - Another class method I implemented a running sum hash code - Another class I just had to pass in the input into another instance field method - Lastly, created a field to hold the input when a specific method was called, then another method wanted me to simply evaluate everything I stored in that ArrayList.