Java, SQL, and JUnit Fundamentals

Denormalization is a database optimization technique where redundant data is intentionally added to one or more tables to improve read performance. This trades write performance and storage space for faster query execution.

Data Definition Language (DDL) consists of SQL commands like CREATE, ALTER, and DROP, which are used to define or modify the structure of database objects such as tables and schemas.

DML stands for Data Manipulation Language, which includes commands like INSERT, UPDATE, and DELETE used to manage and manipulate data within existing database structures.

DDL stands for Data Definition Language, which is used to define and manage database structures like tables, indexes, and views.

The standard SQL command to add new records to a table is 'INSERT INTO'. Other options like 'insert new' or 'add record' are not valid SQL syntax.

SQL Injection is a security vulnerability where an attacker inputs malicious SQL statements into entry fields, forcing the database to execute unintended commands. Using parameterized queries is actually a defense against SQL injection, not the vulnerability itself.

The UPDATE statement can modify multiple rows at once if the WHERE clause matches multiple records, or if no WHERE clause is specified at all. It does not delete rows, which is the job of the DELETE statement.

UPDATE is a Data Manipulation Language (DML) command because it modifies existing data. CREATE, ALTER, and DROP are Data Definition Language (DDL) commands because they modify the database schema structure.

SQL stands for Structured Query Language, which is the standard programming language used to manage and manipulate relational databases.

Constraints (such as NOT NULL, UNIQUE, or CHECK) are typically defined on columns to restrict the type of data that can go into them. While some constraints can be defined at the table level (involving multiple columns), they are not applied to individual rows.

A primary key is a column or set of columns that uniquely identifies each row in a table, ensuring no duplicate records exist and providing a reliable way to reference specific entries.

A third table (junction or association table) is typically used to resolve many-to-many relationships. In a one-to-one relationship, the connection is usually made by placing a foreign key directly in one of the two related tables.

INSERT, UPDATE, and DELETE are Data Manipulation Language (DML) commands because they modify data. Data Definition Language (DDL) commands include CREATE, ALTER, and DROP, which define or modify database structures.

The DROP command is used to completely remove a table structure and all of its data from the database. DELETE and TRUNCATE remove rows from a table but keep the table structure intact.

A subquery is a nested query placed inside another SQL statement (such as SELECT, INSERT, UPDATE, or DELETE) to return data that will be used by the outer query. It is not a command specifically designed to insert records.

The COUNT() aggregate function in SQL is used to return the number of rows that match a specified criterion. SUM() adds up numeric values, while ADD() and TOTAL() are not standard SQL aggregate functions for counting rows.

In SQL, the asterisk (*) wildcard character is used in a SELECT statement to specify that all columns should be retrieved from the target table.

CRUD is an acronym that stands for Create, Read, Update, and Delete. These are the four basic functions of persistent storage in database applications.

The AS keyword is used in SQL to assign a temporary name (an alias) to a column or a table in the query results, making them easier to read or reference.

A JOIN clause in SQL is used to combine rows from two or more tables based on a related column between them, allowing you to query associated data together.

A primary key uniquely identifies each record in a database table. Therefore, it must contain unique values and cannot contain NULL values. It does not have to be an integer, nor does it have to be a foreign key.

A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables. It acts as a cross-reference between tables because it references the primary key or a unique key of another table.

INSERT, UPDATE, and DELETE are part of Data Manipulation Language (DML) because they are used to manage and manipulate data within existing schema objects. DDL is used for defining schemas, DQL for querying, and TCL for transaction control.

Scalar functions perform an operation on a single value and return a single value. For example, UPPER() converts a single string to uppercase, ABS() returns the absolute value of a single number, and CONCAT() joins strings row-by-row, unlike aggregate functions which operate on a set of rows.

Aggregate functions perform a calculation on a set of values and return a single value, typically used in conjunction with the GROUP BY clause. Examples include MAX() to find the maximum value, SUM() to add values, and COUNT() to count rows.

Referential integrity ensures that relationships between tables remain consistent. It requires that any foreign key field in a table must point to a valid, existing primary key or unique key in the referenced table, preventing orphaned records.

In SQL, NULL is a special marker used to indicate that a data value does not exist in the database. It represents missing, unknown, or inapplicable data, rather than a zero or an empty string.

The UNIQUE constraint ensures that all values in a specific column are different across all rows in the table. While this makes the values unique within that column across the table, the most precise description is that the values must be unique within that column.

A sequence is a user-defined schema-bound object that generates a sequence of numeric values according to the specification with which the sequence was created. It is commonly used to automatically generate unique primary key values.

A composite key is a combination of two or more columns in a table that can be used to uniquely identify each row in the table. When multiple columns are combined to form a primary key, it is specifically referred to as a composite primary key.

JUnit is an open-source unit testing framework for Java. It uses annotations to identify test methods and lifecycle phases, and assertions to verify expected outcomes, making all the listed statements correct.

In JUnit 4, the @Before annotation is used on a method to indicate that it should be executed before each @Test method in the class. @BeforeClass runs once before any tests in the class, while @After runs after each test.

HashMap, Hashtable, and TreeMap are all standard classes in the Java Collections Framework that implement the java.util.Map interface. ArrayMap is not a standard JDK class as it is part of Android utilities.

Garbage collection in Java is an automatic memory management process that identifies and discards objects that are no longer needed by the application, thereby freeing up memory resources. This prevents memory leaks without requiring manual de-allocation by the developer.

Test-Driven Development (TDD) is a development methodology where developers write automated test cases before writing the actual production code. The cycle involves writing a failing test, writing the minimum code to make it pass, and then refactoring the code.

Unit testing involves testing the smallest testable parts of an application, such as individual methods or classes, in complete isolation from other parts of the system. This ensures that each individual component functions correctly on its own.

JUnit is a widely-used, open-source unit testing framework specifically designed for the Java programming language. It is not intended for any programming language and is primarily for unit testing rather than integration testing.

In unit testing, mock objects simulate the behavior of complex, real dependencies (such as databases or external APIs). This allows developers to isolate the code being tested and verify its behavior independently of external factors.

Historically and by default, Gradle uses a Domain Specific Language (DSL) based on Groovy for writing build scripts (e.g., build.gradle). While Kotlin DSL is also supported now, Groovy is the traditional and widely recognized language for Gradle configurations.

In the Java Collections Framework, java.util.Map is a self-contained interface that models key-value mappings and does not extend the java.util.Collection interface. In contrast, List, Set, and Queue all extend Collection.

In Java, you can declare a class as abstract even if it contains only concrete methods (methods with implementations) and no abstract methods. This is typically done to prevent the class from being instantiated directly, forcing users to subclass it.

The default implementation of the equals() method in the base java.lang.Object class compares object references using the == operator. Therefore, unless a class overrides equals(), calling obj1.equals(obj2) is functionally identical to obj1 == obj2.

Java's 'Write Once, Run Anywhere' (WORA) capability is enabled by the Java Virtual Machine (JVM). As long as a target machine has a compatible JVM installed, it can execute compiled Java bytecode (.class files) regardless of the underlying operating system or hardware.

The standard entry point for a Java application is public static void main(String args[]) (or String[] args). It must be public so the JVM can access it, static so it can be called without instantiating the class, void because it returns no value, and must accept an array of Strings as arguments.

Unchecked exceptions, which are subclasses of RuntimeException, do not have to be caught or declared in a throws clause. The compiler does not force you to handle them, although you can choose to do so. Only checked exceptions must be handled using a try/catch block or declared in the method signature.

Private members of a class are only accessible within the class in which they are declared. Even though a subclass inherits all members of its superclass, it cannot directly access private fields or methods. Instead, it must use public or protected getter and setter methods to interact with them.

Method overriding occurs when a subclass provides a specific implementation for a method that is already defined in its superclass. The overriding method must have the exact same name, return type, and parameter list (signature) as the method in the parent class. This allows for runtime polymorphism.

In Java, arrays are objects that expose a public, final field named 'length' to store the number of elements. Because it is a field and not a method, you access it without parentheses, unlike the length() method used for Strings or the size() method used for Collections.

The Serializable interface is a marker interface, which means it does not contain any methods or fields. Since there are no methods defined in the interface, a class that implements Serializable does not need to implement any methods.

The Java compiler automatically provides a default, no-argument constructor only if no other constructors are explicitly defined in the class. If you write any constructor yourself, whether it has parameters or not, the compiler will not generate the default constructor.

Instance variables belong to a specific instance (object) of a class. Each object has its own separate copy of instance variables in memory, so modifying the instance variables of one object has no effect on those of another object.

A try-finally block is completely legal in Java. You do not need a catch block if you have a finally block, which is useful when you want to ensure cleanup code runs regardless of whether an exception is thrown, while letting the exception propagate up the call stack.

The Collection interface (java.util.Collection) is the root interface of the collection hierarchy, from which List, Set, and Queue inherit. Note that 'Collections' is a utility class containing static methods, not an interface.

An Integrated Development Environment (IDE) provides a GUI for writing code, compiles code, and manages project structures. However, it does not automatically download arbitrary third-party libraries from the internet on its own; that task is handled by build automation tools like Maven or Gradle.

The Java compiler (javac) translates human-readable Java source code into bytecode, which consists of instructions optimized for execution by the Java Virtual Machine (JVM). The JVM then translates this bytecode into machine-specific instructions for the host operating system.

In Java, variable declaration and assignment must follow the syntax: data type, followed by the variable name, the assignment operator (=), and finally the value. Therefore, 'int x = 5;' is the only syntactically correct option.

The single equals sign '=' is the assignment operator, which stores the value on its right into the variable on its left. The double equals sign '==' is a comparison operator used to evaluate whether two values are equal.

Inheritance represents an 'is-a' relationship between classes. If class B is a specific example of class A (for example, a Dog is an Animal), then class B can inherit from class A to reuse and extend its functionality.