Java OOP Fundamentals Quiz

Java and OOP Fundamentals

• Java, developed by Sun Microsystems (now Oracle), in 1995, is designed for platform independence (WORA - Write Once, Run Anywhere). It evolved from Oak, a language for embedded systems.
• Object-Oriented Programming (OOP) in Java promotes modularity, reusability, and maintainability by modeling real-world entities using classes and objects. It supports encapsulation, inheritance, and polymorphism.
• Classes act as blueprints for creating objects, defining attributes (fields) and behaviors (methods).
• Objects are instances of classes, representing real-world entities with their attributes (state) and behaviors (methods).
• Java is a compiled language (translating Java code into bytecode) and an interpreted Language (executing bytecode on the JVM), which makes it platform independent.
• The compiler converts Java code into bytecode (.class files). The JVM executes this bytecode and provides the runtime environment.
• Java identifiers are names for variables, methods, or classes following specific rules: begin with a letter, underscore _, or dollar sign $; and cannot be Java keywords.
• Reserved words in Java are keywords with predefined meanings (e.g., class, public, static).
• Examples of valid identifiers include myVariable, count1, and _name.
• To declare and instantiate an object, you define the variable or object first and then you create the object using the new keyword. For instance, int x; declares an integer variable and MyClass obj = new MyClass(); instantiates an object of type MyClass.

Basic Java Concepts

• Java is statically typed, meaning variable types are determined at compile time.

• Errors in Java can be compiler errors (syntax issues, missing semicolons), runtime errors (during execution, e.g., division by zero), or logical errors (incorrect program logic).

• A variable's scope denotes where it can be accessed (local, instance, or class).

• The final keyword prevents modification or inheritance. static variables belong to the class, not object instances.

• Primitive Data Types: Java has basic data types like int, double, char, boolean, and others, each with specific memory sizes (int = 4 bytes, double = 8 bytes).

• Type Conversion: Widening is automatic (e.g., int to double), but narrowing requires explicit casting (e.g., double to int).

• Arithmetic Operations: Java supports standard arithmetic operations (+, -, *, /, %) with PEMDAS (standard order of operations).

• Assignment Operators: Java provides shorthand assignment operators like +=, -=, *=, /=, and a %= to modify the value of a variable using an arithmetic operation.

• Strings: Strings are immutable sequences of characters, with useful methods like length(), charAt(), substring(), and equals().

• Java has a string constant pool to store unique string literals.

• Reference Variables: These variables point to objects in memory, and use multiple references could potentially be treated as aliases to the same object.

• Garbage Collection: Java uses automatic garbage collection to free up memory occupied by unused objects.

Input/Output

• The Scanner class reads input from the command line. Methods include nextLine(), nextInt(), hasNext(), etc.
• Packages like java.lang (e.g., String, System) and java.util (e.g., Scanner, Arrays) are used for standard functions.
• System.out.printf() and String.format() provide formatted output. DecimalFormat is a useful subclass used to format numbers using patterns (e.g., currency). DecimalFormat and NumberFormat are useful for formatting numbers (e.g., currency).

Control Flow Statements

• Conditional Statements: if-else statements (including else if) handle conditional branching, and the ternary operator offers shorthand for simple if-else conditions.
• Looping: while, do-while, and for loops implement iterative execution. break exits a loop, and continue skips to the next iteration within the loop. switch statements handle multi-way branches with keyword break and default.
• String comparisons to utilize the equals() method and compareTo() for lexicographic comparisons.
• Logical Operators (&&, ||, !) control conditional expressions.

Arrays

• Arrays are fixed-size collections. 1D and 2D arrays are used. They have default values (e.g., 0 for numbers, null for objects).
• Arrays can be used for collections of elements useful to loop through. The Arrays class provides utility methods like sort() and binarySearch().
• Wrapper classes (e.g., Integer, Double) are used for primitive types when needed.

Methods

• Methods are reusable blocks of code with a header (returnType methodName(parameters)) and body. They can be overloaded through method names, but utilizing different parameters for each method.