Java and Object-Oriented Programming (OOP)

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Questions and Answers

What does the term 'Write Once, Run Anywhere' (WORA) refer to in Java?

  • Java source code requires a different syntax for each operating system.
  • Java applications can only run on the Java Development Kit (JDK).
  • Java code is compiled into machine code for each operating system.
  • Java applications can be run on multiple operating systems without modification. (correct)

Which of the following is NOT a benefit of Object-Oriented Programming (OOP)?

  • Requires the use of more global variables. (correct)
  • Models real-world entities effectively.
  • Supports encapsulation and inheritance.
  • Encourages reusability of code.

What is the primary function of the Java compiler?

  • To translate bytecode into high-level source code.
  • To convert Java source code into machine code.
  • To execute bytecode and provide runtime environment.
  • To check syntax and convert code into bytecode. (correct)

Which statement about Java Virtual Machine (JVM) is incorrect?

<p>JVM is platform-dependent and tailored for each operating system. (D)</p> Signup and view all the answers

What do valid identifiers in Java NOT have to start with?

<p>A number. (C)</p> Signup and view all the answers

Which of the following describes a class in Java?

<p>A blueprint for creating objects with specific attributes and methods. (A)</p> Signup and view all the answers

Which of these statements accurately describes Java's type system?

<p>Java is statically typed, with variable types determined at compile time. (B)</p> Signup and view all the answers

What does encapsulation in OOP primarily promote?

<p>Hiding the internal states of objects from the outside. (A)</p> Signup and view all the answers

What does the nextInt() method of the Scanner class do?

<p>Reads an integer value from the input (B)</p> Signup and view all the answers

Which line of code correctly prints the value of pi up to two decimal places?

<p>System.out.printf(&quot;Value of pi: %.2f\n&quot;, pi); (B)</p> Signup and view all the answers

What will the output be when the age is set to 15 in the following code: if (age >= 18) { System.out.println("You are an adult."); } else { System.out.println("You are a minor."); }?

<p>You are a minor. (D)</p> Signup and view all the answers

In the switch statement, if day is equal to 4, what will be the output?

<p>Invalid day (C)</p> Signup and view all the answers

Which code snippet correctly demonstrates the use of a for loop?

<p>for (int i = 0; i &lt; 5; i++) { System.out.println(i); } (C)</p> Signup and view all the answers

What will be the output of the following code: int[] numbers = {1, 2, 3, 4, 5}; System.out.println(numbers);?

<p>The address of the array (B)</p> Signup and view all the answers

What will happen if a 2D array is traversed using the following code snippet? for (int[] row : matrix) { for (int num : row) { System.out.print(num + " "); } }

<p>It will print all elements in the matrix (A)</p> Signup and view all the answers

Which of the following function signatures demonstrates method overloading?

<p>All of the above (D)</p> Signup and view all the answers

What is the output of greet("Alice", 2); if defined as public static void greet(String name, int times)?

<p>Hello, Alice! Hello, Alice! (A)</p> Signup and view all the answers

Which statement accurately describes the difference between syntax and semantics in programming?

<p>Syntax is the set of rules for writing code, whereas semantics is the meaning of that code. (B)</p> Signup and view all the answers

What is the primary characteristic of a constant in programming?

<p>It cannot be modified once assigned a value. (C)</p> Signup and view all the answers

What will happen if you perform a narrowing conversion without casting?

<p>A compile-time error will occur. (A)</p> Signup and view all the answers

Which of the following correctly describes the behavior of the 'break' statement in loops?

<p>It exits the nearest enclosing loop. (D)</p> Signup and view all the answers

What is the purpose of garbage collection in programming?

<p>To free up memory from objects that have not been referenced. (A)</p> Signup and view all the answers

In a two-dimensional array, how are elements typically organized?

<p>As an array of arrays. (B)</p> Signup and view all the answers

What will the output of the following code segment be if 'x' is equal to 5? String result = (x > 3) ?"Greater" : "Lesser"; System.out.println(result);

<p>Greater (C)</p> Signup and view all the answers

Which of the following statements about reference variables is incorrect?

<p>They always refer to a unique object. (B)</p> Signup and view all the answers

What does the 'equals()' method do when comparing strings?

<p>It compares the content of the strings. (B)</p> Signup and view all the answers

Which of the following statements about loops is true?

<p>A 'do-while' loop can never execute zero times. (A)</p> Signup and view all the answers

In Java, which primitive data type can represent a boolean value?

<p>boolean (C)</p> Signup and view all the answers

Which type of variables can hold values that can be changed during execution?

<p>Variables (B)</p> Signup and view all the answers

When using 'System.out.printf()', which of the following is a correct format specifier for an integer?

<p>%d (D)</p> Signup and view all the answers

What will happen if you assign a string literal to a variable declared as an integer?

<p>A compile-time error will occur. (C)</p> Signup and view all the answers

Flashcards

Java Overview

Java is a programming language developed by Sun Microsystems in 1995, designed for platform independence (WORA).

Object-Oriented Programming (OOP)

OOP is a programming paradigm that uses objects and classes to model real-world entities and supports modularity, reusability, and maintainability.

Classes

Classes are blueprints for creating objects, defining attributes (fields) and behaviors (methods).

Objects

Objects are instances of classes that represent real-world entities with state (attributes) and behavior (methods).

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Compiled vs. Interpreted Languages

Compiled languages are translated into machine code before execution; interpreted languages execute line-by-line. Java uses both: compiled to bytecode, then interpreted by the JVM.

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Java Virtual Machine (JVM)

The JVM executes Java bytecode and provides the runtime environment for Java applications.

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Identifiers

Identifiers are names for variables, methods, classes, etc., following rules like starting with a letter or underscore and avoiding reserved words.

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Statically-Typed Language

In statically-typed languages like Java, variable types are determined at compile time, minimizing runtime type errors.

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Runtime Errors

Errors occurring during program execution, e.g., division by zero.

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Logical Errors

Mistakes in the program's logic leading to incorrect outputs.

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Syntax

Rules governing the structure of code writing.

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Semantics

The meaning of written code or expressions.

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Variables

Values that can change, defined by data types (e.g., int x = 5;).

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Constants

Values that cannot change once defined (e.g., final int MAX = 100;).

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Type Conversion

Changing a value from one data type to another, either widening or narrowing.

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Casting

Explicitly converting one data type to another, like from double to int.

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Primitive Types

Basic data types like int, double, char, etc., each with specific memory sizes.

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Scanner Class

Used to read input from the user via command line in Java.

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if Statement

A conditional statement that executes code based on a true or false condition.

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Loops

Control structures for repeating actions, such as while, do-while, for.

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Arrays

Fixed-size collections of elements, with default values for each type.

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Method Overloading

Multiple methods with the same name but different parameters for flexibility.

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Garbage Collection

Automatic memory management that frees unused objects in Java.

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Scanner for Input

A Java class to read user input from the console.

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nextLine() Method

Reads a full line of text input from the user.

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Formatted Output

Display output with specific formatting using printf().

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if-else Statement

Conditional statement that executes code based on true/false.

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Ternary Operator

A shorthand way to write an if-else statement in Java.

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switch Statement

Selects a block of code to execute based on a variable's value.

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for Loop

Repeats a block of code a fixed number of times.

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while Loop

Repeats a block of code as long as a condition is true.

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1D Array

A collection of elements accessed by a single index.

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Study Notes

Java and Object-Oriented Programming (OOP)

  • Java, developed by Sun Microsystems (now Oracle) in 1995, aims for platform independence (WORA).

  • Evolved from Oak, designed for embedded systems.

  • Object-Oriented Programming (OOP): Enhances modularity, reusability, and maintainability. Models real-world entities using classes and objects. Supports encapsulation, inheritance, and polymorphism.

  • Classes: Blueprints for objects, defining attributes and behaviors.

  • Objects: Instances of classes, representing real-world entities with state (attributes) and behavior (methods).

  • Compiled vs. Interpreted Languages: Java compiles to bytecode—interpreted by the Java Virtual Machine (JVM).

  • Compiler (Java): Converts Java code into bytecode (.class files), checks syntax, and optimizes.

  • Interpreter (JVM): Executes bytecode, providing a runtime environment.

  • Platform Independence: Bytecode runs on any system with a JVM.

  • Performance: Bytecode is optimized for execution on the JVM.

  • Identifiers: Names for variables, methods, and classes. Rules: start with a letter, underscore (_), or dollar sign ($); cannot be reserved words.

  • Reserved Words: Keywords with predefined meanings (e.g., class, public, static).

  • Valid Identifiers: Examples: myVariable, count1, _name.

  • Declaring vs. Instantiating: Declare a variable or object (e.g., int x;) and instantiate an object using new (e.g., MyClass obj = new MyClass();).

  • Java is statically-typed: Variable types determined at compile time.

  • Designing a Class: Defines attributes (e.g., String name) and behaviors (e.g., void speak()).

Java Basics

  • Errors:

    • Compiler errors: Syntax issues (e.g., missing semicolon).
    • Runtime errors: Occur during execution (e.g., division by zero).
    • Logical errors: Incorrect program logic (e.g., wrong output).

  • Syntax vs. Semantics: Syntax are code rules; Semantics are code meaning.

  • Variables vs. Constants: Variables change value; constants don't (e.g., final int MAX = 100;).

  • Scope of a Variable: Where a variable can be accessed (local, instance, class).

  • Keywords:

    • final: Prevents modification or inheritance.
    • static: Belongs to the class, not the instance.

  • Primitives: Basic data types like int, double, char, boolean.

    • Memory sizes (examples): int (4 bytes), double (8 bytes).

  • Legal Assignment of Literals: Assigning values to variables (e.g., int x = 10;).

  • Type Conversion:

    • Widening: Automatic conversion (e.g., int to double).
    • Narrowing: Requires casting (e.g., double to int).

  • Casting: Explicitly converting types (e.g., int x = (int) 3.14;).

  • Purpose of Multiple Primitive Types: Optimize memory usage and precision.

  • Primitive Operations: Arithmetic (+, -, *, /, %).

    • Order of Operations: PEMDAS.
    • Assignment Operators: =, +=, -=, *=, etc.

  • Strings: Immutable sequences of characters.

    • String methods: length(), charAt(), substring(), etc.
    • String constant pool: Stores unique string literals).

  • Reference Variables: Point to objects in memory.

    • Aliases: Multiple references to the same object.

  • Garbage Collection: Automatically frees memory from unused objects.

Input/Output and Formatting

  • Scanner: Reads input from the console. Methods: nextLine(), nextInt(), etc..

  • Packages: java.lang (String, System); java.util (Scanner, Arrays).

  • Formatting:

    • System.out.printf(): Formatted output.
    • String.format(): Format strings.
    • DecimalFormat: Format numbers.

Decision-Making Statements

  • Conditional Statements:

    • if, else if, else
    • Ternary operator: Shorthand for if-else.
    • switch: Multi-way branching.
    • break: Exits switch.
    • default: Executes if no case matches.

  • String Comparison: equals(), compareTo().

  • Logical Operators: && (AND), || (OR), ! (NOT).

Iteration

  • Loops:

    • while: Repeats while a condition is true.
    • do-while: Executes at least once.
    • for: Compact loop with initialization, condition, and update.

  • break: Exits the loop.

  • continue: Skips to the next iteration.

Arrays

  • Arrays: Fixed-size collections.

    • Default values: 0 for numbers, null for objects.

  • 2D Arrays: Arrays of arrays.

    • Row-major vs. column-major traversal.

  • Wrapper Classes: (e.g., Integer, Double) for primitives.

  • Arrays Class: Utility methods (e.g., sort(), binarySearch()).

Methods

  • Writing Methods: Defining returnType methodName(parameters).

  • Method Overloading: Multiple methods with the same name but different parameters.

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