Java Class Structure Flashcards

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Questions and Answers

What are the three structures of a Java class?

Person.java source code file (correct)

class Person (correct)

Person.class bytecode (correct)

class Animal

What components does a class contain?

Package statement, Import statements, Comments, Class declaration, Variables, Constructors, Methods, Nested classes, Nested interfaces, Enum

What is a package statement in Java?

Defines what package a class is in.

What is an import statement in Java?

Allows the use of a class or interface from another package without using the fully qualified name. Signup and view all the answers

Single line comments in Java can be placed anywhere.

False Signup and view all the answers

What is the minimum requirement for a class declaration in Java?

class keyword, name of class, curly bracket body Signup and view all the answers

What defines the class in Java?

Class is defined by class declaration. Signup and view all the answers

What are instance variables?

Variables declared in a class but outside of methods that store the state of an object. Signup and view all the answers

What are class variables?

Variables declared in a class but outside of methods that store the state of the class. Signup and view all the answers

What are instance methods?

Non-static methods that cannot be called unless an object of the class is created. Signup and view all the answers

What are class methods used for?

Used to manipulate class variables. Signup and view all the answers

What is the purpose of a constructor in Java?

Initializes variables and creates objects of a class. Signup and view all the answers

What is the structure and components of a Java source code file?

Defines all Java code entities like class, interface, enum, and application of import and package statements. Signup and view all the answers

What is the name of the Java compiler?

javac.exe (Windows) or javac (Mac, Linux, UNIX) Signup and view all the answers

What is an interface in Java?

A grouping of related methods and constants that specifies a contract for classes to implement. Signup and view all the answers

You can define multiple classes and interfaces in any order in a single Java source code file.

True Signup and view all the answers

You can define multiple classes and interfaces as public in a single source code file.

False Signup and view all the answers

If you declare a class or interface as public in a source code file, the file must share the same name.

True Signup and view all the answers

What does the import statement import.pkgname.* do?

Imports all classes and interfaces from a single package. Signup and view all the answers

Study Notes

Structure of a Java Class

A Java class consists of three main structures: the class definition (class Person), the source code file (Person.java), and the bytecode (Person.class).
The class definition is initiated with the class keyword followed by the class name and body enclosed in curly braces.
The source code file uses the same name as the class with a .java extension and contains the class declaration.

Class Components

Key components of a Java class include:
- Package statement: denotes the package the class belongs to.
- Import statements: allows the use of other classes/interfaces from different packages without their fully qualified names.
- Comments: provide explanations and can be single-line or multi-line.
- Class declaration: contains variables, constructors, methods, and can include nested classes/interfaces and enums (though not covered in the exam).

Package Statement

The package statement defines the package a class is assigned to and is mandatory if not using the default unnamed package.
It must be the first statement in the class and cannot be duplicated; example: package certification;.

Import Statement

The import statement allows access to classes/interfaces from other packages without specifying the full path.
Must appear after the package statement and before the class definition; example: import certification.ExamQuestion;.

Comments

Single line comments are marked with // and can be placed anywhere outside of double-quoted strings.

Class Declaration

Class declaration, also known as class signature, includes various components as the first line of a class.
Minimum syntax: class ClassName { ... }. More elements like access modifiers and inheritance can be added (e.g., public final class Runner extends Person).

Class Definition

The class is defined by its declaration, encapsulating both attributes and methods.
Attributes are represented as instance variables that represent the state of an object.

Instance Variables

Instance variables, or attributes, are defined inside a class but outside methods and represent the state of individual objects.
Each object maintains its own copy of these variables.

Class Variables

Class variables, also known as static variables, are defined outside methods and shared across all instances of the class.
They hold the same value for all objects of that class and cannot be changed independently.

Instance Methods

Instance methods are non-static and can only be invoked by creating an object of the class in which they reside.
They typically manipulate instance variables.

Class Methods

Class methods, also referred to as static methods, operate on class variables and can be called without creating an object.

Constructor

A constructor initializes objects and shares the same name as the class.
A class may have multiple constructors with different parameter sets to instantiate objects differently.

Java Source Code File Structure

Java source code files define one or more Java entities like classes and interfaces, and must end with a .java extension.
The source code file can contain package and import statements.

Java Compiler

The Java compiler is named javac on Mac, Linux, and UNIX systems, while on Windows it is javac.exe.

Interface Definition

An interface is a collection of related methods and constants serving as a contract for classes to implement.

Multiple Class and Interface Declarations

Multiple classes and interfaces can be defined in any order within a single Java source code file, adhering to visibility rules.

Public Class and Interface Naming Rules

Only one public class or interface is allowed per source file, and if declared public, the file name must match the public entity's name.

Importing Packages

The syntax import pkgname.* is used to import all classes and interfaces from a specified package.

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Description

These flashcards cover the fundamental structures of a Java class and its associated source code file. Learn about the components of a Java class, including class definitions and bytecode conversion. Ideal for beginners in Java programming.