Java Class Structure Flashcards

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.