Enumerations

Overview

An enumeration represents a fixed set of named constants. Defined using the enum keyword, enumerations declare a collection of related constants separated by commas. Common examples include days of the week, calendar months, or cardinal directions.

Declaration

Declare enumerations with the enum keyword, specifying the type name and its members. If no explicit base type is defined, the underlying type defaults to int.

Usage Example

enum Hue {
    RED, CYAN, MAGENTA, BLACK;
}

enum Indicator {
    GREEN, AMBER, RED
}

class Stoplight {
    Indicator signal = Indicator.RED;
    
    void transition() {
        switch(signal) {
            case RED:
                signal = Indicator.GREEN;
                break;
            case AMBER:
                signal = Indicator.RED;
                break;
            case GREEN:
                signal = Indicator.AMBER;
                break;
        }
    }
}

Enum Class Internals

Each enumeration implicit extends java.lang.Enum. Members are treated as static final instances of the enum type. Key inherited methods include:

• values(): Returns all enum constants as an array
• valueOf(String): Converts a string to the corresponding enum constant
• compareTo(E): Compares declaration order of two constants
• ordinal(): Returns the zero-based postiion of the constant

Method Demonstrations

Iterating through constants:

enum Status { ACTIVE, INACTIVE, PENDING }

class Processor {
    public static void main(String[] args) {
        for (Status s : Status.values()) {
            System.out.println("Constant: " + s);
        }
    }
}

Comparing constants:

enum Category { PRIMARY, SECONDARY }

class Comparator {
    static void evaluate(Category c) {
        for (Category ref : Category.values()) {
            System.out.println(c + " vs " + ref + ": " + c.compareTo(ref));
        }
    }
}

Getting declartaion indices:

enum Priority { HIGH, MEDIUM, LOW }

class Indexer {
    public static void main(String[] args) {
        for (Priority p : Priority.values()) {
            System.out.println(p.ordinal() + ": " + p);
        }
    }
}

Adding Functionality to Enums

Enums can contain constructors, fields, and methods. Constants must be declared before any additional members.

enum Day {
    MON("Monday"), TUE("Tuesday"), WED("Wednesday"),
    THU("Thursday"), FRI("Friday"), SAT("Saturday"), SUN("Sunday");
    
    private final String fullName;
    
    Day(String name) {
        this.fullName = name;
    }
    
    String getFullName() {
        return fullName;
    }
    
    static void display(int num) {
        switch(num) {
            case 1 -> System.out.println(MON);
            case 2 -> System.out.println(TUE);
            // Additional cases omitted for brevity
        }
    }
}

Specialized Collections

• EnumMap: Optimized map implementation for enum keys
• EnumSet: High-performance set for enum constants

System Interaction Classes

Console Operations

The Console class facilitates character-based console interactions.

import java.io.Console;

class ConsoleHandler {
    public static void main(String[] args) {
        Console terminal = System.console();
        if (terminal != null) {
            String username = terminal.readLine("Username: ");
            char[] password = terminal.readPassword("Password: ");
            terminal.printf("Authenticated: %s%n", username);
        }
    }
}

Input Processing with Scanner

The Scanner class parses primitive types and strings from input sources.

Token vs Line Input

import java.util.Scanner;

class InputDemo {
    public static void main(String[] args) {
        Scanner reader = new Scanner(System.in);
        
        System.out.print("Token input: ");
        if (reader.hasNext()) {
            String token = reader.next();
            System.out.println("Received: " + token);
        }
        
        System.out.print("Line input: ");
        if (reader.hasNextLine()) {
            String line = reader.nextLine();
            System.out.println("Received: " + line);
        }
        reader.close();
    }
}

Type-Specific Parsing

import java.util.Scanner;

class DataReader {
    public static void main(String[] args) {
        Scanner parser = new Scanner(System.in);
        String name = parser.nextLine();
        int age = parser.nextInt();
        double balance = parser.nextDouble();
        parser.close();
        
        System.out.printf("Name: %s, Age: %d, Balance: %.2f%n", name, age, balance);
    }
}

Aggregation Example

import java.util.Scanner;

class StatisticsCalculator {
    public static void main(String[] args) {
        Scanner collector = new Scanner(System.in);
        double total = 0;
        int count = 0;
        
        while (collector.hasNextDouble()) {
            total += collector.nextDouble();
            count++;
        }
        
        System.out.printf("%d values | Sum: %.2f | Average: %.2f%n", 
                          count, total, total/count);
        collector.close();
    }
}

System Utilities

Core System Class

The System class provides access to system resources and environment properties.

Array Manipulation

class ArrayUtils {
    public static void main(String[] args) {
        char[] source = {'X', 'Y', 'Z', 'W'};
        char[] target = {'A', 'B', 'C', 'D'};
        
        System.arraycopy(source, 1, target, 2, 2);
        // Result: target becomes ['A', 'B', 'Y', 'Z']
    }
}

Performance Measurement

class Benchmark {
    public static void main(String[] args) {
        long start = System.currentTimeMillis();
        // Operation to measure
        long duration = System.currentTimeMillis() - start;
        System.out.println("Execution time: " + duration + "ms");
    }
}

Environment Properties

class EnvironmentInspector {
    public static void main(String[] args) {
        System.out.println("Java Version: " + System.getProperty("java.version"));
        System.out.println("OS Name: " + System.getProperty("os.name"));
        System.out.println("User Home: " + System.getProperty("user.home"));
    }
}

Process Control

class ProcessManager {
    public static void terminate(int statusCode) {
        System.exit(statusCode);
    }
    
    public static void requestCleanup() {
        System.gc();
    }
}