[Java] Encapsulation

What is encapsulation?

• ** Do not show anything that is not related to the function you want to use **
• ex: TV. The circuit inside is a black box, and only the functions such as power supply and screen are visible to the user.
• Avoid inadvertently touching irrelevant parts
• Hide the internal functions to realize the functions of the class and show only the functions you want to use.

Access modifier

• Manage whether to show a specific member
• ** Imposing as strong a constraint as possible to meet the requirements **
• Basically package private unless you intend to access it from outside the package (Java 9 or later modules filled the space between public / private)
• ** public **: accessible from all classes
• ** protected **: Only the current class, derived class, and the same package class can be accessed.
• ** None **: Only accessible from the same package class as the current class
• ** private **: Only accessible from the current class

Field permissions

• Publish class data externally in fields
• Instance fields should not be declared public
• ** Read / write permission / prohibition cannot be controlled **
• Fields are variables for managing object state
• I want to limit changes even if I get the value
• Since the field is just a variable, you can get and change its value by deciding whether to access it with a modifier.
• ** Immutable class **: Class that is fixed to the internal state and cannot be changed at the timing of instantiation
• ** Cannot validate the value **
• ** Left and right to change the internal state **
• All code that references the age field affects, such as when the age field is changed to double instead of int
• ** → Use the accessor method! ** **

//Bad example
public class Person {
  public String name;
  public int age;

  public Person(String name, int age) {
    this.name = name;
    this.age = age;
  }

  public String show(){
    return String.format("%s（%d years old)\n", this.name, this.age);
  }
}

Accessor Method

• Make the internal state (= field) of the object inaccessible directly from the outside, and realize a mechanism to get and change via a method.
• The name of the alixer method is ** get / set + capitalized at the beginning of the field **
• Since it is a method, exception handling is also possible (throw instruction, etc.)
• Value processing is possible at the time of acquisition
• Only getter / setter can be prepared to make the field read / write only (read only recommended)

public class Person {
  //Field is private
  private String name;
  private int age;

  //getter in the name field
  public String getName() {
    return this.name;
  }
  //name field setter
  public void setName(String name) {
    this.name = name;
  }
  //age field getter
  public int getAge() {
    return this.age;
  }
  //age field setter
  public void setAge(int age) {
    if (age <= 0) {
      throw new IllegalArgumentException("Please specify the age as a positive number.");
    }
    this.age = age;
  }

  public String show() {
    return String.format("%s（%d).", getName(), getAge());
  }
}

public class AccessorBasic {

  public static void main(String[] args) {
    var p = new Person();
    p.setName("Ichiro");
    p.setAge(47);
    System.out.println(p.show()); //I'm Ichiro (47 years old).
    p.setAge(-30);  //error
  }
}

Immutable class

• A class whose value (field) does not change at all from the place where the object was first created (recommended)

//Declare the class itself as final and do not extend it
public final class Person {
  //All private final (reassignment prohibited)
  private final String name;
  private final int age;

  //Initialize private final field in constructor
  public Person(String name, int age) {
    this.name = name;
    this.age = age;
  }
  //Getter
  public String getName() {
    return this.name;
  }

  public int getAge() {
    return this.age;
  }
}

public class ImmutableBasic {
  public static void main(String[] args) {
    var p = new Person("Satoshi", 10);
    System.out.println(p.getName()); //Satoshi
  }
}

• Example of breaking invariance
• Reference type assignment ** only passes the reference itself **
• If you change the value passed in the actual argument / return value, the content will affect the field as well.
• As an exception, String type is immutable

import java.util.Date;

public final class Person {
  private final String name;
  private final int age;
  private final Date birth;

  public Person(String name, int age, Date birth) {
    this.name = name;
    this.age = age;
    this.birth = birth;
  }

  public String getName() {
    return this.name;
  }

  public int getAge() {
    return this.age;
  }
  
  public Date getBirth() {
    return this.birth;
  }
}

//Example of changing the value passed to the instance later
import java.util.Date;
public class Main {
  public static void main(String[] args) {
    var birth = new Date();
    var p = new Person("Satoshi", 10, birth);
    System.out.println(p.getBirth());  //Tue Nov 03 11:26:33 UTC 2020
    //Update the object passed during instantiation
    birth.setDate(15);
    System.out.println(p.getBirth());  //Sun Nov 15 11:26:33 UTC 2020

//Example of changing the value obtained from the instance
//    var p = new Person("Satoshi", 10, new Date());
//    System.out.println(p.getBirth());  //Tue Nov 03 11:29:05 UTC 2020
//    var birth = p.getBirth();
//
//    birth.setDate(15);
//    System.out.println(p.getBirth());  //Sun Nov 15 11:29:05 UTC 2020
  }
}   

How to prevent invariance from breaking

• ** Defensive copy when passing to arguments / return values **
• Pass the duplicated object instead of passing the object as it is
• ** Convert to immutable when returning as return value **
• Convert a StringBuilder type field to a String type when passing it to the outside

//Constructor argument birth,Extract the time stamp value from the birth field with the getTme method and create a new object
import java.util.Date;

public final class Person {
  private final String name;
  private final int age;
  private final Date birth;

  public Person(String name, int age, Date birth) {
    this.name = name;
    this.age = age;
    //Defensive copy
    this.birth = new Date(birth.getTime());
  }

  public String getName() {
    return this.name;
  }

  public int getAge() {
    return this.age;
  }

  public Date getBirth() {
    //Defensive copy
    return new Date(this.birth.getTime());
  }
}