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Java Encapsulation


Encapsulation

The meaning of Encapsulation, is to make sure that "sensitive" data is hidden from users. To achieve this, you must:

  • declare class variables/attributes as private (only accessible within the same class)
  • provide public setter and getter methods to access and update the value of a private variable

Get and Set

You learned from the previous chapter that private variables can only be accessed within the same class (an outside class have no access to it). However, it is possible to access them if we provide public getter and setter methods.

The get method returns the variable value, and the set method sets the value.

Syntax for both is that they start with either get or set, followed by the name of the variable, with the first letter in upper case:

Example

public class Person {
  private String name; // private = restricted access

  // Getter
  public String getName() {
    return name;
  }

  // Setter
  public void setName(String newName) {
    this.name = newName;
  }
}

Example explained

The get method returns the value of the variable name.

The set method takes a parameter (newName) and assigns it to the name variable. The this keyword is used to refer to the current object.

However, as the name variable is declared as private, we cannot access it from outside this class:

Example

public class MyClass {
  public static void main(String[] args) {
    Person myObj = new Person();
    myObj.name = "John";  // error
    System.out.println(myObj.name); // error 
  }
}
Run example »

If the variable was declared as public, we would expect the following output:

John

However, as we try to access a private variable, we get an error:

MyClass.java:4: error: name has private access in Person
    myObj.name = "John";
         ^
MyClass.java:5: error: name has private access in Person
    System.out.println(myObj.name);
                  ^
2 errors

Instead, we use the getName() and setName() methods to acccess and update the variable:

Example

public class MyClass {
  public static void main(String[] args) {
    Person myObj = new Person();
    myObj.setName("John"); // Set the value of the name variable to "John"
    System.out.println(myObj.getName());
  }
}

// Outputs "John"
Run example »

Why Encapsulation?

  • Better control of class attributes and methods
  • Class variables can be made read-only (if you omit the set method), or write-only (if you omit the get method)
  • Flexible: the programmer can change one part of the code without affecting other parts
  • Increased security of data