getter / setter中的指定方法

Question

To actually understand the meaning of encapsulation, example that class private fields must be accessed through class public methods is as per definition, but actually stll it doesn't make difference as the field is still accessible as it is.
So,I think there should be some processing inside getters/setters to hide how the field is being handled. But it breaks the principal of behind pojos. How can one handle this situation?

Answer 1

I'm not sure what according to you is "the principle of POJOs". The following is a POJO and still hides implementation details behind getters and setters:

public class Example {
    private int thousands;
    private int units;

    public void setValue(int value) {
        thousands = value / 1000;
        units = value % 1000;
    }

    public int getValue() {
        return 1000 * thousands + units;
    }
}

Answer 2

Encapsulation means not to expose the internals of your class.

In the Java context it means that the attributes of your class should NOT be accessible by other classes, instead your class should provide methods that will allow to access the attributes. In cases of POJO classes these methods will only allow to set ( setters ) and get ( getters ) the values of the attributes from the POJO class.

The goal of encapsulation is to protect the attributes of your class from being modified by other classes. Your class is obviously able to do whatever you want with the attributes inside your classes.

Answer 3

No, the field is not accessible as it is. All it takes to make a method a getter or a setter ist the proper signature.

public String getFoo() {
  return null;
}

This is a perfect getter for a String foo , even though it returns null .

public void setFoo(String foo) {
  // do nothing
}

This is a perfect setter for the same member, even though it does nothing.

Answer 4

One of the many uses of getters and setters is to restrict the value of the variable. By making the data members private and keeping the getters and setters public, the programmer can keep a check on the value of the variable. For ex:

class Employee
{
    private int age;

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

    public void setAge(int age)
    {
        if(age<18 || age>60)
            this.age = age;
        else
            System.out.println("The age of the employee should be between 18 and 60");
    }
}

In this case the age of the Employee can never be more less than 18 and more than 60.

Answer 5

Don't know why people usually mix Data Encapsulation with Data hiding. Data encapsulation simply means grouping data together whereas data hiding is a way to store this data so others cannot know it's internal implementation.

Lets say you have a class PersonalInfo in which you have fields like name,gender,age etc. As a programmer(which you are) you will provide user some way to enter these fields, save them in your PersonalInfo object using setter methods.IF user wish to see the information you simple call getters and display information. Your implementation as in you may store this variables in a map may varry. So you can say

public void setName(String name){
  SomeMap.add("name",name);
}

Note * you are the programmer and you will always know the implementatio *n. Keeping the fields private is to allow only your class methods to access your data.

Imagine it this way. User can create an object. He may set all fields using getters and setters. It may be your implementation to calculate age using DOB(Date of Birth) in which case you will not provide setter for age. In this case user cannot say myObj.age=23. This is purely your implementation .Hope this clears the confusion!

Answer 6

It's still accessible but you see when you use it directly (public variable) you can change the value of variable without any restriction. The advantage of using such kind of private variables with setter and getter methods is that you can write code inside the setter method to check whether the value set is in the expected range or not. Or you can even store the value in different form than the apparent view. For example you may store the value of a variable by adding offset to the value of the parameter of the setter method and in getter method you may just revert back the process(Encapsulation). When the value set is not in the expected range you may even throw exceptions.

Example1: Here var1 is a private variable

public void setValue(int var1){
         if(var1<0){
                //throw exception
          }
         this.var1=var1;
 }

Example2:

 public void setValue(int var1){
        this.var1=calculatesomething+var1;
}
public int getValue(){
        return calculatesomething+this.var1;
}

That's the use of encapsulation.....all the best

Answer 7

Encapsulation is to restrict the access to the Class's variables and to regularize the way of editing them.

Class Test
{
 public int a;
 public Test()
 {
  a = 0;
 }
 public getA()
 {
  return a;
 }

 public setA(int a)
 {
   this.a = a
 }
}

Class TestMain
{
  main()
 {
   Test t = new Test();
   System.out.println(t.a);   // This prints 0;
   int a = t.getA();
   a = 10;
   System.out.println(t.a);   // This still prints 0;

   t.a = 20;
   System.out.println(t.a);   // This prints 20;
 }
}

In the above example the programmer may not be intentionally changing the value of ta but the value changes.

If he really intents to change it, then he should use the setter.

Encapsulation is the feature that java provides which solves certain practical problems and helps in extensibility.

If the Test and TestMain classes are written by same person, there wont't be any confusion. But practically that is not the case.