制作不可变的 Java 对象

Question

My goal is to make a Java object immutable. I have a class Student . I coded it in the following way to achieve immutability:

public final class Student {

private String name;
private String age;

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

public String getName() {
    return name;
}

public String getAge() {
    return age;
}

}

My question is, what is the best way to achieve immutability for the Student class?

Answer 1

Your class is not immutable strictly speaking, it is only effectively immutable. To make it immutable, you need to use final :

private final String name;
private final String age;

Although the difference might seem subtle, it can make a significant difference in a multi-threaded context . An immutable class is inherently thread-safe, an effectively immutable class is thread safe only if it is safely published.

Answer 2

There are few things that you must consider for making an immutable class:

• Make your class final - You already have
• Make all the fields private and final - Make appropriate changes in your code
• Don't provide any methods that change the state of your instance
• If you have mutable fields in your class, like List , or Date , making them final won't suffice. You should return a defensive copy from their getters , so that their state isn't mutated by calling methods.

For the 4th point, say you have a Date field in your class, then the getter for that field should look like:

public Date getDate() {
    return new Date(this.date.getTime());
}

Making a defensive copy can become a headache, when your mutable field itself comprises of some mutable field, and that in turn can contain some other mutable field. In that case, you would need to make copy of each of them iteratively. We name this iterative copy of mutable fields as Deep Copy .

Implementing deep copy by yourself may be cumbersome. But,keeping that issue apart, you should consider your class design again, once you see yourself falling into such requirement of making deep defensive copy.

Answer 3

How do you make a mutable object immutable?

1. Declare the class as final so it can't be extended.
2. Make all fields private so that direct access is not allowed.
3. Don't provide setter methods for variables
4. Make all mutable fields final so that it's value can be assigned only once.
5. Initialize all the fields via a constructor performing deep copy.
6. Perform cloning of objects in the getter methods to return a copy rather than returning the actual object reference.

source

Why do we create immutable objects?
Immutable objects are simply objects whose state (the object's data) cannot change after construction.

• are simple to construct, test, and use
• are automatically thread-safe and have no synchronization issues
• don't need a copy constructor
• don't need an implementation of clone
• allow hashCode to use lazy initialization, and to cache its return value
• don't need to be copied defensively when used as a field
• make good Map keys and Set elements (these objects must not change state while in the collection)
• have their class invariant established once upon construction, and it never needs to be checked again
• always have "failure atomicity" (a term used by Joshua Bloch): if an immutable object throws an exception, it's never left in an undesirable or indeterminate state

Source

Answer 4

With final keyword:

private final String name;
private final String age;

Answer 5

Making variables private and no setter methods will work for primitive data types. If my class has any collection of objects?

To making any class immutable with collection object?

Write your own collection object with extends collection class and follow the private variables and no setter methods. or return clone object of your collection object.

public final class Student {

private StudentList names;//Which is extended from arraylist

public Student() {
names = DAO.getNamesList()//Which will return All Student names from Database  its upto you how you want to implement.
}

public StudentList getStudentList(){
return names;//you need to implement your own methods in StudentList class to iterate your arraylist; or you can return Enumeration object.
}

public Enumeration getStudentNamesIterator(
Enumeration e = Collections.enumeration(names);
return e;
}

public class StudentList extends ArrayList {

}

Answer 6

This is fine but I would make the fields final as well.

Also I would make the age an int or double rather than a String.

Answer 7

Expanding on the answer a bit.

final is not the same as Immutable but you can use final to make certain things immutable if you use it in certain ways.

Certain types are immutable, in that they represent unchanging values rather than objects of changeable state. Strings, numbers, etc are immutable. At the end, usually our objects boil down to data structures eventually referencing immutable values, but we change the data structures by assigning new values to the same field names.

So to make something truly immutable you need to make sure that final is used all the way down, until you reach every field reaching every value at the base of your composition tree. Otherwise something could change out from under your the object and it isn't really fully immutable.

Answer 8

Your example is already immutable object, because fields in Student class can only set on instance initialization.

To make object immutable, You must do these steps:

1. Don't use any methods, which can change fields of your class. For example don't use Setters.
2. Avoid to use public non-final fields. If your fields is public then you must declare them as final and initialize them in constructor or directly in the declaration line.

Answer 9

It is too late to answer but may be it help other peoples who have this question.

1. State of immutable object can not be modified after construction, any modification should result in new immutable object.
2. All fields of Immutable class should be final.
3. Object must be properly constructed ie object reference must not leak during construction process.
4. Object should be final in order to restrict sub-class for altering immutability of parent class.

I think this link help more Read more: http://javarevisited.blogspot.com/2013/03/how-to-create-immutable-class-object-java-example-tutorial.html#ixzz40VDQDDL1

Answer 10

It already is immutable -- you can't change the contents once you initialize it, since you haven't made setters. You might add final keywords to the variables.

Answer 11

将所有变量设置为final并在设置某个字段时，使其返回对具有新设置值的新Student对象的引用，如String 。

Answer 12

您只需遵循本示例中显示的指南（谷歌中的第一个结果）： http : //www.javapractices.com/topic/TopicAction.do? Id= 29

Answer 13

Here are few rules, which helps to make a class immutable in Java : 1. State of immutable object can not be modified after construction, any modification should result in new immutable object. 2. All fields of Immutable class should be final. 3. Object must be properly constructed ie object reference must not leak during construction process. 4. Object should be final in order to restrict sub-class for altering immutability of parent class.

Example:

public final class Contacts {

private final String name;
private final String mobile;

public Contacts(String name, String mobile) {
    this.name = name;
    this.mobile = mobile;
}

public String getName(){
    return name;
}

public String getMobile(){
    return mobile;
}

}

Refer this link : http://javarevisited.blogspot.in/2013/03/how-to-create-immutable-class-object-java-example-tutorial.html

Answer 14

According to Strategy for Defining Immutable Objects

1. Don't provide " setter " methods — methods that modify fields or objects referred to by fields.

2. Make all fields final and private .

3. Don't allow subclasses to override methods. The simplest way to do this is to declare the class as final .

   a. A more sophisticated approach is to make the constructor private and construct instances in factory methods.

4. If the instance fields include references to mutable objects, don't allow those objects to be changed:

   a. Don't provide methods that modify the mutable objects.

   b.Don't share references to the mutable objects. Never store references to external, mutable objects passed to the constructor; if necessary, create copies, and store references to the copies. Similarly, create copies of your internal mutable objects when necessary to avoid returning the originals in your methods.

Answer 15

Java SE 16

You can use JEP 395: Records feature, introduced as part of Java SE 16, to create an immutable class in a succinct manner.

If you have already gone through the above link, you must have figured out that you can do it simply as

record Student(String name, String age) { }

What you get in turn are:

1. A final class Student .
2. A canonical constructor whose signature is the same as the header, Student(String name, String age) .
3. private final fields, name and age and their corresponding public accessor method with the same name and return type.
4. Automatically created equals , hashCode and toString methods.

Demo:

Student.java

record Student(String name, String age) { }

Main.java

class Main{
    public static void main(String[] args) {
        Student s1 = new Student("Bharat", "10 Years");
        Student s2 = new Student("Arvind", "10 Years");

        System.out.println(s1);
        System.out.println(s1.equals(s2));
        System.out.println(s1.age().equals(s2.age()));
    }
}

Output:

Student[name=Bharat, age=10 Years]
false
true

Answer 16

Make the class or variable as final that's more than enough

Public final class constants
{
  private final String name;
  private final String mobile;

  // code
}