确保字段是从构造函数初始化的

Question

Edit: Sorry for my previous question, the final keyword will confusing you, It's not appropriate. And sorry for my poor English too.

-----Question------

Assume I have a model class without any constructor, because I need to initialize it within many different logic. But i have a notnull keyword.

public class Model {
    public notnull String name;
    public notnull String address;
    public notnull int age;
}

And I need to use it in different place but keep checking if all notnull fields were initialized in compile time

Model m = new Model();
m.name = "Hello";
m.age = 15;
// syntax error, you should set Model.address

Or

Model m1 = new Model();
m1.address = "World";
m1.age = 20;
// syntax error, you should set Model.name

How can I achieve that in java? Thx.

Answer 1

assume I have a model class without any constractor

This is not possible. Every class has a constructor. If you do not define one you end up with the default constructor. In your case:

public Model() {
    super();
}

checking if all final field were initialized

First, final fields must be initialized, otherwise the compiler will not compile. Hence you have to have to initialize final fields either:

• in the constructor
• when declaring the final field

Constructor Initialization

public final String name;
public final int age;

public Model() {
    this.name = "Hello";
    this.age = 13;
}

Declaration Initialization

public final String name = "Hello";
public final int age = 13;

public Model() {
    super();
}

Answer 2

You can achieve what you seems to want with something like this:

public class Model {
    private final String name;
    private final String address;
    private final int age;

    public Model(final String name, final String address, final int age) {
        if (name == null) {
            throw new IllegalArgumentException("please provide a name");
        }
        if (address == null) {
            throw new IllegalArgumentException("please provide an address");
        }
        this.name = name;
        this.address = address;
        this.age = age;
    }

    // getters
}

Usage:

new Model("World", "Some address", 20) // OK
new Model("World", null, 20) // Exception because null address.

Answer 3

You should look at the Builder Pattern. There are number of articles that you can find on the internet on how to use it. This is one example https://jlordiales.me/2012/12/13/the-builder-pattern-in-practice/ . Using the Builder pattern you can validate if certain fields are set or not and throw exception if there are not.

Answer 4

If only final : You can either instantiate them where you declare or instantiate inside the constructor.

If final + static: You MUST instantiate them where you declared.

So since they are not static you need to instantiate where you specified them or in the constructor.

Answer 5

Assume I have a model class without any constructor because I need to initialize it within many different logic. But I have a notnull keyword.

You need to note that even if you don't write any constructor, the compiler will provide a default no-argument constructor which will initialize all fields in the object to the default values automatically (ie, String types to null and int types to zero values, etc..)

---

The class constructor is a right place to validate all mandatory fields are provided or not , so create your own constructor to check all mandatory fields are passed in during the object creation:

public class Model {
    private String name;
    private String address;
    private int age;

    public Model(String name, String address, int age) {
         if(name == null) { 
               throw new IllegalArgumentException(" Name is mandatory ");
         } 
         if(address == null) { 
               throw new IllegalArgumentException(" Address is mandatory ");
         }
         if(age < 0) { 
               throw new IllegalArgumentException(" Age is Invalid ");
         }
         this.name = name;
         this.address = address;
         this.age = age;
    }
   //Add getters and setters
}

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Also, encapsulate the data of your Model class by marking your fields private (shown above).