Initializing Fields Value in Constructor Versus in Fields Declaration

Question

As we know, in java and some other object-oriented programming languages, fields value can be set in constructor or be initialized in fields declaration statements. I want to know the essential differences between the two ways above. And what conditions I should initialze fields by constructor and what condtions I should't. Thanks for your help.

If the field in question is `final` (and not static at the same time), you have to initialize it in the constructor. — ppeterka, Aug 13 '13 at 08:48

score 5 · Accepted Answer · answered Aug 13 '13 at 08:47

The advantage with an argumented constructors is that you can set the field values as per the inputs but you cant do the same with initialized fields.

So if you want to create different objects with different attribute values then go for constructors with arguements. And assign the values of instance variables in your constructor.

If you simply want all the instance variables to have some default value, then assign the values at declaration.

score 1 · Answer 2 · answered Aug 13 '13 at 08:47

The convention is to initalize a field in the constructor. A constructor is there to essentially build the object. It only follows that building implies the instanciation, or assignment, of it's members. It also means that all your logic for creating the object is in one place, which is desirable. For example..

public NewObj(String something, int somethingElse)
{
    this.something = something;
    this.somethingElse = somethingElse;
    // All the logic is in one place. This is especially useful 
    // when dealing with huge classes.
}

We follow the conventions like this so when another programmer looks at our work, they will know instantly where to look for the logic that creates the class. By placing it in the field declarations (or worse, in both the field declarations and the constructor) you are confusing that convention.

Exceptions I've seen

When creating static final variables in a class, I've found that they are usually created in the field declaration. For example..

public class NewObj
{
     public static final int EMPTY_OBJ = 1;
     public static final int SPECIAL_OBJ = 2;
}

score 1 · Answer 3 · answered Aug 13 '13 at 08:51

final fields are preferred as they are easier to reason about and final fields can only be set in the constructor. I suggest fields which mandatory should be in the constructor.

If you have fields which you want to be able to change later, use setters.

If you have to set the fields by name, there is no easy way to this with a constructor. There is the @ConstructorProperties but few class have implemented this. Java 8 wills support access to argument names via reflection, but for now setters are easier.

Note: there are two alternatives.

You can use static factory methods. This can not only have meaningful names but can return sub-classes
You can use a builder class. This gives you more options for how you define and constructor your object. This is particularly useful if you have lots of mandatory fields.

score 0 · Answer 4 · answered Aug 13 '13 at 08:51

Using Constructors with Arguments , you can initialize the fields during the object creation with the input custom values provided.

This is often helpful when you want fields to have different values .

For Example:

class Employee{

String mName;

Employee(String name)
{
mName=name;
}

This can help you to initialize Employee names easily while creating Employee Object.

Initializing Fields Value in Constructor Versus in Fields Declaration

4 Answers4