Should I initialize variable within constructor or outside constructor [duplicate]

When I use Java based on my C++ knowledge, I love to initialize variable using the following way.

public class ME {
    private int i;

    public ME() {
         this.i = 100;
    }
}

After some time, I change the habit to

public class ME {
    private int i = 100;

    public ME() {
    }
}

I came across others source code, some are using 1st convention, others are using 2nd convention.

May I know which convention do you all recommend, and why?

Solution 1:

I find the second style (declaration + initialization in one go) superior. Reasons:

• It makes it clear at a glance how the variable is initialized. Typically, when reading a program and coming across a variable, you'll first go to its declaration (often automatic in IDEs). With style 2, you see the default value right away. With style 1, you need to look at the constructor as well.
• If you have more than one constructor, you don't have to repeat the initializations (and you cannot forget them).

Of course, if the initialization value is different in different constructors (or even calculated in the constructor), you must do it in the constructor.

Solution 2:

I have the practice (habit) of almost always initializing in the contructor for two reasons, one in my opinion it adds to readablitiy (cleaner), and two there is more logic control in the constructor than in one line. Even if initially the instance variable doesn't require logic, having it in the constructor gives more flexibility to add logic in the future if needed.

As to the concern mentioned above about multiple constructors, that's easily solved by having one no-arg constructor that initializes all the instance variables that are initilized the same for all constructors and then each constructor calls this() at the first line. That solves your reduncancy issues.

Solution 3:

I tend to use the second one to avoid a complicated constructor (or a useless one), also I don't really consider this as an initialization (even if it is an initialization), but more like giving a default value.

For example in your second snippet, you can remove the constructor and have a clearer code.

Solution 4:

If you initialize in the top or in constructor it doesn't make much difference .But in some case initializing in constructor makes sense.

class String
{
    char[] arr/*=char [20]*/; //Here initializing char[] over here will not make sense.
    String()
    {
        this.arr=new char[0];
    }
    String(char[] arr)
    {
        this.arr=arr;
    }
}

So depending on the situation sometime you will have to initialize in the top and sometimes in a constructor.

FYI other option's for initialization without using a constructor :

class Foo
{
    int i;
    static int k;

    //instance initializer block
    {
        //run's every time a new object is created
        i=20;
    }

    //static initializer block
    static{
        //run's only one time when the class is loaded
        k=18;
    }    
}