Java Variable Initialization: Constructor vs. Declaration – Which is Better?

Variable Initialization in Constructors

When creating Java objects, you have two options for initializing instance variables:

Instantiation on Declaration

class A {
    B b = new B();
}

Instantiation in the Constructor

class A {
    B b;

    A() {
         b = new B();
    }
}

Advantages of Each Approach:

Identical Behavior:
The compiler automatically generates equivalent initialization code for either approach, so there is no functional difference.

Readability:
Some developers prefer the first approach as it provides more immediate clarity on which variables are being initialized.

Exception Handling:
In the second approach, you can use exception handling in the constructor to handle potential errors during variable initialization.

Initialization Blocks:
In addition to the above, you can also use initialization blocks to initialize variables. Similar to field declarations, initialization blocks are automatically placed in constructors.

{
    a = new A();
}

Lazily Initialization:
If initializing a variable is expensive, you may opt for lazy initialization by setting it within a getter method that only creates the object when needed.

ExpensiveObject o;

public ExpensiveObject getExpensiveObject() {
    if (o == null) {
        o = new ExpensiveObject();
    }
    return o;
}

Dependency Management:
To enhance dependency management, consider avoiding direct instantiation with the new operator. Instead, adopt dependency injection, where another class or framework handles object creation and dependency injection.

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