Should I initialize variable within constructor or outside constructor

java

best-practices

constructor

initialization

byAnton Shumikhin·Sep 13, 2024

Initialize static or default instance variables outside the constructor when assigning common or constant values. Constructor initialization should be utilized when variables depend on constructing parameters or require complex setup.

Example:

public class Car {
    private static final String DEFAULT_COLOR = "Red"; // Static constant, best initialized inline
    private int serialNumber;

    public Car(int year) {
        this.serialNumber = computeSerialNumber(year); // Varying with parameter, inside the constructor
    }
}

DEFAULT_COLOR is a static constant which is best initialized inline. serialNumber is dependent on year, thus it's initialized within the constructor post computation. This brings clarity and utilizes parameterization.

Constructor initialization: When and why?

Using constructors to initialize variables becomes valuable when these variables need to be assigned values based on construction parameters or dynamic variables. This method allows for custom logic which may be necessary for correctly configuring an object.

If your code involves inheritance, where sub-constructors may depend on super-constructors execution to establish a consistent state, constructor initialization ensures precedence handling, i.e., parent class execution before the child class specific initialization.

Readability and maintenance of your Java code can progress leaps and bounds with variable initialization inside the constructor, keeping it close to the context of usage.

Juggling multiple constructors

In the scenario where a class has multiple constructors, you can circumvent initialization redundancy by using constructor chaining. This is facilitated by a this() call. An initial no-arg constructor sets the default values, then chained to parameterized constructors.

Example of constructor chaining:

public class House {
    private String address;
    private int squareFeet;

    public House() {
        this("Nowhere Land", 0); // Default values, aka hide and seek champions.
    }

    public House(String address, int squareFeet) {
        this.address = address;
        this.squareFeet = squareFeet;
    }
}

Resource shared initialization logic across all constructors might call for an instance initializer block.

Inline initialization for simpler cases

For static variables or when default values can be defined without external correlation, initializing outside the constructor is beneficial. It keeps your code cleaner and reduces constructor clutter.

Remember, static initializer blocks can deal with complex static variable initialization that inline initialization might not handle. These are executed once when the class is loaded, optimizing resource allocation.

In complex frameworks, like those using ORM, initializing variables judiciously is paramount for compatibility with persistence mechanisms like Hibernate or Eclipselink. This ensures that entities behave appropriately within the ORM once set up.

Weighing the options

Choosing where to initialize variables is a trade-off between immediacy (inline initialization) and flexibility (constructor initialization).

Proper encapsulation is critical. Sometimes, constructor initialization is the most effective way to prevent variables from entering unwarranted states. This ensures the creation of an object always in a valid state.

Utilization strategies for effective initialization

Opt for these best practices to optimize usage:

Constructor overloading and chaining - Establishes a hierarchy.
Instance initializer blocks - Useful for anonymous classes; use cautiously, not a party trick.
Separation of concerns - Complex logic in initialization = factory methods or builder pattern's calling.
Static initialization blocks - Don't ignore class loading behaviors, they're like the in-laws of Java.