Should I Initialize Variables Within a Constructor or Outside in Java?

What is the best practice for initializing variables in Java: within the constructor or outside of it?

public class ME {
    private int i;

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

// vs 

public class ME {
    private int i = 100;

    public ME() {
    }
}

In Java, the decision to initialize instance variables within the constructor or directly at the declaration can impact code readability, maintainability, and functionality. Both approaches have their merits and use cases, depending on the specific needs of the class design.

public class Example {
    private int a; // Declaration without immediate initialization
    private int b = 20; // Direct initialization

    public Example(int a) {
        this.a = a; // Initialization in constructor
    }
}

Causes

• Maintainability: Initializing variables outside the constructor can make it easier to see default values at a glance.
• Consistency: Using one technique across a codebase can help with consistency and readability.
• Initialization Logic: When complex initialization logic is required, a constructor may be preferred.

Solutions

• Use direct initialization for simple, default values to improve readability.
• Reserve constructors for initializations requiring logic or the use of multiple parameters.

Mistake: Initializing a variable in both the declaration and the constructor.

Solution: Choose one method of initialization to avoid confusion and potential bugs.

Mistake: Neglecting to initialize variables leading to 'null' or default values unexpectedly.

Solution: Always initialize variables explicitly to maintain predictable behavior.