Java Constructor Initialization: Inside or Outside the Constructor?

Initializer Placement in Constructors: Inside vs. Outside

When initializing instance variables in Java, developers encounter the choice between placing the initialization within the constructor (e.g., this.i = 100;) or directly in the variable declaration (e.g., private int i = 100;). This article explores the recommended convention and the reasons behind it.

Initializer Placement Recommendations

The recommended practice is to declare and initialize variables in one line outside the constructor, as in the example below:

public class ME {
    private int i = 100;

    public ME() {
    }
}

Reasons for Initialization Outside Constructor

• Improved Clarity: Initializing variables inline provides immediate insight into their default values. In IDEs, accessing variable declarations often triggers the display of the declared value. In contrast, constructor-initialized variables require additional navigation to determine their values.
• Consistency Across Constructors: If the initialization value remains consistent across constructors, declaring the variable with an initial value reduces the potential for errors and ensures that the initialization is not overlooked in alternative constructors.
• Simplified Maintenance: When making code modifications, it is easier to locate and manage initializations when they are declared with the variables themselves.

Exceptions to the Rule

The recommendation to initialize variables outside the constructor does not apply in all cases. When the initialization value varies based on the constructor or is dynamically calculated within the constructor, the initialization must occur within the constructor. For example:

public class ME {
    private int i;

    public ME(int initialValue) {
        this.i = initialValue;
    }
}

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