Why Are My Java Fields Null Despite Initialization in the Constructor?

Confusion in Java Field Initialization: The Role of Shadowing

When declaring and initializing fields in a class constructor, you might encounter a puzzling occurrence: the fields remain null or retain their default values despite your initialization efforts. This happens due to a phenomenon known as shadowing.

Scope and Shadows

Every entity in Java has a scope, which determines where it can be referenced using its simple name. When you declare fields within a class, they have a scope that encompasses the entire class body, including nested declarations.

Local Variables vs. Fields

In the constructor, you may inadvertently introduce local variables with the same names as your fields. Local variables have a narrower scope, only extending within the constructor body. When local variables are declared with the same names as fields, they shadow the fields, meaning the local variable takes precedence within its scope.

Example of Shadowing:

Consider this code:

public class Sample {

    private String[] elements;
    private int capacity;

    public Sample() {
        int capacity = 10;
        String[] elements;
        elements = new String[capacity];
    }
}

Here, the constructor shadowing is causing the confusion. The local variables capacity and elements take precedence over the fields, resulting in the field members remaining null or initialized to their default values (0 for int, null for references).

Potential Solution:

To avoid shadowing, consider using unique names for local variables or explicitly refer to the field using its qualified name (e.g., this.elements, this.capacity). For example:

public Sample() {
    this.capacity = 10;
    this.elements = new String[capacity];
}

Other Instances of Shadowing

Shadowing can also occur with constructor parameters. If a constructor parameter has the same name as a field, the parameter takes precedence within the constructor body. To access the field in such cases, use the qualified name with this.

Avoidance Recommendation:

To prevent unexpected behavior due to shadowing, it's generally advisable to use distinct names for fields, local variables, and constructor parameters.

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