Why Are My Java Instance Variables Uninitialized Despite Appearing to Be Initialized in the Constructor?

Confusion over Field Initialization Due to Shadowing

In Java, shadowing refers to the phenomenon where a variable declared within a scope obscures variables with the same name declared in an outer scope. In the provided code snippet:

class StringArray {
    private String[] elements;
    private int capacity;

    public StringArray() {
        int capacity = 10;  // Local variable shadowing field
        String[] elements;  // Local variable shadowing field
        elements = new String[capacity];
    }

    // Getters omitted for brevity
}

Local Variable Shadowing

The first and second statements inside the constructor body declare local variables named capacity and elements. These local variables shadow the instance variables of the same name. Consequently, any assignments or references to capacity and elements within the constructor scope exclusively affect the local variables.

Shadowing Rules

Shadowing rules dictate that local variables have higher precedence than instance variables within their scope. This means that the local variable capacity, initialized to 10, takes precedence over the instance variable capacity, leaving the instance variable uninitialized. Similarly, the assignment to elements initializes the local variable, not the instance variable.

Field Initialization

Since the instance variables are not explicitly initialized in the constructor, they are assigned the default values for their respective types: 0 for capacity and null for elements. This is why the getter methods in the class return these default values.

Resolution

To resolve this issue, eliminate the local variable declarations and refer to the instance variables directly:

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

Avoidance

To prevent such confusion, adopt the practice of using unique names for local variables and instance variables.

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