Why Are My Java Constructor Fields Returning Null or Default Values?

Confusion over Field Initialization in Java Constructor

Developers may encounter situations where fields declared and initialized in a class constructor return null or default values when queried. This behavior stems from the concept of shadowing in Java.

Shadowing with Local Variables

When local variables are declared within a constructor, they possess the same names as instance variables. However, Java prioritizes local variables within their scope, overshadowing the instance variables. Consider the following example:

public class Sample {
    private String[] elements;
    private int capacity;

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

In this constructor, the local variable capacity is initialized to 10, but the instance variable capacity remains uninitialized, resulting in a default value of 0. Similarly, the local variable elements is assigned an array reference, but the instance variable elements remains null.

Shadowing with Constructor Parameters

Constructor parameters can also shadow instance variables with the same name. The parameter's declaration takes precedence, making it impossible to access the instance variable directly. To refer to the instance variable, use the qualified name with the this primary expression, as in:

public Sample(int capacity) {
    this.capacity = capacity;
}

Recommendation

To avoid confusion, it is best practice to use unique names for local variables, constructor parameters, and instance variables whenever possible. This prevents accidental shadowing and ensures that fields are initialized as intended.

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