How to Resolve the Java 'Blank Final Field May Not Have Been Initialized' Exception?

What causes the 'blank final field may not have been initialized' exception in Java?

public class Example {
    private final int value;

    public Example(int value) {
        this.value = value; // This initializes the final field
    }
}

The 'blank final field may not have been initialized' exception occurs in Java when a final variable is declared but not initialized in all constructors. A final variable must be assigned exactly once, and this rule must be adhered to, even when dealing with constructors. If you try to use a blank final variable before it has been initialized, the Java compiler will throw this exception.

public class Product {
    private final String name;
    private final double price;

    public Product(String name, double price) {
        this.name = name;
        this.price = price; // Both fields initialized correctly
    }

    public Product(String name) {
        this(name, 0.0); // Calls the other constructor to avoid initialization issues
    }
}

Causes

• A final field is declared but not initialized in the constructor.
• Multiple constructors exist and not all of them initialize the final field.
• The final field is assigned conditionally, but not in all execution paths of the constructor.

Solutions

• Ensure that all constructors in the class initialize the final field.
• Use an initialization block to set the final field if constructors vary.
• Provide default values for final fields directly during declaration.

Mistake: Not initializing the final field in one of the constructors.

Solution: Ensure all constructors initialize the final field or make use of a common constructor.

Mistake: Conditionally assigning the final field in the constructor.

Solution: Rearrange the logic to guarantee the field is set in all paths of the constructor execution.