How to Properly Initialize a Final Field in an Abstract Class in Java?

How can I properly initialize a final field in an abstract class in Java to avoid compiler warnings?

public abstract class Test {
    protected final ArrayList<Object> objects;

    // Constructor that subclasses will call
    protected Test(int size) {
        this.objects = new ArrayList<>(size);
    }
}

public class TestSubA extends Test {
    public TestSubA() {
        super(20);
        // Other initialization code
    }
}

public class TestSubB extends Test {
    public TestSubB() {
        super(100);
        // Other initialization code
    }
}

In Java, final fields must be initialized once, either at the point of declaration or within a constructor. When using an abstract class with a final field, it's crucial to ensure that initialization is handled correctly to avoid compiler warnings.

public abstract class Test {
    protected final ArrayList<Object> objects;

    // Constructor that subclasses will call
    protected Test(int size) {
        this.objects = new ArrayList<>(size);
    }
}

public class TestSubA extends Test {
    public TestSubA() {
        super(20);
        // Other initialization code
    }
}

public class TestSubB extends Test {
    public TestSubB() {
        super(100);
        // Other initialization code
    }
}

Causes

• The field is not initialized at its declaration.
• The final modifier enforces that it must only be assigned once, typically via a constructor.

Solutions

• Declare a constructor in the abstract class that takes parameters for initializing the final field.
• Call this constructor from the subclasses to provide the necessary values.

Mistake: Ignoring the need to initialize the final field in the abstract class.

Solution: Always define a constructor in the abstract class that initializes the final field.

Mistake: Not calling the constructor of the abstract class from the subclasses.

Solution: Make sure to use the 'super()' statement to call the abstract class constructor.