Calling Java method references Standard Pattern

I have ClassA object that sets method references inside of ClassB1 and ClassB2 objects. ClassB1 and ClassB2 objects later use this method reference while running their methods. But, sometimes we do not set the method reference:

public class ClassA {
    public ClassA() {
        ClassB1 objB1 = new ClassB1();
        ClassB2 objB2 = new ClassB2();
        objB1.setFuncitonA(this::functionA);
        objB1.setFuncitonA(this::functionA);
        objB1.functionB();
        objB2.functionB();
    }
    public void functionA(Integer x) {
        x *= 2;
    }
}

public class ClassB1 {
    private Integer intObjB = new Integer(2);
    private Consumer<Integer> functionA = null;
    public void functionB() {
        if(functionA != null) {
            functionA.accept(intObjB);
        }
    }
    public void setFuncitonA(Consumer<Integer> functionA) {
        this.functionA = functionA;
    }
}

public class ClassB2 {
    private Integer intObjB = new Integer(2);
    private Consumer<Integer> functionA = this::defaultFunctionA;
    public void functionB() {
        functionA.accept(intObjB);
    }
    public void setFuncitonA(Consumer<Integer> functionA) {
        this.functionA = functionA;
    }
    public void defaultFunctionA(Integer intObj) {
        return;
    }
}

Should it be like in ClassB1 or like in ClassB2, or, it does not matter at all? What is the standard pattern of writing such code, if there is any?