How to avoid error "Variable is accessed from within inner class, needs to be declared final."

Question

I was working on a floating action button activity and I encountered this error. How can this type of errors be avoided from next time?

Also, If you have any other ways to code the floating action button activity, please do let me know.

public class FabActivity extends AppCompatActivity {
    FloatingActionButton fab_cross, fab_plus, fab_list, fab_del, fab_upd;
    Animation fab_open, fab_close, rotate_clockwise, rotate_anticlockwise;
    boolean isOpen = false;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_fab);
    }

    public void setFab_cross(FloatingActionButton fab_cross) {
        this.fab_cross = fab_cross;

        fab_cross = (FloatingActionButton) findViewById(R.id.fab_cross);
        fab_cross.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                if(isOpen){
                    fab_plus.startAnimation(fab_close);
                    fab_list.startAnimation(fab_close);
                    fab_del.startAnimation(fab_close);
                    fab_upd.startAnimation(fab_close);
                    fab_cross.startAnimation(rotate_anticlockwise);
                    fab_plus.setClickable(false);
                    fab_list.setClickable(false);
                    fab_del.setClickable(false);
                    fab_upd.setClickable(false);
                    isOpen = false;

                }
                else
                {
                    fab_plus.startAnimation(fab_open);
                    fab_list.startAnimation(fab_open);
                    fab_del.startAnimation(fab_open);
                    fab_upd.startAnimation(fab_open);
                    fab_cross.startAnimation(rotate_clockwise);
                    fab_plus.setClickable(true);
                    fab_list.setClickable(true);
                    fab_del.setClickable(true);
                    fab_upd.setClickable(true);
                    isOpen = true;


                }
            }
        });
    }

    public void setFab_plus(FloatingActionButton fab_plus) {
        this.fab_plus = fab_plus;

        fab_plus = (FloatingActionButton) findViewById(R.id.fab_plus);
    }

    public void setFab_list(FloatingActionButton fab_list) {
        this.fab_list = fab_list;

        fab_list = (FloatingActionButton) findViewById(R.id.fab_list);
    }

    public void setFab_del(FloatingActionButton fab_del) {
        this.fab_del = fab_del;

        fab_del = (FloatingActionButton) findViewById(R.id.fab_del);
    }

    public void setFab_upd(FloatingActionButton fab_upd) {
        this.fab_upd = fab_upd;

        fab_upd = (FloatingActionButton) findViewById(R.id.fab_upd);
    }

    public void setFab_open(Animation fab_open) {
        this.fab_open = fab_open;

        fab_open = AnimationUtils.loadAnimation(getApplicationContext(),R.anim.fab_open);
    }

    public void setFab_close(Animation fab_close) {
        this.fab_close = fab_close;

        fab_close = AnimationUtils.loadAnimation(getApplicationContext(),R.anim.fab_close);
    }

    public void setRotate_clockwise(Animation rotate_clockwise) {
        this.rotate_clockwise = rotate_clockwise;

        rotate_clockwise = AnimationUtils.loadAnimation(getApplicationContext(),R.anim.rotate_clockwise);
    }

    public void setRotate_anticlockwise(Animation rotate_anticlockwise) {
        this.rotate_anticlockwise = rotate_anticlockwise;

        rotate_anticlockwise = AnimationUtils.loadAnimation(getApplicationContext(),R.anim.rotate_anticlockwise);
    }
}