我无法访问objects属性

Question

我做了一个叫Point的课。我在MainActivity上创建了一个名为"A“的对象。现在，我想要一些MainActivity上的方法，这些方法应该读取"A“的属性，但这是不可能的。为什么我不能访问我的自制对象"A“的类点？例如，“加法”方法不能读取对象"A“的属性"x”和"y“。为什么？

public class Point {

    public int x;
    public int y;
    public Map <String, Integer> paths = new HashMap<String, Integer>();

}

lll

public class MainActivity extends AppCompatActivity {

Point A, B, C, D;
int i;
TextView tv;


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

        tv = findViewById(R.id.tv);

        Point A = new Point();
        A.x = 1;
        A.y = 1;
        A.paths.put("B", 1);
        A.paths.put("C", 2);

        addition();

    }

    private void addition() {


    tv.setText(""+A.x);

    }
}

编辑:tv.setText(“+A.x)；现在给我一个NullpointerExeption

 Caused by: java.lang.NullPointerException: Attempt to read from field 'int com.example.georg.pathfinding.Point.x' on a null object reference
        at com.example.georg.pathfinding.MainActivity.addition(MainActivity.java:57)
        at com.example.georg.pathfinding.MainActivity.onCreate(MainActivity.java:49)

Answer 1

尝试将您的Point A声明为全局的

public class MainActivity extends AppCompatActivity {

        Point A, B, C, D;
        int i;
        TextView tv;
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);

            tv = findViewById(R.id.tv);

             A = new Point();
            A.x = 1;
            A.y = 1;
            A.paths.put("B", 1);
            A.paths.put("C", 2);

            addition();

        }

        private void addition() {

             tv.setText(""+A.x);

        }

    }

Answer 2

将值传递给您的方法：-

public class MainActivity extends AppCompatActivity {

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

        Point A = new Point();
        A.x = 1;
        A.y = 1;
        A.paths.put("B", 1);
        A.paths.put("C", 2);

        addition(A.x, A.y);

    }

    private void addition(int a, int b) {

         int i;
         i = a + b;

    }

}