Question

我做了一个叫Point的课。我在MainActivity上创建一个名为“ A”的对象。现在，我想要MainActivity上的一些方法，该方法应该读取“ A”的属性，但是这是不可能的。为什么我不能访问Point类的自制对象“ A”？例如，方法“加法”无法读取对象“ 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>();

}

ll

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

尝试此操作将您的 Point A 声明为全局

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;

    }

}

我无法访问对象属性

2 个答案: