java - 为什么我的 Intent 无法传递数据并接收intent.getintExtra空对象失败？

Question

我已经声明了我的变量并收集了有关我的主要 Activity 的数据。我已经设定了 Intent ，并在我的结果 Activity 中收集了 Intent 。问题似乎是数据没有从主 Activity 传递到所需的屏幕。

主要目标是在新的 Activity 屏幕上显示结果。

public class MainActivity extends AppCompatActivity {

  //variables
    Button btnCalculate, btnReset;
    Spinner spinSpeaker;
    EditText editLength, editWidth, editHeight;
    String t1,t2,t3;

    int numLength, numWidth, numHeight, numResult=0;


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

        //connecting variables to GUI elements
        btnCalculate= findViewById(R.id.btnCalculate);
        btnReset = findViewById(R.id.btnReset2);
        spinSpeaker = findViewById(R.id.spinSpeaker);
        editLength= findViewById(R.id.editLength);
        editWidth = findViewById(R.id.editWidth);
        editHeight = findViewById(R.id.editHeight);




        //adding function to the calculate button
        btnCalculate.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //parsing text to integers
                t1=editLength.getText().toString();
                t2=editWidth.getText().toString();
                t3=editHeight.getText().toString();

                if (t3.equals("")) {
                    editHeight.setText("11");
                }else if (t2.equals("")) {
                    editWidth.setText("11");
                }else if (t1.equals("")) {
                    editLength.setText("11");
                }else{
                    numLength = Integer.parseInt(t1);
                    numWidth = Integer.parseInt(t2);
                    numHeight = Integer.parseInt(t3);
                }

                //alert to contact Pro design
                if ((numLength>=999)||(numWidth>=999)||(numHeight>=20)) {

                    Toast.makeText(MainActivity.this, R.string.contactPro,
                            Toast.LENGTH_LONG).show();
                }else {
                    //calculating volume
                    numResult = numHeight * numWidth * numLength;

                    //sending data to other activities
                    Intent intent = new Intent(MainActivity.this, Result.class);
                    intent.putExtra("varHeight", numHeight);
                    intent.putExtra("varWidth", numWidth);
                    intent.putExtra("varLength", numLength);
                    intent.putExtra("varResult", numResult);
                    startActivity(intent);
                }
            }
        });
 //the following is the result activity
public class Result extends AppCompatActivity {

    //Pulling variables from main activity
    Intent variables = getIntent();
    //int length = variables.getIntExtra("varLength",0);
    //int width =variables.getIntExtra("varWidth",0);
    int totalResult = variables.getIntExtra("varResult",0);

    String result = String.valueOf(totalResult);
    TextView tim;

   Canvas canvas;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        canvas = new Canvas(this);
        setContentView(R.layout.activity_result);
        //setContentView(canvas);

        tim = findViewById(R.id.txtResult);
        tim.setText(result);

我预计 R.id.txtResult 会显示 numResult = numHeight * numWidth * numLength;就在第二个屏幕上。

我不断收到的错误消息是::

引起:java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法“int android.content.Intent.getIntExtra(java.lang.String, int)” 在 com.example.project.Result.(Result.java:23)

Answer 1

getIntent() 必须在 onCreate() 中调用。将 Result Activity 更改为:

public class Result extends AppCompatActivity {

    TextView tim;

    Canvas canvas;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        canvas = new Canvas(this);
        setContentView(R.layout.activity_result);
        //setContentView(canvas);

        Intent variables = getIntent();
        //int length = variables.getIntExtra("varLength",0);
        //int width =variables.getIntExtra("varWidth",0);
        int totalResult = variables.getIntExtra("varResult",0);

        String result = String.valueOf(totalResult);

        tim = findViewById(R.id.txtResult);
        tim.setText(result);
    }