Question

当我尝试启动此活动时，它不会运行。

LogCat显示以下错误：

07-25 12:10:46.813: E/AndroidRuntime(797): FATAL EXCEPTION: main
07-25 12:10:46.813: E/AndroidRuntime(797): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.testingandroid/com.example.testingandroid.DoSums}: java.lang.NullPointerException

我的代码：

public class DoSums extends Activity implements OnClickListener {
    public RadioButton r1 = (RadioButton) findViewById(R.id.doradioButton1);
    public RadioButton r2 = (RadioButton) findViewById(R.id.doradioButton2);
    public RadioButton r3 = (RadioButton) findViewById(R.id.doradioButton3);
    public RadioButton r4 = (RadioButton) findViewById(R.id.doradioButton4);

    public EditText et = (EditText) findViewById(R.id.doeditText1);
    //Button btnNextScreen = (Button) findViewById(R.id.button1);

    timeanddistanceM pen = new timeanddistanceM();

    public String Q, A, finalvalue, scale;
    public double fv;

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

        objCreator();
        //btnNextScreen.setOnClickListener(this);
        r1.setOnClickListener(this);
        r2.setOnClickListener(this);
        r3.setOnClickListener(this);
        r4.setOnClickListener(this);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.do_sums, menu);
        return true;
    }

    public void objCreator() {

        pen.timeanddistanceM();
        // Setting question
        String Q = pen.Q;
        et.setText("Question" + Q);
        // Setting options

        // 1. Getting options
        options ops = new options();
        ops.optionsCreator(pen.fv, pen.scale);

        // 2.Assigning options
        r1.setText(ops.jRadioButton1);
        r2.setText(ops.jRadioButton2);
        r3.setText(ops.jRadioButton3);
        r4.setText(ops.jRadioButton4);
    }

    public void RadiobuttonAction(RadioButton testRadio) {

        if (testRadio.getText().toString().equals(pen.finalvalue)) {
            objCreator();
        }
    }

    @Override
    public void onClick(View arg0) {

        switch (arg0.getId()) {
        /*case R.id.doradioButton1:
            pen.timeanddistanceM();
            String Q = pen.Q;
            et.setText("Question" + Q);
            break;*/
        case R.id.doradioButton1:
            RadiobuttonAction(r1);
            break;

        case R.id.doradioButton2:
            RadiobuttonAction(r2);
            break;

        case R.id.doradioButton3:
            RadiobuttonAction(r3);
            break;

        case R.id.doradioButton4:
            RadiobuttonAction(r4);
            break;
        }
    }
}

Answer 1

在将内容视图设置为Activity之后，您应该找到自己的观点：

public RadioButton r1; public RadioButton r2; public RadioButton r3; public RadioButton r4; public EditText et; protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_do_sums); //here you should retreive your views by its ids r1 = (RadioButton) findViewById(R.id.doradioButton1); r2 = (RadioButton) findViewById(R.id.doradioButton2); r3 = (RadioButton) findViewById(R.id.doradioButton3); r4 = (RadioButton) findViewById(R.id.doradioButton4); et = (EditText) findViewById(R.id.doeditText1);

Answer 2

必须在findViewById之后调用所有setContentView。如果在活动的视图层次结构不存在之前调用。移动

public RadioButton r1 = (RadioButton) findViewById(R.id.doradioButton1);
public RadioButton r2 = (RadioButton) findViewById(R.id.doradioButton2);
public RadioButton r3 = (RadioButton) findViewById(R.id.doradioButton3);
public RadioButton r4 = (RadioButton) findViewById(R.id.doradioButton4);

public EditText et = (EditText) findViewById(R.id.doeditText1);


 protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_do_sums);
    r1 = (RadioButton) findViewById(R.id.doradioButton1);
    // etc
 }

Answer 3

在EditText中初始化您的RadioButtons和onCreate()，如下所示，并确保在您的清单中添加您的活动

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_do_sums);
        r1 = (RadioButton) findViewById(R.id.doradioButton1);
        r2 = (RadioButton) findViewById(R.id.doradioButton2);
        r3 = (RadioButton) findViewById(R.id.doradioButton3);
        r4 = (RadioButton) findViewById(R.id.doradioButton4);
        et = (EditText) findViewById(R.id.doeditText1); 
    }

Answer 4

由于未创建布局，因此无法在findViewById之前使用setContentView。

Answer 5

好的它没有用，因为你之前在oncreate方法上不能这样做。

public EditText et = (EditText) findViewById(R.id.doeditText1);
//Button btnNextScreen = (Button) findViewById(R.id.button1);

timeanddistanceM pen = new timeanddistanceM();

必须是

public EditText et;
timeanddistanceM pen;

关于你的oncreate方法

et = (EditText) findViewById(R.id.doeditText1);
pen = new timeanddistanceM();

对您的单选按钮执行相同的操作

Android活动无法运行。这是代码

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