Question

我无法在我的应用程序中找到崩溃的来源。我做了一个干净/构建。我是Android开发人员的新手，所以我还在学习。从查看logcat，我认为可能存在一些可能与calculate函数有关的null错误异常。任何帮助或指导表示赞赏。

java代码：

    package com.example.rectangle;

    import java.text.DecimalFormat;

    import android.support.v7.app.ActionBarActivity;
    import android.support.v7.app.ActionBar;
    import android.support.v4.app.Fragment;
    import android.content.Context;
    import android.os.Bundle;
    import android.view.KeyEvent;
    import android.view.LayoutInflater;
    import android.view.Menu;
    import android.view.MenuItem;
    import android.view.MotionEvent;
    import android.view.View;
    import android.view.ViewGroup;
    import android.view.inputmethod.InputMethodManager;
    import android.widget.Button;
    import android.widget.EditText;
    import android.widget.TextView;
    import android.widget.TextView.OnEditorActionListener;
    import android.os.Build;

    public class MainActivity extends ActionBarActivity implements OnEditorActionListener {

    //graphical variables
    private EditText widthInput;
    private EditText heightInput;
    private TextView areaOutput;
    private TextView perimeterOutput;
    private Button calculateButton;


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

        //assign ids to graphical variables
        widthInput = (EditText) findViewById(R.id.widthEditText);
        heightInput = (EditText) findViewById(R.id.heightEditText);
        areaOutput = (TextView) findViewById(R.id.areaOutput);
        perimeterOutput = (TextView) findViewById(R.id.perimterOutput);
        calculateButton = (Button) findViewById(R.id.calculateButton);

        widthInput.setOnEditorActionListener(this);
        heightInput.setOnEditorActionListener(this);

       //calculate and display
        calculateAreaAndPerimeter();

    }

  //hide keyboard
    //source: http://stackoverflow.com/questions/8697499/hide-keyboard-when-user-taps-anywhere-else-on-the-screen-in-android
    @Override
    public boolean onTouchEvent(MotionEvent event) {
        InputMethodManager imm = (InputMethodManager)getSystemService(Context.
                                                        INPUT_METHOD_SERVICE);
        imm.hideSoftInputFromWindow(getCurrentFocus().getWindowToken(), 0);
        return true;
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container, false);
            return rootView;
        }
    }



//display 
public void display(String area, String perimeter)
{
    areaOutput.setText(area);
    perimeterOutput.setText(perimeter);
}

//algorithms
private void calculateAreaAndPerimeter()
{
    //get width and height
    String widthInputString = widthInput.getText().toString();
    String heightInputString = heightInput.getText().toString();
    float width = Float.parseFloat(widthInputString);
    float height = Float.parseFloat(heightInputString);

    //calculate and display
    DecimalFormat df = new DecimalFormat();
    df.setMaximumFractionDigits(2);
    float area;
    area = width * height;
    String areaString = (df.format(area)) + "";
    float perimeter;
    perimeter = (width * 2) + (height * 2);
    String perimeterString = (df.format(perimeter)) + "";
    display(areaString, perimeterString);

}

@Override
public boolean onEditorAction(TextView v, int actionId, KeyEvent event) {
    // TODO Auto-generated method stub
    calculateAreaAndPerimeter();
    return false;
}

}

logcat：

11-12 13:05:14.049: E/AndroidRuntime(10535): FATAL EXCEPTION: main
11-12 13:05:14.049: E/AndroidRuntime(10535): Process: com.example.rectangle, PID: 10535
11-12 13:05:14.049: E/AndroidRuntime(10535): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.rectangle/com.example.rectangle.MainActivity}: java.lang.NullPointerException
11-12 13:05:14.049: E/AndroidRuntime(10535):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2198)
11-12 13:05:14.049: E/AndroidRuntime(10535):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2257)
11-12 13:05:14.049: E/AndroidRuntime(10535):    at android.app.ActivityThread.access$800(ActivityThread.java:139)
11-12 13:05:14.049: E/AndroidRuntime(10535):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210)
11-12 13:05:14.049: E/AndroidRuntime(10535):    at android.os.Handler.dispatchMessage(Handler.java:102)
11-12 13:05:14.049: E/AndroidRuntime(10535):    at android.os.Looper.loop(Looper.java:136)
11-12 13:05:14.049: E/AndroidRuntime(10535):    at android.app.ActivityThread.main(ActivityThread.java:5086)
11-12 13:05:14.049: E/AndroidRuntime(10535):    at java.lang.reflect.Method.invokeNative(Native Method)
11-12 13:05:14.049: E/AndroidRuntime(10535):    at java.lang.reflect.Method.invoke(Method.java:515)
11-12 13:05:14.049: E/AndroidRuntime(10535):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
11-12 13:05:14.049: E/AndroidRuntime(10535):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
11-12 13:05:14.049: E/AndroidRuntime(10535):    at dalvik.system.NativeStart.main(Native Method)
11-12 13:05:14.049: E/AndroidRuntime(10535): Caused by: java.lang.NullPointerException
11-12 13:05:14.049: E/AndroidRuntime(10535):    at com.example.rectangle.MainActivity.onCreate(MainActivity.java:46)
11-12 13:05:14.049: E/AndroidRuntime(10535):    at android.app.Activity.performCreate(Activity.java:5248)
11-12 13:05:14.049: E/AndroidRuntime(10535):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1110)
11-12 13:05:14.049: E/AndroidRuntime(10535):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2162)
11-12 13:05:14.049: E/AndroidRuntime(10535):    ... 11 more

Fragment_main

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity$PlaceholderFragment" >

    <TextView
        android:id="@+id/heightTextView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/widthTextView"
        android:layout_below="@+id/widthTextView"
        android:layout_marginTop="29dp"
        android:text="Height"
        android:textAppearance="?android:attr/textAppearanceMedium" />

    <TextView
        android:id="@+id/areaTextView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/heightTextView"
        android:layout_below="@+id/heightTextView"
        android:layout_marginTop="30dp"
        android:text="Area"
        android:textAppearance="?android:attr/textAppearanceMedium" />

    <TextView
        android:id="@+id/perimeterTextView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/areaTextView"
        android:layout_below="@+id/areaTextView"
        android:layout_marginTop="40dp"
        android:text="Perimeter"
        android:textAppearance="?android:attr/textAppearanceMedium" />

    <EditText
        android:id="@+id/heightEditText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBottom="@+id/heightTextView"
        android:layout_alignLeft="@+id/widthEditText"
        android:layout_alignRight="@+id/widthEditText"
        android:ems="10"
        android:inputType="numberSigned"
        android:text="0.0" />

    <TextView
        android:id="@+id/widthTextView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true"
        android:layout_marginTop="35dp"
        android:text="Width"
        android:textAppearance="?android:attr/textAppearanceMedium" />

    <EditText
        android:id="@+id/widthEditText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_above="@+id/heightTextView"
        android:layout_alignParentRight="true"
        android:ems="10"
        android:inputType="numberDecimal"
        android:text="0.0" />

    <TextView
        android:id="@+id/areaOutput"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBottom="@+id/areaTextView"
        android:layout_marginLeft="38dp"
        android:layout_toRightOf="@+id/perimeterTextView"
        android:text="0.0"
        android:textAppearance="?android:attr/textAppearanceMedium" />

    <TextView
        android:id="@+id/perimterOutput"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/perimeterTextView"
        android:layout_alignBottom="@+id/perimeterTextView"
        android:layout_alignLeft="@+id/areaOutput"
        android:text="0.0"
        android:textAppearance="?android:attr/textAppearanceMedium" />

</RelativeLayout>

Answer 1

试试这个：

TextView.OnEditorActionListener listener = new TextView.OnEditorActionListener() {

   @Override
   public boolean onEditorAction(TextView v, int actionId, KeyEvent event) {
      // TODO Auto-generated method stub
      calculateAreaAndPerimeter();
     return false;
    }
};

编译可能仍然想要你拥有的那个，如果，那么创建一个如上所述。

我认为你必须告诉编译器它是一个监听器，即使它可能已经创建了你的方法。 Android Studio不能很好地创建侦听器语法，我通常必须返回并使用侦听器的东西包围一个新的公共方法。希望有用！

应用程序崩溃（无法启动ComponentInfo错误）

1 个答案: