为什么displayCounts会崩溃我的应用？

Question

我无法理解为什么displayCounts会让我的应用程序崩溃.....它说我在空对象上调用虚方法....有人可以帮我吗？

基本上每次运行其中一个方法时它都会更新一个计数器，但每次我在displayCounts位中添加它都会崩溃。

package course.labs.activitylab;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;

import org.w3c.dom.Text;

public class ActivityOne extends Activity {

	// Use these as keys when you're saving state between reconfigurations
	private static final String RESTART_KEY = "restart";
	private static final String RESUME_KEY = "resume";
	private static final String START_KEY = "start";
	private static final String CREATE_KEY = "create";

	// String for LogCat documentation
	private final static String TAG = "Lab-ActivityOne";

	// Lifecycle counters

	// TODO:
  private  int mCreate = 0;
    private  int mRestart = 0;
    private int mResume = 0;
    private int mStart = 0;


    private TextView mTvCreate;
    private TextView mTvRestart;
    private TextView mTvStart;
    private TextView mTvResume;

	// Create variables named
	// mCreate, mRestart, mStart and mResume
	// to count calls to onCreate(), onRestart(), onStart() and
	// onResume(). These variables should not be defined as static.

	// You will need to increment these variables' values when their
	// corresponding lifecycle methods get called.

	// TODO: Create variables for each of the TextViews
	// named mTvCreate, mTvRestart, mTvStart, mTvResume.
	// for displaying the current count of each counter variable

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

		// TODO: Assign the appropriate TextViews to the TextView variables
        mTvCreate = (TextView)  findViewById(mCreate);
        mTvRestart = (TextView) findViewById(mRestart);
        mTvStart = (TextView)  findViewById(mStart);
        mTvResume = (TextView)   findViewById(mResume);

		// Hint: Access the TextView by calling Activity's findViewById()
		// textView1 = (TextView) findViewById(R.id.textView1);

		Button launchActivityTwoButton = (Button) findViewById(R.id.bLaunchActivityTwo);
		launchActivityTwoButton.setOnClickListener(new OnClickListener() {

			@Override
			public void onClick(View v) {



				// Launch Activity Two
				// Hint: use Context's startActivity() method

				// Create an intent stating which Activity you would like to
				// start
               Intent intent = new Intent(ActivityOne.this, ActivityTwo.class);
                startActivity(intent);

                // Launch the Activity using the intent

			}
		});

		// Has previous state been saved?
		if (savedInstanceState != null) {

			// TODO:
            mCreate = savedInstanceState.getInt(CREATE_KEY);
            mStart =savedInstanceState.getInt(START_KEY);
            mRestart = savedInstanceState.getInt(RESTART_KEY);
            mResume = savedInstanceState.getInt(RESUME_KEY);

			// Restore value of counters from saved state
			// Only need 4 lines of code, one for every count variable

		}

		// Emit LogCat message
		Log.i(TAG, "Entered the onCreate() method");

		// TODO:
        mCreate++;
       //displayCounts();
		// Update the appropriate count variable
		// Update the user interface via the displayCounts() method

	}

	// Lifecycle callback overrides

	@Override
	public void onStart() {
		super.onStart();

		// Emit LogCat message
		Log.i(TAG, "Entered the onStart() method");

		// TODO:
        mStart++;
        //displayCounts();
		// Update the appropriate count variable
		// Update the user interface

	}

	@Override
	public void onResume() {
		super.onResume();

		// Emit LogCat message
		Log.i(TAG, "Entered the onResume() method");

		// TODO:
        mResume++;
        //displayCounts();
		// Update the appropriate count variable
		// Update the user interface

	}

	@Override
	public void onPause() {
		super.onPause();

		// Emit LogCat message
		Log.i(TAG, "Entered the onPause() methodc");
	}

	@Override
	public void onStop() {
		super.onStop();

		// Emit LogCat message
		Log.i(TAG, "Entered the onStop() method");
	}

	@Override
	public void onRestart() {
		super.onRestart();

		// Emit LogCat message
		Log.i(TAG, "Entered the onRestart() method");

		// TODO:
        mRestart++;
        //displayCounts();
		// Update the appropriate count variable
		// Update the user interface

	}

	@Override
	public void onDestroy() {
		super.onDestroy();

		// Emit LogCat message
		Log.i(TAG, "Entered the onDestroy() method");
	}

	@Override
	public void onSaveInstanceState(Bundle savedInstanceState) {
		// TODO:
        savedInstanceState.putInt(RESUME_KEY, mResume);
        savedInstanceState.putInt(RESTART_KEY, mRestart);
        savedInstanceState.putInt(CREATE_KEY, mCreate);
        savedInstanceState.putInt(START_KEY, mStart);
		// Save state information with a collection of key-value pairs
		// 4 lines of code, one for every count variable

	}

	// Updates the displayed counters
	// This method expects that the counters and TextView variables use the
	// names
	// specified above
	public void displayCounts() {

		mTvCreate.setText("onCreate() calls: " + mCreate);
		mTvStart.setText("onStart() calls: " + mStart);
		mTvResume.setText("onResume() calls: " + mResume);
		mTvRestart.setText("onRestart() calls: " + mRestart);

	}
}

Answer 1

在onCreate中，您将mTvCreate，mTvStart等视图分配给findViewById（0），该视图在成员变量声明中被注释为TODO。在您正在膨胀的activity_one.xml文件中为这些视图分配ID，并将这些ID传递给findViewById。那么你应该得到正确的视图引用而不是null。

Answer 2

看起来正在运行的Coursera类中的代码。调用findViewById（）时，你必须引用布局文件中的ID;将采用“R.id.create”格式。

Answer 3

似乎你必须写：

mTvCreate = (TextView) findViewById(R.id.create); 
mTvRestart = (TextView) findViewById(R.id.restart); 
mTvResume = (TextView) findViewById(R.id.resume); 
mTvStart = (TextView) findViewById(R.id.start);