您正在尝试
**xml**创建实例之前访问实例，这会使应用程序崩溃。
要检查由于您的应用崩溃而引起的故障，您可以参考检 Logcat
入您的IDE。
将您的实例化代码放入
onCreate()回调中：-
package com.example.android.testapplication;import android.support.v7.app.AppCompatActivity;import android.os.Bundle;import android.view.View;import android.widget.Button;import android.widget.TextView;import android.widget.Toast;public class MainActivity extends AppCompatActivity {final TextView mShowCounter;final Button button;private int mCount = 0;@Overrideprotected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); mShowCounter = (TextView) findViewById(R.id.text_view_counter); button = (Button) findViewById(R.id.toast_button);}public void showToast(View view) { Toast toast = Toast.makeText(this, R.string.toast_popup,Toast.LENGTH_LONG); toast.show();}public void counterUp(View view) { mCount++; if(mShowCounter != null) { mShowCounter.setText(Integer.toString(mCount)); }}}
