[已解决] 没有setcontentview 就使用findViewById 创建的变量会出现null exception

 

package bjpkten.permissionrelateddemo;

import android.Manifest;
import android.content.pm.PackageManager;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.widget.TextView;
import android.widget.Toast;

import java.security.Permission;

public class MainActivity extends AppCompatActivity {

    private TextView mTextView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // setContentView(R.layout.activity_main);

        mTextView = (TextView) findViewById(R.id.text);

    }

    @Override
    protected void onResume() {
        super.onResume();
        mTextView.setText(R.string.app_name);

    }
}

 

 

报错：

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: bjpkten.permissionrelateddemo, PID: 23988
    java.lang.RuntimeException: Unable to resume activity {bjpkten.permissionrelateddemo/bjpkten.permissionrelateddemo.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(int)' on a null object reference
        at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3784)
        at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3816)
        at android.app.servertransaction.ResumeActivityItem.execute(ResumeActivityItem.java:51)
        at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:145)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:70)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(int)' on a null object reference
        at bjpkten.permissionrelateddemo.MainActivity.onResume(MainActivity.java:37)
        at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1412)
        at android.app.Activity.performResume(Activity.java:7292)
        at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3776)
        at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3816) 
        at android.app.servertransaction.ResumeActivityItem.execute(ResumeActivityItem.java:51) 
        at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:145) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:70) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:193) 
        at android.app.ActivityThread.main(ActivityThread.java:6669) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)