java.lang.RuntimeException : Unable to start activity ComponentInfo Android Manifest 错误

Question

我遇到了一个奇怪的错误，我完全不确定它是从哪里来的，android list 似乎没有任何我能看到的问题。我还添加了主要 Activity 。

错误日志

11-27 09:53:12.364 2553-2553/? E/AndroidRuntime: FATAL EXCEPTION: main
                                                 Process: com.example.capstoneproject.capstoneproject, PID: 2553
                                                 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.capstoneproject.capstoneproject/com.example.capstoneproject.capstoneproject.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.widget.Button.findViewById(int)' on a null object reference
                                                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
                                                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
                                                     at android.app.ActivityThread.access$800(ActivityThread.java:151)
                                                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)

Android list

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name"
            android:theme="@style/AppTheme.NoActionBar">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

主要 Activity

package com.example.capstoneproject.capstoneproject;

import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.Button;
import android.widget.TextView;


public class MainActivity extends AppCompatActivity {
    Button ConnectButton;
    TextView StatusText;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        ConnectButton.findViewById(R.id.button);
        StatusText.findViewById(R.id.textView2);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();


            }
        });
    }

    public void CButton (View view){

        StatusText.setText("Connected");
    }

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

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

Answer 1

改变

ConnectButton.findViewById(R.id.button);
StatusText.findViewById(R.id.textView2);

到

ConnectButton = (Button) findViewById(R.id.button);
StatusText = (TextView) findViewById(R.id.textView2);