Android - 无法开始活动

Posted 2023-04-19

【中文标题】Android - 无法开始活动

【英文标题】：Android - Unable to start activity

【发布时间】：2015-12-21 21:45:39

【问题描述】：

我正在尝试为 android 开发一个简单的应用程序。我试图制作一个侦听器，当单击按钮时将打开另一个活动。

监听器在 MainActivity。当我将应用程序启动到模拟器中并按下按钮时，应用程序立即崩溃.. 花了最后 2 个小时试图找出原因。

错误

09-24 09:25:17.086：E/AndroidRuntime(2445)：java.lang.RuntimeException：无法启动活动 ComponentInfocom.idanofek.locationshare/com.idanofek.locationshare.SecondActivity：java.lang。空指针异常

MainActivity.class

package com.idanofek.locationshare;
import android.support.v7.*;
import android.app.Activity;
import android.graphics.Color;
import android.os.Bundle;
import android.widget.*;
import android.graphics.Color; /* Colors */
import android.content.res.*;
import android.util.TypedValue;
import android.text.method.*;
import android.content.*;
import android.view.*;  /* OnClickListener */
import android.view.View.OnClickListener;
import android.R.id;   /* Connect XML and JAVA */

public class MainActivity extends Activity

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

    RelativeLayout layout;
    setContentView(R.layout.activity_main);
    layout = (RelativeLayout)findViewById(R.id.MainLayout);

    final Button loginB = (Button)findViewById(R.id.button1);
    loginB.setOnClickListener(new Button.OnClickListener()
        public void onClick(View v)
        
            Intent intent = new Intent(getApplicationContext(), SecondActivity.class);
            startActivity(intent);
        
    );

    TextView loginT = (TextView)findViewById(R.id.textView2);
    loginT.setOnClickListener(new View.OnClickListener() 

        @Override
        public void onClick(View v) 
            // TODO Auto-generated method stub
            Intent intent = new Intent(MainActivity.this, SecondActivity.class);
            startActivity(intent);
        
    );


@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);

SecondActivity.class

package com.idanofek.locationshare
import android.app.ActionBar;
import android.app.Activity;
import android.content.Intent;
import android.graphics.Color;
import android.os.Bundle;
import android.view.MenuItem;
import android.view.View;
import android.view.Window;
import android.widget.Button;
import android.widget.EditText;
import android.widget.RelativeLayout;
import android.widget.TextView;

public class SecondActivity extends Activity 

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) 
    super.onCreate(savedInstanceState);

    final RelativeLayout layout;
    setContentView(R.layout.second_activity);
    layout = (RelativeLayout)findViewById(R.id.SecondLayout);

    /* Text - email sent with password recover */
    final TextView rec_text = (TextView)findViewById(R.id.textView2);
    rec_text.setVisibility(View.INVISIBLE);
    /* field - enter your email for recover */
    final EditText mail_recover = (EditText)findViewById(R.id.mail_recover);
    mail_recover.setVisibility(View.INVISIBLE);

    ActionBar actionBar = getActionBar();
    actionBar.setHomeButtonEnabled(true);

    TextView forgot_pass = (TextView)findViewById(R.id.forgot_pass);
    forgot_pass.setOnClickListener(new View.OnClickListener() 

        @Override
        public void onClick(View v) 
            // TODO Auto-generated method stuba
            mail_recover.setVisibility(View.VISIBLE);
            final Button login = (Button)findViewById(R.id.button1);

            RelativeLayout.LayoutParams LOGIN = new RelativeLayout.LayoutParams(
                    RelativeLayout.LayoutParams.WRAP_CONTENT,
                    RelativeLayout.LayoutParams.WRAP_CONTENT
            );

            final RelativeLayout.LayoutParams REC_TEXT = new RelativeLayout.LayoutParams(
                    RelativeLayout.LayoutParams.WRAP_CONTENT,
                    RelativeLayout.LayoutParams.WRAP_CONTENT
            );              

            LOGIN.setMargins(0, 125, 0, 0);
            login.setText("Recover");
            login.setOnClickListener(new View.OnClickListener() 

                @Override
                public void onClick(View v) 
                    // TODO Auto-generated method stub
                    REC_TEXT.setMargins(0, 125, 0, 0);
                    rec_text.setVisibility(View.VISIBLE);
                    rec_text.setText("Email sent!\nCheck your email for your new password.");
                
            );
        
    );


@Override
public boolean onOptionsItemSelected(MenuItem item) 
    switch (item.getItemId()) 
        case android.R.id.home:
            // app icon in action bar clicked; go home
            Intent intent = new Intent(SecondActivity.this, MainActivity.class);
            intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
            startActivity(intent);
            return true;
        default:
            return super.onOptionsItemSelected(item);
    

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.idanofek.locationshare"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="14"
        android:targetSdkVersion="17" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />s
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".SecondActivity"
                  android:label="@string/app_name">
        </activity>
    </application>

</manifest>

谁能帮忙？

【问题讨论】：

请分享完整的堆栈跟踪

getActionBar(); 可能为空

是的，用 ActionBarActivity 扩展 secondActivity，我希望它会修复。

非常感谢！ ActionBar 确实返回了 null.. 现在可以工作了！

ActionBarActivity 已弃用。请改用AppCompatActivity

【参考方案1】：

这里是问题ActionBar actionBar = getActionBar();

请扩展 SecondActivity 和 AppCompatActivity 或 ActionBarActivity 以获得 ActionBar 。