Android按钮意图崩溃

Question

我在ActionBar中创建按钮，这将打开我另一个类。这是我的MainActivity代码:

package com.example.menu;
import android.os.Bundle;
import android.app.ActionBar;
import android.app.Activity;
import android.content.Intent;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
public class MainActivity extends Activity  {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button add = (Button) findViewById(R.id.add);
        Button mest = (Button) findViewById(R.id.miskaste);
        ActionBar actionBar = getActionBar();
        String dateString = (String) android.text.format.DateFormat.format("yyyy/MM/dd", new java.util.Date());
        actionBar.setTitle(dateString);
        add = (Button)findViewById(R.id.add);
        add.setOnClickListener(new View.OnClickListener() {
              @Override
              public void onClick(View view) {
                Intent intent = new Intent(MainActivity.this, Add.class);
                startActivity(intent);
              }
        });
    }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the myouare going to die alone with enu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;

}
}

这是我的Add类:

package com.example.menu;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
public class Add extends Activity{
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        setContentView(R.layout.add);
        Button orderButton = (Button) findViewById(R.id.but);
        orderButton.setOnClickListener(new View.OnClickListener() {
          @Override
          public void onClick(View view) {
            finish();
          }
        });
    }
}

和清单:

<activity android:name=".Add" />

我的计划，你在代码中看到的是按下按钮，运行另一个活动，但当我运行我的应用程序-它崩溃了。我已经为这个工作了两个小时了，什么都没有……

这是我的Logcat:

05-21 11:57:39.596: E/AndroidRuntime(622):  at android.os.Handler.dispatchMessage(Handler.java:99)
05-21 11:57:39.596: E/AndroidRuntime(622):  at android.os.Looper.loop(Looper.java:137)
05-21 11:57:39.596: E/AndroidRuntime(622):  at android.app.ActivityThread.main(ActivityThread.java:4424)
05-21 11:57:39.596: E/AndroidRuntime(622):  at java.lang.reflect.Method.invokeNative(Native Method)
05-21 11:57:39.596: E/AndroidRuntime(622):  at java.lang.reflect.Method.invoke(Method.java:511)
05-21 11:57:39.596: E/AndroidRuntime(622):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
05-21 11:57:39.596: E/AndroidRuntime(622):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
05-21 11:57:39.596: E/AndroidRuntime(622):  at dalvik.system.NativeStart.main(Native Method)
05-21 11:57:39.596: E/AndroidRuntime(622): Caused by: java.lang.NullPointerException
05-21 11:57:39.596: E/AndroidRuntime(622):  at com.example.menu.MainActivity.onCreate(MainActivity.java:27)
05-21 11:57:39.596: E/AndroidRuntime(622):  at android.app.Activity.performCreate(Activity.java:4465)
05-21 11:57:39.596: E/AndroidRuntime(622):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
05-21 11:57:39.596: E/AndroidRuntime(622):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920)

Answer 1

如果你想开始这个添加活动，你必须在AndroidManifest.xml中添加这个添加活动的意图过滤器:

<intent-filter>
   <action android:name="android.intent.action.MAIN" />
</intent-filter>

Answer 2

假设您正在尝试调用动作栏中的按钮，正如您提到的

i need it to be in actionbar

和你提到的选项菜单项

    <item android:id="@+id/add" 
android:icon="@drawable/add" 
android:showAsAction="ifRoom|withText" />

尝试在你的代码中调用这个

public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
    case R.id.add:
      Intent intent = new Intent(MainActivity.this, Add.class);
                startActivity(intent);
      break;  
    default:     
      break;
    }
    return true;
  }

Answer 3

在你的'layout'文件夹中有一个文件activity_main，你应该在那里创建你的按钮，像这样:

<Button
            android:id="@+id/btn_next"
            style="@style/CameraButtonText"
            android:layout_marginRight="5dp"
            android:background="@layout/nextbutton"
            android:onClick="overviewClick"
            android:text="@string/next" />

和你必须膨胀它如果你想在动作栏:http://developer.android.com/guide/topics/ui/actionbar.html

链接