无法从 FragmentActivity 调用意图

Question

我有我的登录屏幕，在 ViewPager 的其中一个页面上有一个登录按钮。我试图让这个按钮直接进入主要活动，因为我还没有整理好所有的服务器端。但是，当我这样做时，会出现此错误：

01-21 14:51:33.499: E/AndroidRuntime(2256): FATAL EXCEPTION: main
01-21 14:51:33.499: E/AndroidRuntime(2256): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.fbla.sap/com.fbla.sap.LoginActivity}: java.lang.NullPointerException
01-21 14:51:33.499: E/AndroidRuntime(2256):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2304)
01-21 14:51:33.499: E/AndroidRuntime(2256):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2358)
01-21 14:51:33.499: E/AndroidRuntime(2256):     at android.app.ActivityThread.access$700(ActivityThread.java:165)
01-21 14:51:33.499: E/AndroidRuntime(2256):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1326)
01-21 14:51:33.499: E/AndroidRuntime(2256):     at android.os.Handler.dispatchMessage(Handler.java:99)
01-21 14:51:33.499: E/AndroidRuntime(2256):     at android.os.Looper.loop(Looper.java:137)
01-21 14:51:33.499: E/AndroidRuntime(2256):     at android.app.ActivityThread.main(ActivityThread.java:5450)
01-21 14:51:33.499: E/AndroidRuntime(2256):     at java.lang.reflect.Method.invokeNative(Native Method)
01-21 14:51:33.499: E/AndroidRuntime(2256):     at java.lang.reflect.Method.invoke(Method.java:525)
01-21 14:51:33.499: E/AndroidRuntime(2256):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1187)
01-21 14:51:33.499: E/AndroidRuntime(2256):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)
01-21 14:51:33.499: E/AndroidRuntime(2256):     at dalvik.system.NativeStart.main(Native Method)
01-21 14:51:33.499: E/AndroidRuntime(2256): Caused by: java.lang.NullPointerException
01-21 14:51:33.499: E/AndroidRuntime(2256):     at com.fbla.sap.LoginActivity.onCreate(LoginActivity.java:23)
01-21 14:51:33.499: E/AndroidRuntime(2256):     at android.app.Activity.performCreate(Activity.java:5369)
01-21 14:51:33.499: E/AndroidRuntime(2256):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1104)
01-21 14:51:33.499: E/AndroidRuntime(2256):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2267)
01-21 14:51:33.499: E/AndroidRuntime(2256):     ... 11 more

这是我的 LoginActivity.java 的源代码：

package com.fbla.sap;

import android.os.Bundle;
import android.app.Activity;
import android.content.Intent;
import android.support.v4.view.ViewPager;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.view.Menu;
import android.view.View;
import android.widget.Button;

public class LoginActivity extends FragmentActivity {

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);

        ViewPager viewPager = (ViewPager) findViewById(R.id.pager);
        viewPager.setAdapter(new ViewPagerAdapter(getSupportFragmentManager()));

        Button loginBtn = (Button) findViewById(R.id.loginBtn);
        loginBtn.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                Intent intent = new Intent(LoginActivity.this, InterfaceActivity.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.login, menu);
        return true;
    }

}

Answer 1

Button loginBtn = (Button) findViewById(R.id.loginBtn);

loginBtn 可能为空。