Android:未分配区段页面适配器

Question

我有以下代码：

package com.example.me.echoboard;

import android.support.design.widget.TabLayout;
import android.support.v7.app.AppCompatActivity;
import android.support.v4.view.ViewPager;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

    private SectionsPageAdapter mSectionsPageAdapter;
    private ViewPager mViewPager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mSectionsPageAdapter = new SectionsPageAdapter(getSupportFragmentManager());
        mViewPager = findViewById(R.id.container);
        setupViewPager(mViewPager);

        TabLayout tabLayout = findViewById(R.id.tabs);
        tabLayout.setupWithViewPager(mViewPager);
    }

    private void setupViewPager(ViewPager viewPager){
        SectionsPageAdapter adapter = new SectionsPageAdapter(getSupportFragmentManager());
        adapter.addFragment(new Tab1Fragment(), "Message Board");
        adapter.addFragment(new Tab2Fragment(), "Statistics");
        adapter.addFragment(new Tab3Fragment(), "My Notes");
        viewPager.setAdapter(adapter);
    }
}

我以某种方式破坏了我的计划。调试的错误是，在最后一行中，我将viewPager设置为适配器，据说该适配器为null。

IDE确实证实了这一点。在mSectionsPageAdapter上徘徊在private SectionsPageAdapter mSectionsPageAdapter;上时，我被告知它是赋值的，但从未被访问过(这就说明了为什么它是null)。然而，这显然是不正确的，它被指定为mSectionsPageAdapter = new SectionsPageAdapter(getSupportFragmentManager());。

我看不出问题出在哪里。同样，代码的结构也很好，在我开始使用一些数据可视化工具之前，它运行得非常完美，随后出现了错误消息。

有人知道从哪里开始找吗？

谢谢!

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.example.me.echoboard, PID: 16132
                  java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.me.echoboard/com.example.me.echoboard.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.view.ViewPager.setAdapter(android.support.v4.view.PagerAdapter)' on a null object reference
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2924)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2985)
                      at android.app.ActivityThread.-wrap14(ActivityThread.java)
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1635)
                      at android.os.Handler.dispatchMessage(Handler.java:102)
                      at android.os.Looper.loop(Looper.java:154)
                      at android.app.ActivityThread.main(ActivityThread.java:6692)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1468)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1358)
                   Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.view.ViewPager.setAdapter(android.support.v4.view.PagerAdapter)' on a null object reference
                      at com.example.me.echoboard.MainActivity.setupViewPager(MainActivity.java:31)
                      at com.example.me.echoboard.MainActivity.onCreate(MainActivity.java:20)
                      at android.app.Activity.performCreate(Activity.java:6912)
                      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1126)
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2877)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2985) 
                      at android.app.ActivityThread.-wrap14(ActivityThread.java) 
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1635) 
                      at android.os.Handler.dispatchMessage(Handler.java:102) 
                      at android.os.Looper.loop(Looper.java:154) 
                      at android.app.ActivityThread.main(ActivityThread.java:6692) 
                      at java.lang.reflect.Method.invoke(Native Method) 
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1468) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1358)

Answer 1

未使用的分配给mSectionsPageAdapter的问题，虽然是正确的，但却是一个“鲱鱼”问题。根据您发布的异常文本，在将该适配器分配给NullPointerException期间将抛出一个ViewPager，这意味着ViewPager的引用为null。

该空引用起源于行mViewPager = findViewById(R.id.container);，因此请确保在您的布局文件中，该视图寻呼机的id确实是id/container。