无法启动活动 ComponentInfo java.lang.NullPointerException：[重复]答案

【问题标题】：Unable to start activity ComponentInfo java.lang.NullPointerException: [duplicate]无法启动活动 ComponentInfo java.lang.NullPointerException：[重复]
【发布时间】：2017-12-25 07:57:34
【问题描述】：

当我运行应用程序时出现以下错误

Unable to start activity ComponentInfo java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setAdapter(android.support.v7.widget.RecyclerView$Adapter)' on a null object reference

这是我的 mainactivity.java 类

public class MainActivity extends AppCompatActivity {

    ArrayList<Contact> contacts;

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

        RecyclerView rvContacts = findViewById(R.id.rvContacts);
        contacts = Contact.createContactsList(20);
        ContactsAdapter adapter = new ContactsAdapter();items
        rvContacts.setAdapter(adapter);
        rvContacts.setLayoutManager(new LinearLayoutManager(this));
    }
}

【问题讨论】：

标签： java android nullpointerexception android-adapter

【解决方案1】：

你需要像这样投射RecyclerView..

RecyclerView rvContacts = (RecyclerView)findViewById(R.id.rvContacts);

您还需要致电setContentView(R.id.your_layout) 来设置您的布局。

【讨论】：

【解决方案2】：

您在 MainActivity 的 onCreate 中缺少 setContentView()

所以在super.onCreate(savedInstanceState);下面添加行setContentView(R.layout.activity_main);校验码

 public class MainActivity extends AppCompatActivity {

    ArrayList<Contact> contacts;

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

        RecyclerView rvContacts = findViewById(R.id.rvContacts);

        // Initialize contacts
        contacts = Contact.createContactsList(20);
        // Create adapter passing in the sample user data
        ContactsAdapter adapter = new ContactsAdapter();
        // Attach the adapter to the recyclerview to populate items
        rvContacts.setAdapter(adapter);
        // Set layout manager to position the items
        rvContacts.setLayoutManager(new LinearLayoutManager(this));
        // That's all!
    }
}

【讨论】：