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

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What is a NullPointerException, and how do I fix it? 12 answers

什么是NullPointerException,我该如何解决? 12个答案

after writing the app when i ran it iam getting following error

在我运行它时编写应用程序后,我得到以下错误

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

here is my mainactivity.java class

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);

        // 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!
    }
}

2 个解决方案

#1

you need to cast RecyclerView like this..

你需要像这样投射RecyclerView ..

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

and also you need to call setContentView(R.id.your_layout) to set your layout.

你还需要调用setContentView(R.id.your_layout)来设置你的布局。

#2

you are missing setContentView() in your onCreate of MainActivity

你在MainActivity的onCreate中缺少setContentView()

so add line setContentView(R.layout.activity_main); below super.onCreate(savedInstanceState); check code

所以添加行setContentView(R.layout.activity_main);在super.onCreate(savedInstanceState)下面;检查代码

 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!
    }
}

#1