遇到错误 Attempt to invoke virtual method 'RecyclerView.setAdapter(RecyclerView$Adapter)' on a null object reference in Android Studio

Question

I encounter the problem " Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setAdapter(androidx.recyclerview.widget.RecyclerView$Adapter)' on a null object reference" when I run my code. I've searched for similar solutions but it doesn't seem to fix my error.

How should I revise my code?

This is my MainActivity.java code:

package endlesstaste;

import android.os.Bundle;

import com.google.android.material.floatingactionbutton.FloatingActionButton;
import com.google.android.material.snackbar.Snackbar;

import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.view.View;
import android.view.Menu;
import android.view.MenuItem;

import java.util.LinkedList;

public class MainActivity extends AppCompatActivity {
    private final LinkedList<String> mImagePathList = new LinkedList<>();
    private String mDrawableFilePath =
            "android.resource://endlesstaste/drawable/";
    private RecyclerView mRecyclerView;
    ImageListAdapter mAdapter;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        for (int i = 1; i <= 5; i++) {
            mImagePathList.addLast(mDrawableFilePath + "image" + i);
        }
        // Get a handle to the RecyclerView.
        mRecyclerView = findViewById(R.id.recyclerview);
        // Create an adapter and supply the data to be displayed.
        mAdapter = new ImageListAdapter(this, mImagePathList);
        // Connect the adapter with the RecyclerView.
        mRecyclerView.setAdapter(mAdapter);
        // Give the RecyclerView a default layout manager.
        mRecyclerView.setLayoutManager(new LinearLayoutManager(this));

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

This is my ImageListAdapter.java code:

package endlesstaste;

import android.os.Bundle;

import com.google.android.material.floatingactionbutton.FloatingActionButton;
import com.google.android.material.snackbar.Snackbar;

import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.view.View;
import android.view.Menu;
import android.view.MenuItem;

import java.util.LinkedList;

public class MainActivity extends AppCompatActivity {
    private final LinkedList<String> mImagePathList = new LinkedList<>();
    private String mDrawableFilePath =
            "android.resource://endlesstaste/drawable/";
    private RecyclerView mRecyclerView;
    ImageListAdapter mAdapter;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        for (int i = 1; i <= 5; i++) {
            mImagePathList.addLast(mDrawableFilePath + "image" + i);
        }
        // Get a handle to the RecyclerView.
        mRecyclerView = findViewById(R.id.recyclerview);
        // Create an adapter and supply the data to be displayed.
        mAdapter = new ImageListAdapter(this, mImagePathList);
        // Connect the adapter with the RecyclerView.
        mRecyclerView.setAdapter(mAdapter);
        // Give the RecyclerView a default layout manager.
        mRecyclerView.setLayoutManager(new LinearLayoutManager(this));

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

Answer 1

You cannot call

mRecyclerView = findViewById(R.id.recyclerview);

before calling

setContentView(R.layout.activity_main);

When you do, the call to findViewById returns null. Move setContentView (and super.onCreate while you're at it) to the start of onCreate before you start getting views.