如何修复Java中的空指针异常错误？

问：

当我尝试登录时（在 LoginActivity 文件之后），我的应用程序不断崩溃。这是代码。我真的很感激这方面的一些帮助。 MainActivity（打开注册/登录屏幕）

package com.jovanovic.stefan.sqlitetutorial;
import androidx.appcompat.app.AppCompatActivity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    EditText username, password, repassword;
    Button signup, signin;
    DBHelper DB;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        username = (EditText) findViewById(R.id.username);
        password = (EditText) findViewById(R.id.password);
        repassword = (EditText) findViewById(R.id.repassword);
        signup = (Button) findViewById(R.id.btnsignup);
        signin = (Button) findViewById(R.id.btnsignin);
        DB = new DBHelper(this);

        signup.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                String user = username.getText().toString();
                String pass = password.getText().toString();
                String repass = repassword.getText().toString();

                if(user.equals("")||pass.equals("")||repass.equals(""))
                    Toast.makeText(MainActivity.this, "Please enter all the fields", Toast.LENGTH_SHORT).show();
                else{
                    if(pass.equals(repass)){
                        Boolean checkuser = DB.checkusername(user);
                        if(checkuser==false){
                            Boolean insert = DB.insertData(user, pass);
                            if(insert==true){
                                Toast.makeText(MainActivity.this, "Registered successfully", Toast.LENGTH_SHORT).show();
                                Intent intent = new Intent(getApplicationContext(),HomeActivity.class);
                                startActivity(intent);
                            }else{
                                Toast.makeText(MainActivity.this, "Registration failed", Toast.LENGTH_SHORT).show();
                            }
                        }
                        else{
                            Toast.makeText(MainActivity.this, "User already exists! please sign in", Toast.LENGTH_SHORT).show();
                        }
                    }else{
                        Toast.makeText(MainActivity.this, "Passwords not matching", Toast.LENGTH_SHORT).show();
                    }
                } }
        });

        signin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(getApplicationContext(), LoginActivity.class);
                startActivity(intent);
            }
        });
    }
}

LoginActivity（注册后继续登录）

package com.jovanovic.stefan.sqlitetutorial;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class LoginActivity extends AppCompatActivity {
    EditText username, password;
    Button btnlogin;
    DBHelper DB;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);

        username = (EditText) findViewById(R.id.username1);
        password = (EditText) findViewById(R.id.password1);
        btnlogin = (Button) findViewById(R.id.btnsignin1);
        DB = new DBHelper(this);

        btnlogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                String user = username.getText().toString();
                String pass = password.getText().toString();

                if(user.equals("")||pass.equals(""))
                    Toast.makeText(LoginActivity.this, "Please enter all the fields", Toast.LENGTH_SHORT).show();
                else{
                    Boolean checkuserpass = DB.checkusernamepassword(user, pass);
                    if(checkuserpass==true){
                        Log.d("mytag","1");
                        Toast.makeText(LoginActivity.this, "Sign in successfull", Toast.LENGTH_SHORT).show();
                        Log.d("mytag","2");
                        Intent intent  = new Intent(getApplicationContext(), HomeActivity.class);
                        Log.d("mytag","3");
                        startActivity(intent);
                    }else{
                        Toast.makeText(LoginActivity.this, "Invalid Credentials", Toast.LENGTH_SHORT).show();
                    }
                }
            }
        });
    }
}

HomeActivity（这应该会打开应用程序的主屏幕。该应用程序有一个添加按钮，该按钮使用 recyclerview 添加项目）

package com.jovanovic.stefan.sqlitetutorial;

import androidx.annotation.Nullable;
import androidx.appcompat.app.AlertDialog;
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.app.Activity;
import android.content.DialogInterface;
import android.content.Intent;
import android.database.Cursor;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.widget.ImageView;
import android.widget.TextView;

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

import java.util.ArrayList;

public class HomeActivity extends Activity {

    RecyclerView recyclerView;
    FloatingActionButton add_button;
    ImageView empty_imageview;
    TextView no_data;

    MyDatabaseHelper myDB;
    ArrayList<String> book_id, book_title, book_author, book_pages;
    CustomAdapter customAdapter;

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

        recyclerView = findViewById(R.id.recyclerView);
        add_button = findViewById(R.id.add_button);
        empty_imageview = findViewById(R.id.empty_imageview);
        no_data = findViewById(R.id.no_data);
        add_button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(HomeActivity.this, AddActivity.class);
                startActivity(intent);
            }
        });

        myDB = new MyDatabaseHelper(HomeActivity.this);
        book_id = new ArrayList<>();
        book_title = new ArrayList<>();
        book_author = new ArrayList<>();
        book_pages = new ArrayList<>();

        storeDataInArrays();

        customAdapter = new CustomAdapter(HomeActivity.this,this, book_id, book_title, book_author,
                book_pages);
        recyclerView.setAdapter(customAdapter);
        recyclerView.setLayoutManager(new LinearLayoutManager(HomeActivity.this));

    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if(requestCode == 1){
            recreate();
        }
    }

    void storeDataInArrays(){
        Cursor cursor = myDB.readAllData();
        if(cursor.getCount() == 0){
            empty_imageview.setVisibility(View.VISIBLE);
            no_data.setVisibility(View.VISIBLE);
        }else{
            while (cursor.moveToNext()){
                book_id.add(cursor.getString(0));
                book_title.add(cursor.getString(1));
                book_author.add(cursor.getString(2));
                book_pages.add(cursor.getString(3));
            }
            empty_imageview.setVisibility(View.GONE);
            no_data.setVisibility(View.GONE);
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.my_menu, menu);
        return super.onCreateOptionsMenu(menu);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        if(item.getItemId() == R.id.delete_all){
            confirmDialog();
        }
        return super.onOptionsItemSelected(item);
    }

    void confirmDialog(){
        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        builder.setTitle("Delete All?");
        builder.setMessage("Are you sure you want to delete all Data?");
        builder.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialogInterface, int i) {
                MyDatabaseHelper myDB = new MyDatabaseHelper(HomeActivity.this);
                myDB.deleteAllData();
                //Refresh Activity
                Intent intent = new Intent(HomeActivity.this, HomeActivity.class);
                startActivity(intent);
                finish();
            }
        });
        builder.setNegativeButton("No", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialogInterface, int i) {

            }
        });
        builder.create().show();
    }
}

清单文件

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.jovanovic.stefan.sqlitetutorial">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".UpdateActivity"
            android:parentActivityName=".HomeActivity"/>
        <activity android:name=".HomeActivity"
            android:label="GDSC Items App"/>
        <activity android:name=".LoginActivity"
            android:label="GDSC Items App"/>
        <activity
            android:name=".AddActivity"
            android:label="Add Item"
            android:parentActivityName=".HomeActivity" />
        <activity
            android:name=".MainActivity"
            android:label="GDSC Items App">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <action android:name="android.intent.action.VIEW" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

错误日志

2022-01-27 20:31:01.505 3354-3354/com.jovanovic.stefan.sqlitetutorial E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.jovanovic.stefan.sqlitetutorial, PID: 3354
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.jovanovic.stefan.sqlitetutorial/com.jovanovic.stefan.sqlitetutorial.HomeActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.material.floatingactionbutton.FloatingActionButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2946)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3081)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1831)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:201)
        at android.app.ActivityThread.main(ActivityThread.java:6810)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.material.floatingactionbutton.FloatingActionButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at com.jovanovic.stefan.sqlitetutorial.HomeActivity.onCreate(HomeActivity.java:45)
        at android.app.Activity.performCreate(Activity.java:7224)
        at android.app.Activity.performCreate(Activity.java:7213)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1272)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2926)