Android:java.lang.NullPointerException on.setOnClickListener

我对按钮的

.setOnClickListener

有问题

每次我点击按钮时，我都会发现应用程序崩溃的问题

错误：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
    at com.example.xxx.uberclone.DriverLoginActivity.onCreate(DriverLoginActivity.java:75)

package com.example.xxx.uberclone;

import android.content.Intent;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.auth.AuthResult;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;

public class DriverLoginActivity extends AppCompatActivity {

    private Button mLogin, mRegistration;
    private EditText mEmail, mPassword;

    private FirebaseAuth mAuth;
    private FirebaseAuth.AuthStateListener firebaseAuthListener;

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

        mAuth = FirebaseAuth.getInstance();

        firebaseAuthListener = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
                if (user!=null){
                    Intent intent = new Intent(DriverLoginActivity.this,MapActivity.class);
                    startActivity(intent);
                    finish();
                    return;
                }
            }
        };

        mEmail = (EditText) findViewById(R.id.email);
        mPassword = (EditText) findViewById(R.id.password);

        mLogin = (Button) findViewById(R.id.driver);
        mRegistration = (Button) findViewById(R.id.registration);

        mRegistration.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                final String email = mEmail.getText().toString();
                final String password = mPassword.getText().toString();
                mAuth.createUserWithEmailAndPassword(email, password).addOnCompleteListener(DriverLoginActivity.this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        if (!task.isSuccessful()){
                            Toast.makeText(DriverLoginActivity.this, "Sing-up error", Toast.LENGTH_SHORT).show();
                        }else{
                            String user_id = mAuth.getCurrentUser().getUid();
                            DatabaseReference current_user_db = FirebaseDatabase.getInstance().getReference().child("Users").child("Drivers");
                            current_user_db.setValue(true);
                        }
                    }
                });

            }
        });

        mLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                final String email = mEmail.getText().toString();
                final String password = mPassword.getText().toString();
                mAuth.signInWithEmailAndPassword(email, password).addOnCompleteListener(DriverLoginActivity.this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        if (!task.isSuccessful()) {
                            Toast.makeText(DriverLoginActivity.this, "Sing-up error", Toast.LENGTH_SHORT).show();
                        }
                    }
                });
            }
        });


    }

    @Override
    protected void onStart() {
        super.onStart();
        mAuth.addAuthStateListener(firebaseAuthListener);
    }

    @Override
    protected void onStop() {
        super.onStop();
        mAuth.removeAuthStateListener(firebaseAuthListener);
    }
}

mLogin.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        final String email = mEmail.getText().toString();
        final String password = mPassword.getText().toString();
        mAuth.signInWithEmailAndPassword(email, password).addOnCompleteListener(CustomerLoginActivity.this, new OnCompleteListener<AuthResult>() {
            @Override
            public void onComplete(@NonNull Task<AuthResult> task) {
                if(!task.isSuccessful()){
                    Toast.makeText(CustomerLoginActivity.this, "sign in error", Toast.LENGTH_SHORT).show();
                }
            }
        });

    }
});

下面是一段Java代码：

DriverLoginActivity.java:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
    at com.example.xxx.uberclone.DriverLoginActivity.onCreate(DriverLoginActivity.java:75)

package com.example.xxx.uberclone;

import android.content.Intent;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.auth.AuthResult;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;

public class DriverLoginActivity extends AppCompatActivity {

    private Button mLogin, mRegistration;
    private EditText mEmail, mPassword;

    private FirebaseAuth mAuth;
    private FirebaseAuth.AuthStateListener firebaseAuthListener;

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

        mAuth = FirebaseAuth.getInstance();

        firebaseAuthListener = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
                if (user!=null){
                    Intent intent = new Intent(DriverLoginActivity.this,MapActivity.class);
                    startActivity(intent);
                    finish();
                    return;
                }
            }
        };

        mEmail = (EditText) findViewById(R.id.email);
        mPassword = (EditText) findViewById(R.id.password);

        mLogin = (Button) findViewById(R.id.driver);
        mRegistration = (Button) findViewById(R.id.registration);

        mRegistration.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                final String email = mEmail.getText().toString();
                final String password = mPassword.getText().toString();
                mAuth.createUserWithEmailAndPassword(email, password).addOnCompleteListener(DriverLoginActivity.this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        if (!task.isSuccessful()){
                            Toast.makeText(DriverLoginActivity.this, "Sing-up error", Toast.LENGTH_SHORT).show();
                        }else{
                            String user_id = mAuth.getCurrentUser().getUid();
                            DatabaseReference current_user_db = FirebaseDatabase.getInstance().getReference().child("Users").child("Drivers");
                            current_user_db.setValue(true);
                        }
                    }
                });

            }
        });

        mLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                final String email = mEmail.getText().toString();
                final String password = mPassword.getText().toString();
                mAuth.signInWithEmailAndPassword(email, password).addOnCompleteListener(DriverLoginActivity.this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        if (!task.isSuccessful()) {
                            Toast.makeText(DriverLoginActivity.this, "Sing-up error", Toast.LENGTH_SHORT).show();
                        }
                    }
                });
            }
        });


    }

    @Override
    protected void onStart() {
        super.onStart();
        mAuth.addAuthStateListener(firebaseAuthListener);
    }

    @Override
    protected void onStop() {
        super.onStop();
        mAuth.removeAuthStateListener(firebaseAuthListener);
    }
}

mLogin.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        final String email = mEmail.getText().toString();
        final String password = mPassword.getText().toString();
        mAuth.signInWithEmailAndPassword(email, password).addOnCompleteListener(CustomerLoginActivity.this, new OnCompleteListener<AuthResult>() {
            @Override
            public void onComplete(@NonNull Task<AuthResult> task) {
                if(!task.isSuccessful()){
                    Toast.makeText(CustomerLoginActivity.this, "sign in error", Toast.LENGTH_SHORT).show();
                }
            }
        });

    }
});

package com.example.xxx.uberclone；
导入android.content.Intent；
导入android.support.annotation.NonNull；
导入android.support.v7.app.AppActivity；
导入android.os.Bundle；
导入android.view.view；
导入android.widget.Button；
导入android.widget.EditText；
导入android.widget.Toast；
导入com.google.android.gms.tasks.OnCompleteListener；
导入com.google.android.gms.tasks.Task；
导入com.google.firebase.auth.AuthResult；
导入com.google.firebase.auth.FirebaseAuth；
导入com.google.firebase.auth.FirebaseUser；
导入com.google.firebase.database.DatabaseReference；
导入com.google.firebase.database.FirebaseDatabase；
公共类DriverLoginActivity扩展了AppCompatActivity{
专用按钮登录、注册；
私人编辑文本mEmail，mPassword；
私人消防队；
私有FirebaseAuth.AuthStateListener firebaseAuthListener；
@凌驾
创建时受保护的void（Bundle savedInstanceState）{
super.onCreate（savedInstanceState）；
setContentView（R.layout.activity\u driver\u login）；
mAuth=FirebaseAuth.getInstance（）；
firebaseAuthListener=新的FirebaseAuth.AuthStateListener（）{
@凌驾
AuthStateChanged上的公共void（@NonNull FirebaseAuth FirebaseAuth）{
FirebaseUser=FirebaseAuth.getInstance（）.getCurrentUser（）；
如果（用户！=null）{
Intent Intent=新的Intent（DriverLoginActivity.this，MapActivity.class）；
星触觉（意向）；
完成（）；
返回；
}
}
};
mEmail=（EditText）findViewById（R.id.email）；
mPassword=（EditText）findViewById（R.id.password）；
mLogin=（按钮）findViewById（R.id.driver）；
mrRegistration=（按钮）findViewById（R.id.registration）；
mrRegistration.setOnClickListener（新视图.OnClickListener（）{
@凌驾
公共void onClick（视图v）{
最后一个字符串email=mEmail.getText（）.toString（）；
最终字符串密码=mPassword.getText（）.toString（）；
mAuth.createUserWithEmailAndPassword（电子邮件，密码）.addOnCompleteListener（DriverLoginActivity.this，new OnCompleteListener（））{
@凌驾
未完成的公共void（@NonNull任务）{
如果（！task.issusccessful（））{
Toast.makeText（DriverLoginActivity.this，“唱出错误”，Toast.LENGTH_SHORT.show（）；
}否则{
字符串user_id=mAuth.getCurrentUser（）.getUid（）；
DatabaseReference当前_user_db=FirebaseDatabase.getInstance（）.getReference（）.child（“用户”）.child（“驱动程序”）；
当前用户设置值（真）；
}
}
});
}
});
mLogin.setOnClickListener（新视图.OnClickListener（）{
@凌驾
公共void onClick（视图v）{
最后一个字符串email=mEmail.getText（）.toString（）；
最终字符串密码=mPassword.getText（）.toString（）；
mAuth.signInWithEmailAndPassword（电子邮件，密码）。addOnCompleteListener（DriverLoginActivity.this，new OnCompleteListener（）{
@凌驾
未完成的公共void（@NonNull任务）{
如果（！task.issusccessful（））{
Toast.makeText（DriverLoginActivity.this，“唱出错误”，Toast.LENGTH_SHORT.show（）；
}
}
});
}
});
}
@凌驾
受保护的void onStart（）{
super.onStart（）；
mAuth.addAuthStateListener（firebaseAuthListener）；
}
@凌驾
受保护的void onStop（）{
super.onStop（）；
mAuth.removeAuthStateListener（firebaseAuthListener）；
}
}

这里是带有按钮的XML文件：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    android:orientation="vertical">

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="I'm a driver"
        android:id="@+id/driver"/>
    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="I'm a costumer"
        android:id="@+id/customer"/>

</LinearLayout>

以下是驱动程序XML代码：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".DriverLoginActivity"
    android:orientation="vertical">


    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="E-mail"
        android:id="@+id/email"/>
    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Password"
        android:id="@+id/password"/>
    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Login"
        android:id="@+id/login"/>
    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Registration"
        android:id="@+id/registration"/>

</LinearLayout>

错误指向这一行：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
    at com.example.xxx.uberclone.DriverLoginActivity.onCreate(DriverLoginActivity.java:75)

package com.example.xxx.uberclone;

import android.content.Intent;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.auth.AuthResult;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;

public class DriverLoginActivity extends AppCompatActivity {

    private Button mLogin, mRegistration;
    private EditText mEmail, mPassword;

    private FirebaseAuth mAuth;
    private FirebaseAuth.AuthStateListener firebaseAuthListener;

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

        mAuth = FirebaseAuth.getInstance();

        firebaseAuthListener = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
                if (user!=null){
                    Intent intent = new Intent(DriverLoginActivity.this,MapActivity.class);
                    startActivity(intent);
                    finish();
                    return;
                }
            }
        };

        mEmail = (EditText) findViewById(R.id.email);
        mPassword = (EditText) findViewById(R.id.password);

        mLogin = (Button) findViewById(R.id.driver);
        mRegistration = (Button) findViewById(R.id.registration);

        mRegistration.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                final String email = mEmail.getText().toString();
                final String password = mPassword.getText().toString();
                mAuth.createUserWithEmailAndPassword(email, password).addOnCompleteListener(DriverLoginActivity.this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        if (!task.isSuccessful()){
                            Toast.makeText(DriverLoginActivity.this, "Sing-up error", Toast.LENGTH_SHORT).show();
                        }else{
                            String user_id = mAuth.getCurrentUser().getUid();
                            DatabaseReference current_user_db = FirebaseDatabase.getInstance().getReference().child("Users").child("Drivers");
                            current_user_db.setValue(true);
                        }
                    }
                });

            }
        });

        mLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                final String email = mEmail.getText().toString();
                final String password = mPassword.getText().toString();
                mAuth.signInWithEmailAndPassword(email, password).addOnCompleteListener(DriverLoginActivity.this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        if (!task.isSuccessful()) {
                            Toast.makeText(DriverLoginActivity.this, "Sing-up error", Toast.LENGTH_SHORT).show();
                        }
                    }
                });
            }
        });


    }

    @Override
    protected void onStart() {
        super.onStart();
        mAuth.addAuthStateListener(firebaseAuthListener);
    }

    @Override
    protected void onStop() {
        super.onStop();
        mAuth.removeAuthStateListener(firebaseAuthListener);
    }
}

mLogin.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        final String email = mEmail.getText().toString();
        final String password = mPassword.getText().toString();
        mAuth.signInWithEmailAndPassword(email, password).addOnCompleteListener(CustomerLoginActivity.this, new OnCompleteListener<AuthResult>() {
            @Override
            public void onComplete(@NonNull Task<AuthResult> task) {
                if(!task.isSuccessful()){
                    Toast.makeText(CustomerLoginActivity.this, "sign in error", Toast.LENGTH_SHORT).show();
                }
            }
        });

    }
});

mLogin.setOnClickListener（新视图.OnClickListener（）{
@凌驾
公共void onClick（视图v）{
最后一个字符串email=mEmail.getText（）.toString（）；
最终字符串密码=mPassword.getText（）.toString（）；
mAuth.signInWithEmailAndPassword（电子邮件，密码）.addOnCompleteListener（CustomerLoginActivity.this，new OnCompleteListener（））{
@凌驾
未完成的公共void（@NonNull任务）{
如果（！task.issusccessful（））{
Toast.makeText（CustomerLoginActivity.this，“登录错误”，Toast.LENGTH_SHORT.show（）；
}
}
});
}
});

在您的XML中没有ID为

R.ID.registration

的

按钮，只有
@+ID/driver
和
@+ID/customer
所以将
R.ID.registration
更改为Java代码中的
R.ID.customer
。
我想您的问题已经得到了回答！