Android 如何解决；NullPointerException:Can'；t为参数传递null'；路径字符串'；在child（）中；？

我不明白我的代码哪里出错了

我尝试了一些解决方案，但没有一个在我的情况下起作用

代码：

package com.example.kiit.questionme2;

import android.content.DialogInterface;
import android.content.Intent;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

import com.example.kiit.questionme2.Model.User;
import com.google.firebase.FirebaseApp;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;
import com.rengwuxian.materialedittext.MaterialEditText;

public class Activity3 extends AppCompatActivity {

MaterialEditText edtnewpassword, edtnewemail, edtuser;//for signingup

FirebaseDatabase database;
DatabaseReference users;

Button signup;

public void goback(View view) {


    Intent intenta = new Intent(getApplicationContext(), MainActivity.class);

    startActivity(intenta);

}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_3);

    edtnewemail = (MaterialEditText) findViewById(R.id.newemail);
    edtnewpassword = (MaterialEditText) findViewById(R.id.newpassword);
    edtuser = (MaterialEditText) findViewById(R.id.newuser);

    database = FirebaseDatabase.getInstance();
    users = database.getReference("Users");

    FirebaseApp.initializeApp(this);
    signup = (Button) findViewById(R.id.signup);
    signup.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            signUp(edtnewemail.getText().toString(), edtnewpassword.getText().toString(), edtuser.getText().toString());
        }
    });


}

private void signUp(String emu, String pwd, final String use) {

    final User user = new User(edtuser.getText().toString());
    edtnewpassword.getText().toString();
    edtnewemail.getText().toString();

    users.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            if (dataSnapshot.child(use).exists()) {
                if (!use.isEmpty()) {
                    if (dataSnapshot.child(use).exists())

                    {
                        Toast.makeText(Activity3.this, "User already exists.", Toast.LENGTH_SHORT).show();
                    } else

                    {
                        users.child(user.getUsername())
                                .setValue(user);
                        Toast.makeText(Activity3.this, "User registration successful.", Toast.LENGTH_SHORT).show();
                    }
                }
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

}

    E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.kiit.questionme2, PID: 6946
java.lang.NullPointerException: Can't pass null for argument 'pathString' in child()
    at com.google.firebase.database.DatabaseReference.child(Unknown Source)
    at com.google.firebase.database.DataSnapshot.child(Unknown Source)
    at com.example.kiit.questionme2.Activity3$2.onDataChange(Activity3.java:76)
    at com.google.firebase.database.Query$1.onDataChange(Unknown Source)
    at com.google.android.gms.internal.zzbpx.zza(Unknown Source)
    at com.google.android.gms.internal.zzbqx.zzZS(Unknown Source)
    at com.google.android.gms.internal.zzbra$1.run(Unknown Source)
    at android.os.Handler.handleCallback(Handler.java:751)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6077)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
Application terminated.

错误：

package com.example.kiit.questionme2;

import android.content.DialogInterface;
import android.content.Intent;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

import com.example.kiit.questionme2.Model.User;
import com.google.firebase.FirebaseApp;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;
import com.rengwuxian.materialedittext.MaterialEditText;

public class Activity3 extends AppCompatActivity {

MaterialEditText edtnewpassword, edtnewemail, edtuser;//for signingup

FirebaseDatabase database;
DatabaseReference users;

Button signup;

public void goback(View view) {


    Intent intenta = new Intent(getApplicationContext(), MainActivity.class);

    startActivity(intenta);

}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_3);

    edtnewemail = (MaterialEditText) findViewById(R.id.newemail);
    edtnewpassword = (MaterialEditText) findViewById(R.id.newpassword);
    edtuser = (MaterialEditText) findViewById(R.id.newuser);

    database = FirebaseDatabase.getInstance();
    users = database.getReference("Users");

    FirebaseApp.initializeApp(this);
    signup = (Button) findViewById(R.id.signup);
    signup.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            signUp(edtnewemail.getText().toString(), edtnewpassword.getText().toString(), edtuser.getText().toString());
        }
    });


}

private void signUp(String emu, String pwd, final String use) {

    final User user = new User(edtuser.getText().toString());
    edtnewpassword.getText().toString();
    edtnewemail.getText().toString();

    users.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            if (dataSnapshot.child(use).exists()) {
                if (!use.isEmpty()) {
                    if (dataSnapshot.child(use).exists())

                    {
                        Toast.makeText(Activity3.this, "User already exists.", Toast.LENGTH_SHORT).show();
                    } else

                    {
                        users.child(user.getUsername())
                                .setValue(user);
                        Toast.makeText(Activity3.this, "User registration successful.", Toast.LENGTH_SHORT).show();
                    }
                }
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

}

    E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.kiit.questionme2, PID: 6946
java.lang.NullPointerException: Can't pass null for argument 'pathString' in child()
    at com.google.firebase.database.DatabaseReference.child(Unknown Source)
    at com.google.firebase.database.DataSnapshot.child(Unknown Source)
    at com.example.kiit.questionme2.Activity3$2.onDataChange(Activity3.java:76)
    at com.google.firebase.database.Query$1.onDataChange(Unknown Source)
    at com.google.android.gms.internal.zzbpx.zza(Unknown Source)
    at com.google.android.gms.internal.zzbqx.zzZS(Unknown Source)
    at com.google.android.gms.internal.zzbra$1.run(Unknown Source)
    at android.os.Handler.handleCallback(Handler.java:751)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6077)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
Application terminated.

我需要将用户名密码和电子邮件作为输入，并检查它是否存在，如果存在，则显示相应的消息，如果不存在，则使用保存在Firebase中的数据成功注册。

而不是

如果（dataSnapshot.child（use）.exists（））

---

如果（use！=null&&！use.isEmpty（）&&dataSnapshot.child（use.exists（））此错误/崩溃是由于

use

在放置它的代码中为null，特别是在这一行代码中：

dataSnapshot.child（use.exists）（）

因此，在调用此方法之前，您可以检查

use

是否不为null。这将确保代码中的字段不会为null

因此，上述方法的代码如下所示：

if（使用！=null）{
//在这里调用此方法
}
否则{
//做点别的
}

---

这将确保您的代码在方法执行期间，而不是在方法执行之前，不会捕获

nullPointer

。因此，您的应用程序的流程更加流畅。

很抱歉，我发布了错误的代码。这是有错误的代码。@prernamitra，根据您的IDE，这是代码中的第76行？第76行只是一个空格嗯，它上面的那一行是什么，最接近76的代码行？75 if（dataSnapshot.child（use.exists（））很抱歉我发布了错误的代码。这是出现错误的代码。也可能是重复的