java - Android Studio 按钮导致崩溃

Question

在应用程序中，我单击一个链接，该链接应将我带到注册页面。应用程序最终崩溃，我在运行日志中收到以下错误。

错误消息 - 已更新

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.alanlyne.tbm, PID: 17423
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.alanlyne.tbm/com.alanlyne.tbm.SignUpActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6669)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
    at com.alanlyne.tbm.SignUpActivity.onCreate(SignUpActivity.java:37)
    at android.app.Activity.performCreate(Activity.java:7136)
    at android.app.Activity.performCreate(Activity.java:7127)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) 
    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$H.handleMessage(ActivityThread.java:1808) 
    at android.os.Handler.dispatchMessage(Handler.java:106) 
    at android.os.Looper.loop(Looper.java:193) 
    at android.app.ActivityThread.main(ActivityThread.java:6669) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 

导致错误的代码 

   package com.alanlyne.tbm;

import androidx.annotation.NonNull;
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.TextView;
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.FirebaseAuthException;

public class SignUpActivity extends AppCompatActivity {
    public EditText emailId, password;
    Button btnSignUp;
    TextView tvSignIn;
    FirebaseAuth mFirebaseAuth;

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

        mFirebaseAuth = FirebaseAuth.getInstance();
        emailId = findViewById(R.id.editText);
        password = findViewById(R.id.editText2);
        btnSignUp = findViewById(R.id.button2);
        tvSignIn = findViewById(R.id.textView);
        btnSignUp.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String email = emailId.getText().toString();
                String pwd = password.getText().toString();
                if (email.isEmpty()) {
                    emailId.setError("Please enter email");
                    emailId.requestFocus();
                } else if (pwd.isEmpty()) {
                    password.setError("Please enter password");
                    password.requestFocus();
                } else if (email.isEmpty() && pwd.isEmpty()) {
                    Toast.makeText(SignUpActivity.this, "Fields are empty!", Toast.LENGTH_SHORT).show();
                } else if (!(email.isEmpty() && pwd.isEmpty())) {
                    mFirebaseAuth.createUserWithEmailAndPassword(email, pwd).addOnCompleteListener(SignUpActivity.this, new OnCompleteListener<AuthResult>() {
                        @Override
                        public void onComplete(@NonNull Task<AuthResult> task) {
                            if (!task.isSuccessful()) {
                                Toast.makeText(SignUpActivity.this, "Sign Up Unsuccessful, Please Try Again", Toast.LENGTH_SHORT).show();
                            } else {
                                startActivity(new Intent(SignUpActivity.this, HomeActivity.class));
                            }
                        }
                    });
                } else {
                    Toast.makeText(SignUpActivity.this, "Error Occurred!", Toast.LENGTH_SHORT).show();
                }
            }
        });

        tvSignIn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent i = new Intent(SignUpActivity.this, loginActivity.class);
                startActivity(i);
            }
        });
    }
}

我还附上了注册 Activity 的 XML 页面，以防可能导致问题。

按钮 XML - 已更新

 <?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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=".SignUpActivity">

    <EditText
        android:id="@+id/editText"
        android:layout_width="325dp"
        android:layout_height="69dp"
        android:layout_marginStart="85dp"
        android:layout_marginLeft="85dp"
        android:layout_marginTop="231dp"
        android:layout_marginEnd="85dp"
        android:layout_marginRight="85dp"
        android:layout_marginBottom="569dp"
        android:ems="10"
        android:hint="Email"
        android:inputType="textEmailAddress"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/editText2"
        android:layout_width="326dp"
        android:layout_height="66dp"
        android:layout_marginStart="85dp"
        android:layout_marginLeft="85dp"
        android:layout_marginEnd="85dp"
        android:layout_marginRight="85dp"
        android:layout_marginBottom="8dp"
        android:ems="10"
        android:hint="Password"
        android:inputType="textPassword"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.505"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/editText"
        app:layout_constraintVertical_bias="0.074" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="177dp"
        android:layout_marginLeft="177dp"
        android:layout_marginTop="340dp"
        android:layout_marginEnd="177dp"
        android:layout_marginRight="177dp"
        android:layout_marginBottom="97dp"
        android:text="Already have an account? Sign in here"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/editText2" />

    <Button
        android:id="@+id/signUp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="66dp"
        android:layout_marginBottom="226dp"
        android:text="Sign Up"
        app:layout_constraintBottom_toTopOf="@+id/textView"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/editText2" />

    <TextView
        android:id="@+id/textView3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="176dp"
        android:layout_marginLeft="176dp"
        android:layout_marginTop="99dp"
        android:layout_marginEnd="178dp"
        android:layout_marginRight="178dp"
        android:layout_marginBottom="102dp"
        android:text="Sign Up"
        app:layout_constraintBottom_toTopOf="@+id/editText"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

先前 Activity 的代码

package com.alanlyne.tbm;

import androidx.annotation.NonNull;
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.TextView;
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;

public class loginActivity extends AppCompatActivity {
    public EditText emailId, password;
    Button btnSignIn;
    TextView tvSignUp;
    FirebaseAuth mFirebaseAuth;
    private FirebaseAuth.AuthStateListener mAuthStateListener;

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

        mFirebaseAuth = FirebaseAuth.getInstance();
        emailId = findViewById(R.id.editText);
        password = findViewById(R.id. editText2);
        btnSignIn = findViewById(R.id.button2);
        tvSignUp = findViewById(R.id.textView);

        mAuthStateListener = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                FirebaseUser mFirebaseUser = mFirebaseAuth.getCurrentUser();
                if(mFirebaseUser != null){
                    Toast.makeText(loginActivity.this, "You are logged in", Toast.LENGTH_SHORT).show();
                    Intent i = new Intent(loginActivity.this, HomeActivity.class);
                    startActivity(i);
                }
                else{
                    Toast.makeText(loginActivity.this, "Please login", Toast.LENGTH_SHORT).show();
                }
            }
        };
        btnSignIn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                String email = emailId.getText().toString();
                String pwd = password.getText().toString();
                if (email.isEmpty()) {
                    emailId.setError("Please enter email");
                    emailId.requestFocus();
                } else if (pwd.isEmpty()) {
                    password.setError("Please enter password");
                    password.requestFocus();
                } else if (email.isEmpty() && pwd.isEmpty()) {
                    Toast.makeText(loginActivity.this, "Fields are empty!", Toast.LENGTH_SHORT).show();
                } else if (!(email.isEmpty() && pwd.isEmpty())) {
                    mFirebaseAuth.signInWithEmailAndPassword(email, pwd).addOnCompleteListener(loginActivity.this, new OnCompleteListener<AuthResult>() {
                        @Override
                        public void onComplete(@NonNull Task<AuthResult> task) {
                            if(!task.isSuccessful()){
                                Toast.makeText(loginActivity.this, "Login Error, Please Try Again", Toast.LENGTH_SHORT).show();
                            }else{
                                Intent intToHome = new Intent(loginActivity.this,HomeActivity.class);
                                startActivity(intToHome);
                            }
                        }
                    });

                } else {
                    Toast.makeText(loginActivity.this, "Error Occurred!", Toast.LENGTH_SHORT).show();
                }

            }
        });
        tvSignUp.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intSignUp = new Intent(loginActivity.this, SignUpActivity.class);
                startActivity(intSignUp);
            }
        });
    }

    @Override
    protected void onStart() {
        super.onStart();
        mFirebaseAuth.addAuthStateListener(mAuthStateListener);
    }
}

先前 Activity 的 XML

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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=".loginActivity">

    <EditText
        android:id="@+id/editText"
        android:layout_width="325dp"
        android:layout_height="69dp"
        android:layout_marginStart="85dp"
        android:layout_marginLeft="85dp"
        android:layout_marginTop="231dp"
        android:layout_marginEnd="85dp"
        android:layout_marginRight="85dp"
        android:layout_marginBottom="569dp"
        android:ems="10"
        android:hint="Email"
        android:inputType="textEmailAddress"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/editText2"
        android:layout_width="326dp"
        android:layout_height="66dp"
        android:layout_marginStart="85dp"
        android:layout_marginLeft="85dp"
        android:layout_marginEnd="85dp"
        android:layout_marginRight="85dp"
        android:layout_marginBottom="8dp"
        android:ems="10"
        android:hint="Password"
        android:inputType="textPassword"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.505"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/editText"
        app:layout_constraintVertical_bias="0.074" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="177dp"
        android:layout_marginLeft="177dp"
        android:layout_marginTop="340dp"
        android:layout_marginEnd="177dp"
        android:layout_marginRight="177dp"
        android:layout_marginBottom="97dp"
        android:text="Not registered? Sign up here"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/editText2" />

    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="66dp"
        android:layout_marginBottom="226dp"
        android:hint="Sign in"
        android:text="Sign in"
        app:layout_constraintBottom_toTopOf="@+id/textView"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/editText2"
        app:layout_constraintVertical_bias="0.384" />

    <TextView
        android:id="@+id/textView3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="176dp"
        android:layout_marginLeft="176dp"
        android:layout_marginTop="99dp"
        android:layout_marginEnd="178dp"
        android:layout_marginRight="178dp"
        android:layout_marginBottom="102dp"
        android:text="Sign In"
        app:layout_constraintBottom_toTopOf="@+id/editText"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

Answer 1

您对 Button 的引用在内部是错误的SignUpActivity Activity 。

使用

btnSignUp = findViewById(R.id.signUp);

代替

btnSignUp = findViewById(R.id.button2);