java - 在 Android 中的另一个 Activity 中启动 Activity

Question

按注册按钮后，显示错误:不幸的是应用程序已停止

LogCat 错误:java.lang.RuntimeException:无法启动 Activity ComponentInfo{com.example.lolacupcakes/com.example.lolacupcakes.HomeActivity}:java.lang.NullPointerException:尝试调用虚拟方法“void android.widget”。空对象引用上的 Button.setOnClickListener(android.view.View$OnClickListener)'

SignUpActivity:Java 代码

public class SignUpActivity extends AppCompatActivity {

    EditText emailId, password;
    Button btnSignUp;
    TextView tvSignIn;
    FirebaseAuth mFirebaseAuth;

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

        mFirebaseAuth = FirebaseAuth.getInstance();
        emailId = findViewById(R.id.emailIDSignUp);
        password = findViewById(R.id.pwdIDSignUp);
        btnSignUp = findViewById(R.id.btnIDSignUp);
        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 Id");
                    emailId.requestFocus();
                }
                else if (pwd.isEmpty())
                {
                    password.setError("Please Enter Email Id");
                    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, "SignUp 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);
            }
        });



    }
}

注册 Activity XML:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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">


    <Button
        android:id="@+id/btnIDSignUp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="132dp"
        android:text="Sign Up" />

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="62dp"
        android:src="@drawable/logo_lola" />

    <EditText
        android:id="@+id/pwdIDSignUp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="206dp"
        android:ems="10"
        android:hint="Password"
        android:inputType="textPassword" />

    <EditText
        android:id="@+id/emailIDSignUp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="269dp"
        android:ems="10"
        android:hint="Email"
        android:inputType="textEmailAddress" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="87dp"
        android:text="Already have an account? Sign in here." />


</RelativeLayout>

HomeActivity Java 代码:

public class HomeActivity extends AppCompatActivity {

    Button btnlogout;
    FirebaseAuth mFirebaseAuth;

    private FirebaseAuth.AuthStateListener mAuthStateListener;

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

        btnlogout.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                FirebaseAuth.getInstance().signOut();
                Intent intoMain  = new Intent(HomeActivity.this, SignUpActivity.class);
                startActivity(intoMain);
            }
        });
    }
}

HomeActivityXML:

<?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=".HomeActivity">


    <TextView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:text="Welcome"
        android:textSize="50dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="1.0" />

    <Button
        android:id="@+id/btnLogout"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Logout"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        tools:layout_editor_absoluteY="424dp" />

</androidx.constraintlayout.widget.ConstraintLayout>

Answer 1

您错过了初始化btnlogout。在设置 View.OnClickListener

之前，在 HomeActivity onCreate 中添加此行

btnlogout = findViewById(R.id.btnLogout);