即使我在具有相同 ID 的 xml 和 java 文件中对按钮进行编码，也会遇到空异常

Question

找到了我忘记在代码中包含 setContentView 的解决方案

这是我的第一个应用程序，我正在构建一个登录 app.This 是我的登录页面，它通过按钮连接到我的首页和主页面 activity。我已将 java 和 xml 页面编码为我的 Knowldge.My 重置按钮'btn_link_to_register' 和 'btn_action_back_to_homepage' 正在抛出 null 异常。它们都出现在我的 java 和 xml 代码中。使用 Intent 意图并单独调用它没有改变 anything.Please 帮助！！

'LoginActivity.java'

'''package com.bharath.smartdata;

import android.content.Intent;
import android.os.Bundle;
import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import android.text.TextUtils;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ProgressBar;
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;

public class LoginActivity extends AppCompatActivity {

    private EditText inputEmail, inputPassword;
    private FirebaseAuth auth;
    private ProgressBar progressBar;
    private Button btnSignup, btnSignOut, btnReset ,btn_link_to_register, btn_action_back_to_homepage;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        //Get Firebase auth instance
        auth = FirebaseAuth.getInstance();

        if (auth.getCurrentUser() != null) {
            startActivity(new Intent(LoginActivity.this, MainActivity.class));
            finish();
        }

        inputEmail = (EditText) findViewById(R.id.email);
        inputPassword = (EditText) findViewById(R.id.password);
        progressBar = (ProgressBar) findViewById(R.id.progressBar);
        btnSignup = (Button) findViewById(R.id.btn_signup);
        btnSignOut =(Button) findViewById(R.id.btnsignout);
        btn_action_back_to_homepage = (Button) findViewById(R.id.btn_action_back_to_homepage);
        btnReset = (Button) findViewById(R.id.btn_reset_password);
        btn_link_to_register =(Button) findViewById(R.id.btn_link_to_register);

        //Get Firebase auth instance
        auth = FirebaseAuth.getInstance();

        //Redirect to Homepage
        btn_action_back_to_homepage.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                startActivity(new Intent(LoginActivity.this, Frontpage.class));
                finish();
            }
        });

        //Redirect To Signup Page
        btn_link_to_register.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                startActivity(new Intent(LoginActivity.this, SignupActivity.class));
            }
        });


        //Login
        btnSignup.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String email = inputEmail.getText().toString();
                final String password = inputPassword.getText().toString();

                if (TextUtils.isEmpty(email)) {
                    Toast.makeText(getApplicationContext(), "Enter email address!", Toast.LENGTH_SHORT).show();
                    return;
                }

                if (TextUtils.isEmpty(password)) {
                    Toast.makeText(getApplicationContext(), "Enter password!", Toast.LENGTH_SHORT).show();
                    return;
                }

                progressBar.setVisibility(View.VISIBLE);

                //authenticate user
                auth.signInWithEmailAndPassword(email, password)
                        .addOnCompleteListener(LoginActivity.this, new OnCompleteListener<AuthResult>() {
                            @Override
                            public void onComplete(@NonNull Task<AuthResult> task) {
                                // If sign in fails, display a message to the user. If sign in succeeds
                                // the auth state listener will be notified and logic to handle the
                                // signed in user can be handled in the listener.
                                progressBar.setVisibility(View.GONE);
                                if (!task.isSuccessful()) {
                                    // there was an error
                                    if (password.length() < 6) {
                                        inputPassword.setError(getString(R.string.minimum_password));
                                    } else {
                                        Toast.makeText(LoginActivity.this, getString(R.string.auth_failed), Toast.LENGTH_LONG).show();
                                    }
                                } else {
                                    Intent intent = new Intent(LoginActivity.this, MainActivity.class);
                                    startActivity(intent);
                                    finish();
                                }
                            }
                        });

                btnReset.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {
                        startActivity(new Intent(LoginActivity.this, ResetPassowordActivity.class));
                    }
                });

                //SignOut
                btnSignOut.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {
                        signOut();
                    }
                });

            }

            //sign out method
            public void signOut() {
                auth.signOut();
            }


        });
    }

    public void clickFuncTion(View view) {
    }
}'''

'activity_login.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"
    android:fitsSystemWindows="true"
    android:background="#42CDDF"
    tools:context="com.bharath.smartdata.LoginActivity">

    <!-- Link to Login Screen -->

    <TextView
        android:id="@+id/textView"
        android:layout_width="298dp"
        android:layout_height="47dp"
        android:text="Welcome Back!!!"
        android:textSize="36sp"
        app:layout_constraintBottom_toTopOf="@+id/imageView"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.495"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.355" />

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="342dp"
        android:layout_height="193dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.362"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.171"
        app:srcCompat="@drawable/background" />

    <EditText
        android:id="@+id/email"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="@string/email"
        android:inputType="textEmailAddress"
        android:maxLines="1"
        android:singleLine="true"
        android:textColor="@android:color/white"
        app:layout_constraintBottom_toTopOf="@+id/password"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/imageView"
        app:layout_constraintVertical_bias="0.284" />


    <EditText
        android:id="@+id/password"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="76dp"
        android:focusableInTouchMode="true"
        android:hint="@string/hint_password"
        android:imeActionLabel="@+id/login"
        android:imeOptions="actionUnspecified"
        android:inputType="textPassword"
        android:maxLines="1"
        android:singleLine="true"
        android:textColor="@android:color/white"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="@+id/email"
        app:layout_constraintHorizontal_bias="1.0"
        app:layout_constraintStart_toStartOf="@+id/email"
        app:layout_constraintTop_toBottomOf="@+id/imageView"
        app:layout_constraintVertical_bias="0.052" />

    <Button
        android:id="@+id/btn_signup"
        style="?android:textAppearanceSmall"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="@color/colorAccent"
        android:text="@string/btn_login"
        android:textColor="@android:color/black"
        android:textStyle="bold"
        android:onClick="clickFuncTion"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="@+id/password"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintStart_toStartOf="@+id/password"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.635" />

    <Button
        android:id="@+id/btn_reset_password"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:background="@null"
        android:text="@string/btn_forgot_password"
        android:textAllCaps="false"
        android:textColor="@color/colorAccent"
        android:onClick="clickFuncTion"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.806" />

    <ProgressBar
        android:id="@+id/progressBar"
        android:layout_width="30dp"
        android:layout_height="30dp"
        android:layout_gravity="center|bottom"
        android:visibility="gone"
        app:layout_constraintBottom_toTopOf="@+id/textView"
        app:layout_constraintEnd_toEndOf="parent" />

    <Button
        android:id="@+id/btn_link_to_register"
        android:layout_width="411dp"
        android:layout_height="56dp"
        android:text="@string/btn_link_to_register"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        android:onClick="clickFuncTion"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.887" />

    <Button
        android:id="@+id/btn_action_back_to_homepage"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/action_back_to_homepage"
        android:onClick="clickFuncTion"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.976" />

    <Button
        android:id="@+id/btnsignout"
        android:layout_width="414dp"
        android:layout_height="58dp"
        android:text="@string/btn_sign_out"
        android:onClick="clickFuncTion"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.465"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.72" />
</androidx.constraintlayout.widget.ConstraintLayout>
'''

'错误信息'

 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.bharath.smartdata.LoginActivity.onCreate(LoginActivity.java:53)

Answer 1

编辑版本：

您为每个按钮添加了这行代码：

android:onClick="clickFuncTion"

但是在您的 java 文件中（在文件底部），您没有添加移动到另一个 activity 的意图（clickFuncTion 函数是空的）：

public void clickFuncTion(View view) {
}

如果你想使用 onClickListener（像这个）：

btn_action_back_to_homepage.setOnClickListener(new View.OnClickListener(){
}

你必须删除这些：

android:onClick="clickFuncTion"

public void clickFuncTion(View view) {
}

示例：

选项 1（不对按钮使用 onClick 属性和 clickFuntion）：

activity_main.xml 文件:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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=".MainActivity">

    <Button
        android:id="@+id/testButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Test this button"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</android.support.constraint.ConstraintLayout>

MainActivity.java 文件:

package com.example.myapplication;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

    private Button testButton;

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

        testButton = (Button) findViewById(R.id.testButton);

        testButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Log.d("test button", "test button");
            }
        });
    }
}

选项 2（对按钮使用 onClick 属性和 clickFuntion）：

activity_main.xml 文件:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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=".MainActivity">

    <Button
        android:id="@+id/testButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Test this button"
        android:onClick="clickFuncTion"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</android.support.constraint.ConstraintLayout>

MainActivity.java 文件:

package com.example.myapplication;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

    private Button testButton;

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

        testButton = (Button) findViewById(R.id.testButton);
    }

    public void clickFuncTion(View view) {
        Log.d("test button", "test button");
    }
}

两者产生相同的结果：

2021-04-19 14:42:50.727 27886-27886/com.example.myapplication D/test button: test button

即使我在具有相同 ID 的 xml 和 java 文件中对按钮进行编码，也会遇到空异常

Null exeption encountered even though I have coded the button in both xml and java files with same ID

android-button

firebase

android-studio