Android 为什么findByViewId返回null？

我觉得这是一个大脑受损的问题，但就我个人而言，我看不出有什么不对

我的活动具有以下按钮定义（在单独的LinearLayout类中）

emailSignInButton工作正常，而forgotPasswordButton返回null。我生病了，或者这对我来说很明显

我试着抛开logcat并查看信息，但没有发现任何证据

以下是整个xml文件：

<!-- Login progress -->
<ProgressBar
    android:id="@+id/login_progress"
    style="?android:attr/progressBarStyleLarge"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginBottom="8dp"
    android:visibility="gone"/>

<ScrollView
    android:id="@+id/login_form"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <LinearLayout
        android:id="@+id/email_login_form"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical">

        <TextView android:id="@+id/login"
                  android:layout_width="match_parent"
                  android:layout_height="wrap_content"
                  android:gravity="center"
                  android:text="@string/login"
                  android:textColor="@color/colorPrimary"
                  android:textSize="24sp"
            />

        <android.support.design.widget.TextInputLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content">

            <AutoCompleteTextView
                android:id="@+id/email"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:hint="@string/prompt_email"
                android:inputType="textEmailAddress"
                android:maxLines="1"
                android:singleLine="true"/>

        </android.support.design.widget.TextInputLayout>

        <android.support.design.widget.TextInputLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content">

            <EditText
                android:id="@+id/password"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:hint="@string/prompt_password"
                android:imeActionId="@+id/login"
                android:imeActionLabel="@string/action_sign_in_short"
                android:imeOptions="actionUnspecified"
                android:inputType="textPassword"
                android:maxLines="1"
                android:singleLine="true"/>

        </android.support.design.widget.TextInputLayout>


        <android.support.design.widget.TextInputLayout
            android:id="@+id/confirmPasswordLayout"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:visibility="gone">

            <EditText
                android:id="@+id/confirmPassword"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:hint="@string/prompt_confirm_password"
                android:imeActionId="@+id/login"
                android:imeActionLabel="@string/action_sign_in_short"
                android:imeOptions="actionUnspecified"
                android:inputType="textPassword"
                android:maxLines="1"
                android:singleLine="true"
                />

        </android.support.design.widget.TextInputLayout>

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="horizontal"
            >
            <Button
                android:id="@+id/email_sign_in_button"
                style="?android:textAppearanceSmall"
                android:layout_width="250dp"
                android:layout_height="match_parent"
                android:layout_marginTop="16dp"
                android:text="@string/action_sign_in"
                android:textStyle="bold"
                />

            <Button
                android:id="@+id/create_account_button"
                style="?android:textAppearanceSmall"
                android:layout_width="250dp"
                android:layout_height="match_parent"
                android:layout_marginTop="16dp"
                android:text="@string/action_create_account"
                android:textStyle="bold"
                />
        </LinearLayout>

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="horizontal"
            >
            <Button
                android:id="@+id/forgot_password"
                style="?android:textAppearanceSmall"
                android:layout_gravity="center"
                android:layout_width="250dp"
                android:layout_height="match_parent"
                android:layout_marginLeft="125dp"
                android:layout_marginTop="16dp"
                android:text="@string/action_forgot_password"
                android:textStyle="bold"
                />
        </LinearLayout>

    </LinearLayout>
</ScrollView>

---

我试着把这个按钮和另一个按钮放在同一条直线上，看看它是否有不同。它没有。

我认为问题在于，在初始化视图之前初始化方法。 因此，请尝试以下方法：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_login);
    // Set up the login form.
    emailView = (AutoCompleteTextView) findViewById(R.id.email);
    confirmPasswordView = (EditText) findViewById(R.id.confirmPassword);
    confirmPasswordLayout = (LinearLayout)  findViewById(R.id.confirmPasswordLayout);
    mainLayout = (LinearLayout) findViewById(R.id.mainLayout);
    LoginClickListener loginClickListener = new LoginClickListener();
    createAccountButton = (Button) findViewById(R.id.create_account_button);
    createAccountButton.setOnClickListener(loginClickListener);
    emailSignInButton = (Button) findViewById(R.id.email_sign_in_button);
    emailSignInButton.setOnClickListener(loginClickListener);
    forgotPasswordButton = (Button) findViewById(R.id.forgot_password);
    forgotPasswordButton.setOnClickListener(new ForgotPasswordListener());
    loginFormView = findViewById(R.id.login_form);
    progressView = findViewById(R.id.login_progress);
    loginTitle = (TextView) findViewById(R.id.login);
    passwordView = (EditText) findViewById(R.id.password);
    populateAutoComplete();
    passwordView.setOnEditorActionListener(new TextView.OnEditorActionListener() {
        @Override
        public boolean onEditorAction(TextView textView, int id, KeyEvent keyEvent) {
            if (id == R.id.login || id == EditorInfo.IME_NULL) {
                attemptLogin();
                return true;
            }
            return false;
        }
    });

}

我还建议，你应该用黄油刀

---

这将清理您的代码

明白了！在某个时候，我为这个活动创建了多个布局，用于其他大小的设备。我不知道为什么。因此，代码运行的布局与主布局不同

我将我的按钮添加到我所有的布局中，它似乎已经解决了这个问题

---

睡了一夜好觉后感觉好多了。似乎我已经清醒了。

你说的“在单独的线性布局课程中”是什么意思？您是如何在活动中加载两个线性布局的？您所说的分开线性布局是什么意思？请添加完整的xml文件和java活动代码。@AlexTa，我的活动在垂直线性布局中包含多个水平线性布局。此代码在活动中，因此是的，它们都已加载。请将日志cat也发送给我们好吗？现在一切似乎都很好@ThomNo，不是这样。我不确定你认为重新安排代码会有什么帮助。正如我指出的，之前的按钮已正确初始化。

<!-- Login progress -->
<ProgressBar
    android:id="@+id/login_progress"
    style="?android:attr/progressBarStyleLarge"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginBottom="8dp"
    android:visibility="gone"/>

<ScrollView
    android:id="@+id/login_form"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <LinearLayout
        android:id="@+id/email_login_form"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical">

        <TextView android:id="@+id/login"
                  android:layout_width="match_parent"
                  android:layout_height="wrap_content"
                  android:gravity="center"
                  android:text="@string/login"
                  android:textColor="@color/colorPrimary"
                  android:textSize="24sp"
            />

        <android.support.design.widget.TextInputLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content">

            <AutoCompleteTextView
                android:id="@+id/email"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:hint="@string/prompt_email"
                android:inputType="textEmailAddress"
                android:maxLines="1"
                android:singleLine="true"/>

        </android.support.design.widget.TextInputLayout>

        <android.support.design.widget.TextInputLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content">

            <EditText
                android:id="@+id/password"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:hint="@string/prompt_password"
                android:imeActionId="@+id/login"
                android:imeActionLabel="@string/action_sign_in_short"
                android:imeOptions="actionUnspecified"
                android:inputType="textPassword"
                android:maxLines="1"
                android:singleLine="true"/>

        </android.support.design.widget.TextInputLayout>


        <android.support.design.widget.TextInputLayout
            android:id="@+id/confirmPasswordLayout"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:visibility="gone">

            <EditText
                android:id="@+id/confirmPassword"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:hint="@string/prompt_confirm_password"
                android:imeActionId="@+id/login"
                android:imeActionLabel="@string/action_sign_in_short"
                android:imeOptions="actionUnspecified"
                android:inputType="textPassword"
                android:maxLines="1"
                android:singleLine="true"
                />

        </android.support.design.widget.TextInputLayout>

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="horizontal"
            >
            <Button
                android:id="@+id/email_sign_in_button"
                style="?android:textAppearanceSmall"
                android:layout_width="250dp"
                android:layout_height="match_parent"
                android:layout_marginTop="16dp"
                android:text="@string/action_sign_in"
                android:textStyle="bold"
                />

            <Button
                android:id="@+id/create_account_button"
                style="?android:textAppearanceSmall"
                android:layout_width="250dp"
                android:layout_height="match_parent"
                android:layout_marginTop="16dp"
                android:text="@string/action_create_account"
                android:textStyle="bold"
                />
        </LinearLayout>

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="horizontal"
            >
            <Button
                android:id="@+id/forgot_password"
                style="?android:textAppearanceSmall"
                android:layout_gravity="center"
                android:layout_width="250dp"
                android:layout_height="match_parent"
                android:layout_marginLeft="125dp"
                android:layout_marginTop="16dp"
                android:text="@string/action_forgot_password"
                android:textStyle="bold"
                />
        </LinearLayout>

    </LinearLayout>
</ScrollView>

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_login);
    // Set up the login form.
    emailView = (AutoCompleteTextView) findViewById(R.id.email);
    populateAutoComplete();

    loginTitle = (TextView) findViewById(R.id.login);

    passwordView = (EditText) findViewById(R.id.password);
    passwordView.setOnEditorActionListener(new TextView.OnEditorActionListener() {
        @Override
        public boolean onEditorAction(TextView textView, int id, KeyEvent keyEvent) {
            if (id == R.id.login || id == EditorInfo.IME_NULL) {
                attemptLogin();
                return true;
            }
            return false;
        }
    });

    confirmPasswordView = (EditText) findViewById(R.id.confirmPassword);

    confirmPasswordLayout = (LinearLayout) findViewById(R.id.confirmPasswordLayout);
    mainLayout = (LinearLayout) findViewById(R.id.mainLayout);

    LoginClickListener loginClickListener = new LoginClickListener();

    createAccountButton = (Button) findViewById(R.id.create_account_button);
    createAccountButton.setOnClickListener(loginClickListener);

    emailSignInButton = (Button) findViewById(R.id.email_sign_in_button);
    emailSignInButton.setOnClickListener(loginClickListener);

    forgotPasswordButton = (Button) findViewById(R.id.forgot_password);
    forgotPasswordButton.setOnClickListener(new ForgotPasswordListener());

    loginFormView = findViewById(R.id.login_form);
    progressView = findViewById(R.id.login_progress);
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_login);
    // Set up the login form.
    emailView = (AutoCompleteTextView) findViewById(R.id.email);
    confirmPasswordView = (EditText) findViewById(R.id.confirmPassword);
    confirmPasswordLayout = (LinearLayout)  findViewById(R.id.confirmPasswordLayout);
    mainLayout = (LinearLayout) findViewById(R.id.mainLayout);
    LoginClickListener loginClickListener = new LoginClickListener();
    createAccountButton = (Button) findViewById(R.id.create_account_button);
    createAccountButton.setOnClickListener(loginClickListener);
    emailSignInButton = (Button) findViewById(R.id.email_sign_in_button);
    emailSignInButton.setOnClickListener(loginClickListener);
    forgotPasswordButton = (Button) findViewById(R.id.forgot_password);
    forgotPasswordButton.setOnClickListener(new ForgotPasswordListener());
    loginFormView = findViewById(R.id.login_form);
    progressView = findViewById(R.id.login_progress);
    loginTitle = (TextView) findViewById(R.id.login);
    passwordView = (EditText) findViewById(R.id.password);
    populateAutoComplete();
    passwordView.setOnEditorActionListener(new TextView.OnEditorActionListener() {
        @Override
        public boolean onEditorAction(TextView textView, int id, KeyEvent keyEvent) {
            if (id == R.id.login || id == EditorInfo.IME_NULL) {
                attemptLogin();
                return true;
            }
            return false;
        }
    });

}