使用Android Studio，我试图通过传递Toast.makeText()作为第一个参数（应该是一个Context）来扩展AppCompatActivity的类中的this方法，如下所示：
Toast.makeText(this, messageResId, Toast.LENGTH_SHORT).show();
这给了我以下错误：
Cannot resolve method makeText(com.example.myname.geoquiz.QuizActivity, int, int)
似乎它没有将我的QuizActivity识别为Context，但我不明白为什么考虑QuizActivity扩展AppCompatActivity，这是一个Context。下面是我的代码（我省略了所有其他方法）。
public class QuizActivity extends AppCompatActivity {
private void checkAnswer(boolean userPressedTrue) {
boolean answerIsTrue = mQuestionBank[mCurrentIndex].isAnswerTrue();
int messageResId = 0;
if (userPressedTrue == answerIsTrue) {
messageResId = R.string.correct_toast;
} else {
messageResId = R.string.incorrect_toast;
}
Toast.makeText(this, messageResId, Toast.LENGTH_SHORT).show();
}
}
我已尝试查看StackOverflow这样的问题，但我看到的最佳建议是将this替换为getApplicationContext()但我尝试使用该方法时无法识别该方法。有什么想法吗？
我还尝试过Clean Project，Rebuild Project和Invalid Cache / Restart，以确保它不会像Android Studio那样容易修复。
查看我的AppCompatActivity.java文件，发现了31个错误和5个警告。这是代码：
/*
* Copyright (C) 2015 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package android.support.v7.app;
import android.content.Intent;
import android.content.res.Configuration;
import android.content.res.Resources;
import android.os.Build;
import android.os.Bundle;
import android.support.annotation.CallSuper;
import android.support.annotation.IdRes;
import android.support.annotation.LayoutRes;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.annotation.StyleRes;
import android.support.v4.app.ActivityCompat;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.NavUtils;
import android.support.v4.app.TaskStackBuilder;
import android.support.v4.view.KeyEventCompat;
import android.support.v7.view.ActionMode;
import android.support.v7.widget.Toolbar;
import android.support.v7.widget.VectorEnabledTintResources;
import android.util.DisplayMetrics;
import android.view.KeyEvent;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.View;
import android.view.ViewGroup;
/**
* Base class for activities that use the
* <a href="{@docRoot}tools/extras/support-library.html">support library</a> action bar features.
*
* <p>You can add an {@link android.support.v7.app.ActionBar} to your activity when running on API level 7 or higher
* by extending this class for your activity and setting the activity theme to
* {@link android.support.v7.appcompat.R.style#Theme_AppCompat Theme.AppCompat} or a similar theme.
*
* <div class="special reference">
* <h3>Developer Guides</h3>
*
* <p>For information about how to use the action bar, including how to add action items, navigation
* modes and more, read the <a href="{@docRoot}guide/topics/ui/actionbar.html">Action
* Bar</a> API guide.</p>
* </div>
*/
public class AppCompatActivity extends FragmentActivity implements AppCompatCallback,
TaskStackBuilder.SupportParentable, ActionBarDrawerToggle.DelegateProvider {
private AppCompatDelegate mDelegate;
private int mThemeId = 0;
private boolean mEatKeyUpEvent;
private Resources mResources;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
final AppCompatDelegate delegate = getDelegate();
delegate.installViewFactory();
delegate.onCreate(savedInstanceState);
if (delegate.applyDayNight() && mThemeId != 0) {
// If DayNight has been applied, we need to re-apply the theme for
// the changes to take effect. On API 23+, we should bypass
// setTheme(), which will no-op if the theme ID is identical to the
// current theme ID.
if (Build.VERSION.SDK_INT >= 23) {
onApplyThemeResource(getTheme(), mThemeId, false);
} else {
setTheme(mThemeId);
}
}
super.onCreate(savedInstanceState);
}
@Override
public void setTheme(@StyleRes final int resid) {
super.setTheme(resid);
// Keep hold of the theme id so that we can re-set it later if needed
mThemeId = resid;
}
@Override
protected void onPostCreate(@Nullable Bundle savedInstanceState) {
super.onPostCreate(savedInstanceState);
getDelegate().onPostCreate(savedInstanceState);
}
/**
* Support library version of {@link android.app.Activity#getActionBar}.
*
* <p>Retrieve a reference to this activity's ActionBar.
*
* @return The Activity's ActionBar, or null if it does not have one.
*/
@Nullable
public ActionBar getSupportActionBar() {
return getDelegate().getSupportActionBar();
}
/**
* Set a {@link android.widget.Toolbar Toolbar} to act as the
* {@link android.support.v7.app.ActionBar} for this Activity window.
*
* <p>When set to a non-null value the {@link #getActionBar()} method will return
* an {@link android.support.v7.app.ActionBar} object that can be used to control the given
* toolbar as if it were a traditional window decor action bar. The toolbar's menu will be
* populated with the Activity's options menu and the navigation button will be wired through
* the standard {@link android.R.id#home home} menu select action.</p>
*
* <p>In order to use a Toolbar within the Activity's window content the application
* must not request the window feature
* {@link android.view.Window#FEATURE_ACTION_BAR FEATURE_SUPPORT_ACTION_BAR}.</p>
*
* @param toolbar Toolbar to set as the Activity's action bar, or {@code null} to clear it
*/
public void setSupportActionBar(@Nullable Toolbar toolbar) {
getDelegate().setSupportActionBar(toolbar);
}
@Override
public MenuInflater getMenuInflater() {
return getDelegate().getMenuInflater();
}
@Override
public void setContentView(@LayoutRes int layoutResID) {
getDelegate().setContentView(layoutResID);
}
@Override
public void setContentView(View view) {
getDelegate().setContentView(view);
}
@Override
public void setContentView(View view, ViewGroup.LayoutParams params) {
getDelegate().setContentView(view, params);
}
@Override
public void addContentView(View view, ViewGroup.LayoutParams params) {
getDelegate().addContentView(view, params);
}
@Override
public void onConfigurationChanged(Configuration newConfig) {
super.onConfigurationChanged(newConfig);
getDelegate().onConfigurationChanged(newConfig);
if (mResources != null) {
// The real (and thus managed) resources object was already updated
// by ResourcesManager, so pull the current metrics from there.
final DisplayMetrics newMetrics = super.getResources().getDisplayMetrics();
mResources.updateConfiguration(newConfig, newMetrics);
}
}
@Override
protected void onPostResume() {
super.onPostResume();
getDelegate().onPostResume();
}
@Override
protected void onStart() {
super.onStart();
getDelegate().onStart();
}
@Override
protected void onStop() {
super.onStop();
getDelegate().onStop();
}
@Override
public View findViewById(@IdRes int id) {
return getDelegate().findViewById(id);
}
@Override
public final boolean onMenuItemSelected(int featureId, android.view.MenuItem item) {
if (super.onMenuItemSelected(featureId, item)) {
return true;
}
final ActionBar ab = getSupportActionBar();
if (item.getItemId() == android.R.id.home && ab != null &&
(ab.getDisplayOptions() & ActionBar.DISPLAY_HOME_AS_UP) != 0) {
return onSupportNavigateUp();
}
return false;
}
@Override
protected void onDestroy() {
super.onDestroy();
getDelegate().onDestroy();
}
@Override
protected void onTitleChanged(CharSequence title, int color) {
super.onTitleChanged(title, color);
getDelegate().setTitle(title);
}
/**
* Enable extended support library window features.
* <p>
* This is a convenience for calling
* {@link android.view.Window#requestFeature getWindow().requestFeature()}.
* </p>
*
* @param featureId The desired feature as defined in
* {@link android.view.Window} or {@link android.support.v4.view.WindowCompat}.
* @return Returns true if the requested feature is supported and now enabled.
*
* @see android.app.Activity#requestWindowFeature
* @see android.view.Window#requestFeature
*/
public boolean supportRequestWindowFeature(int featureId) {
return getDelegate().requestWindowFeature(featureId);
}
@Override
public void supportInvalidateOptionsMenu() {
getDelegate().invalidateOptionsMenu();
}
/**
* @hide
*/
public void invalidateOptionsMenu() {
getDelegate().invalidateOptionsMenu();
}
/**
* Notifies the Activity that a support action mode has been started.
* Activity subclasses overriding this method should call the superclass implementation.
*
* @param mode The new action mode.
*/
@Override
@CallSuper
public void onSupportActionModeStarted(@NonNull ActionMode mode) {
}
/**
* Notifies the activity that a support action mode has finished.
* Activity subclasses overriding this method should call the superclass implementation.
*
* @param mode The action mode that just finished.
*/
@Override
@CallSuper
public void onSupportActionModeFinished(@NonNull ActionMode mode) {
}
/**
* Called when a support action mode is being started for this window. Gives the
* callback an opportunity to handle the action mode in its own unique and
* beautiful way. If this method returns null the system can choose a way
* to present the mode or choose not to start the mode at all.
*
* @param callback Callback to control the lifecycle of this action mode
* @return The ActionMode that was started, or null if the system should present it
*/
@Nullable
@Override
public ActionMode onWindowStartingSupportActionMode(@NonNull ActionMode.Callback callback) {
return null;
}
/**
* Start an action mode.
*
* @param callback Callback that will manage lifecycle events for this context mode
* @return The ContextMode that was started, or null if it was canceled
*/
@Nullable
public ActionMode startSupportActionMode(@NonNull ActionMode.Callback callback) {
return getDelegate().startSupportActionMode(callback);
}
/**
* @deprecated Progress bars are no longer provided in AppCompat.
*/
@Deprecated
public void setSupportProgressBarVisibility(boolean visible) {
}
/**
* @deprecated Progress bars are no longer provided in AppCompat.
*/
@Deprecated
public void setSupportProgressBarIndeterminateVisibility(boolean visible) {
}
/**
* @deprecated Progress bars are no longer provided in AppCompat.
*/
@Deprecated
public void setSupportProgressBarIndeterminate(boolean indeterminate) {
}
/**
* @deprecated Progress bars are no longer provided in AppCompat.
*/
@Deprecated
public void setSupportProgress(int progress) {
}
/**
* Support version of {@link #onCreateNavigateUpTaskStack(android.app.TaskStackBuilder)}.
* This method will be called on all platform versions.
*
* Define the synthetic task stack that will be generated during Up navigation from
* a different task.
*
* <p>The default implementation of this method adds the parent chain of this activity
* as specified in the manifest to the supplied {@link android.support.v4.app.TaskStackBuilder}. Applications
* may choose to override this method to construct the desired task stack in a different
* way.</p>
*
* <p>This method will be invoked by the default implementation of {@link #onNavigateUp()}
* if {@link #shouldUpRecreateTask(android.content.Intent)} returns true when supplied with the intent
* returned by {@link #getParentActivityIntent()}.</p>
*
* <p>Applications that wish to supply extra Intent parameters to the parent stack defined
* by the manifest should override
* {@link #onPrepareSupportNavigateUpTaskStack(android.support.v4.app.TaskStackBuilder)}.</p>
*
* @param builder An empty TaskStackBuilder - the application should add intents representing
* the desired task stack
*/
public void onCreateSupportNavigateUpTaskStack(@NonNull TaskStackBuilder builder) {
builder.addParentStack(this);
}
/**
* Support version of {@link #onPrepareNavigateUpTaskStack(android.app.TaskStackBuilder)}.
* This method will be called on all platform versions.
*
* Prepare the synthetic task stack that will be generated during Up navigation
* from a different task.
*
* <p>This method receives the {@link android.support.v4.app.TaskStackBuilder} with the constructed series of
* Intents as generated by {@link #onCreateSupportNavigateUpTaskStack(android.support.v4.app.TaskStackBuilder)}.
* If any extra data should be added to these intents before launching the new task,
* the application should override this method and add that data here.</p>
*
* @param builder A TaskStackBuilder that has been populated with Intents by
* onCreateNavigateUpTaskStack.
*/
public void onPrepareSupportNavigateUpTaskStack(@NonNull TaskStackBuilder builder) {
}
/**
* This method is called whenever the user chooses to navigate Up within your application's
* activity hierarchy from the action bar.
*
* <p>If a parent was specified in the manifest for this activity or an activity-alias to it,
* default Up navigation will be handled automatically. See
* {@link #getSupportParentActivityIntent()} for how to specify the parent. If any activity
* along the parent chain requires extra Intent arguments, the Activity subclass
* should override the method {@link #onPrepareSupportNavigateUpTaskStack(android.support.v4.app.TaskStackBuilder)}
* to supply those arguments.</p>
*
* <p>See <a href="{@docRoot}guide/topics/fundamentals/tasks-and-back-stack.html">Tasks and
* Back Stack</a> from the developer guide and
* <a href="{@docRoot}design/patterns/navigation.html">Navigation</a> from the design guide
* for more information about navigating within your app.</p>
*
* <p>See the {@link android.support.v4.app.TaskStackBuilder} class and the Activity methods
* {@link #getSupportParentActivityIntent()}, {@link #supportShouldUpRecreateTask(android.content.Intent)}, and
* {@link #supportNavigateUpTo(android.content.Intent)} for help implementing custom Up navigation.</p>
*
* @return true if Up navigation completed successfully and this Activity was finished,
* false otherwise.
*/
public boolean onSupportNavigateUp() {
Intent upIntent = getSupportParentActivityIntent();
if (upIntent != null) {
if (supportShouldUpRecreateTask(upIntent)) {
TaskStackBuilder b = TaskStackBuilder.create(this);
onCreateSupportNavigateUpTaskStack(b);
onPrepareSupportNavigateUpTaskStack(b);
b.startActivities();
try {
ActivityCompat.finishAffinity(this);
} catch (IllegalStateException e) {
// This can only happen on 4.1+, when we don't have a parent or a result set.
// In that case we should just finish().
finish();
}
} else {
// This activity is part of the application's task, so simply
// navigate up to the hierarchical parent activity.
supportNavigateUpTo(upIntent);
}
return true;
}
return false;
}
/**
* Obtain an {@link android.content.Intent} that will launch an explicit target activity
* specified by sourceActivity's {@link android.support.v4.app.NavUtils#PARENT_ACTIVITY} <meta-data>
* element in the application's manifest. If the device is running
* Jellybean or newer, the android:parentActivityName attribute will be preferred
* if it is present.
*
* @return a new Intent targeting the defined parent activity of sourceActivity
*/
@Nullable
@Override
public Intent getSupportParentActivityIntent() {
return NavUtils.getParentActivityIntent(this);
}
/**
* Returns true if sourceActivity should recreate the task when navigating 'up'
* by using targetIntent.
*
* <p>If this method returns false the app can trivially call
* {@link #supportNavigateUpTo(android.content.Intent)} using the same parameters to correctly perform
* up navigation. If this method returns false, the app should synthesize a new task stack
* by using {@link android.support.v4.app.TaskStackBuilder} or another similar mechanism to perform up navigation.</p>
*
* @param targetIntent An intent representing the target destination for up navigation
* @return true if navigating up should recreate a new task stack, false if the same task
* should be used for the destination
*/
public boolean supportShouldUpRecreateTask(@NonNull Intent targetIntent) {
return NavUtils.shouldUpRecreateTask(this, targetIntent);
}
/**
* Navigate from sourceActivity to the activity specified by upIntent, finishing sourceActivity
* in the process. upIntent will have the flag {@link android.content.Intent#FLAG_ACTIVITY_CLEAR_TOP} set
* by this method, along with any others required for proper up navigation as outlined
* in the Android Design Guide.
*
* <p>This method should be used when performing up navigation from within the same task
* as the destination. If up navigation should cross tasks in some cases, see
* {@link #supportShouldUpRecreateTask(android.content.Intent)}.</p>
*
* @param upIntent An intent representing the target destination for up navigation
*/
public void supportNavigateUpTo(@NonNull Intent upIntent) {
NavUtils.navigateUpTo(this, upIntent);
}
@Override
public void onContentChanged() {
// Call onSupportContentChanged() for legacy reasons
onSupportContentChanged();
}
/**
* @deprecated Use {@link #onContentChanged()} instead.
*/
@Deprecated
public void onSupportContentChanged() {
}
@Nullable
@Override
public ActionBarDrawerToggle.Delegate getDrawerToggleDelegate() {
return getDelegate().getDrawerToggleDelegate();
}
/**
* {@inheritDoc}
*
* <p>Please note: AppCompat uses it's own feature id for the action bar:
* {@link AppCompatDelegate#FEATURE_SUPPORT_ACTION_BAR FEATURE_SUPPORT_ACTION_BAR}.</p>
*/
@Override
public boolean onMenuOpened(int featureId, Menu menu) {
return super.onMenuOpened(featureId, menu);
}
/**
* {@inheritDoc}
*
* <p>Please note: AppCompat uses it's own feature id for the action bar:
* {@link AppCompatDelegate#FEATURE_SUPPORT_ACTION_BAR FEATURE_SUPPORT_ACTION_BAR}.</p>
*/
@Override
public void onPanelClosed(int featureId, Menu menu) {
super.onPanelClosed(featureId, menu);
}
@Override
protected void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
getDelegate().onSaveInstanceState(outState);
}
/**
* @return The {@link AppCompatDelegate} being used by this Activity.
*/
@NonNull
public AppCompatDelegate getDelegate() {
if (mDelegate == null) {
mDelegate = AppCompatDelegate.create(this, this);
}
return mDelegate;
}
@Override
public boolean dispatchKeyEvent(KeyEvent event) {
if (KeyEventCompat.isCtrlPressed(event) &&
event.getUnicodeChar(event.getMetaState() & ~KeyEvent.META_CTRL_MASK) == '<') {
// Capture the Control-< and send focus to the ActionBar
final int action = event.getAction();
if (action == KeyEvent.ACTION_DOWN) {
final ActionBar actionBar = getSupportActionBar();
if (actionBar != null && actionBar.isShowing() && actionBar.requestFocus()) {
mEatKeyUpEvent = true;
return true;
}
} else if (action == KeyEvent.ACTION_UP && mEatKeyUpEvent) {
mEatKeyUpEvent = false;
return true;
}
}
return super.dispatchKeyEvent(event);
}
@Override
public Resources getResources() {
if (mResources == null && VectorEnabledTintResources.shouldBeUsed()) {
mResources = new VectorEnabledTintResources(this, super.getResources());
}
return mResources == null ? super.getResources() : mResources;
}
}
答案 0 :(得分：2)
创建另一项活动 QuizActivity1 并复制 QuizActivity 的正文代码并导入所需的类，删除 {{1 并重构 QuizActivity1 到 QuizActivity ，它可能会解决问题。 ：）
答案 1 :(得分：1)
这是一个古老的问题，但是我今天偶然发现了这个问题。像这样使用getApplicationContext（）代替使用“ this”：
CharSequence text = "I'm a toast.";
Toast.makeText(getApplicationContext(), text, Toast.LENGTH_SHORT).show();
答案 2 :(得分：1)
看来你必须传入“上下文”而不是“这个”。也就是说，如果您在 2020 年阅读本文并使用导航组件。
Toast.makeText(context, 'text', Toast.LENGTH_LONG).show()
答案 3 :(得分：0)
我今天和科特林有类似的经历。 试试这个：
self仅仅是“上下文”