已修改@user919216代码..并使其与WebView兼容

@Override
public void onBackPressed() {
    if (webview.canGoBack()) {
        webview.goBack();

    }
    else
    {
     AlertDialog.Builder builder = new AlertDialog.Builder(this);
builder.setMessage("Are you sure you want to exit?")
       .setCancelable(false)
       .setPositiveButton("Yes", new DialogInterface.OnClickListener() {
           public void onClick(DialogInterface dialog, int id) {
                finish();
           }
       })
       .setNegativeButton("No", new DialogInterface.OnClickListener() {
           public void onClick(DialogInterface dialog, int id) {
                dialog.cancel();
           }
       });
AlertDialog alert = builder.create();
alert.show();
    }

}

在Android 2.0+中，这看起来像:

@Override
public void onBackPressed() {
    new AlertDialog.Builder(this)
        .setIcon(android.R.drawable.ic_dialog_alert)
        .setTitle("Closing Activity")
        .setMessage("Are you sure you want to close this activity?")
        .setPositiveButton("Yes", new DialogInterface.OnClickListener()
    {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            finish();    
        }

    })
    .setNegativeButton("No", null)
    .show();
}

在早期版本中，它看起来像:

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    //Handle the back button
    if(keyCode == KeyEvent.KEYCODE_BACK) {
        //Ask the user if they want to quit
        new AlertDialog.Builder(this)
        .setIcon(android.R.drawable.ic_dialog_alert)
        .setTitle(R.string.quit)
        .setMessage(R.string.really_quit)
        .setPositiveButton(R.string.yes, new DialogInterface.OnClickListener() {

            @Override
            public void onClick(DialogInterface dialog, int which) {

                //Stop the activity
                YourClass.this.finish();    
            }

        })
        .setNegativeButton(R.string.no, null)
        .show();

        return true;
    }
    else {
        return super.onKeyDown(keyCode, event);
    }

}

@Override
public void onBackPressed() {
    new AlertDialog.Builder(this)
           .setMessage("Are you sure you want to exit?")
           .setCancelable(false)
           .setPositiveButton("Yes", new DialogInterface.OnClickListener() {
               public void onClick(DialogInterface dialog, int id) {
                   ExampleActivity.super.onBackPressed();
               }
           })
           .setNegativeButton("No", null)
           .show();
}

已修改@user919216代码..并使其与WebView兼容

@Override
public void onBackPressed() {
    if (webview.canGoBack()) {
        webview.goBack();

    }
    else
    {
     AlertDialog.Builder builder = new AlertDialog.Builder(this);
builder.setMessage("Are you sure you want to exit?")
       .setCancelable(false)
       .setPositiveButton("Yes", new DialogInterface.OnClickListener() {
           public void onClick(DialogInterface dialog, int id) {
                finish();
           }
       })
       .setNegativeButton("No", new DialogInterface.OnClickListener() {
           public void onClick(DialogInterface dialog, int id) {
                dialog.cancel();
           }
       });
AlertDialog alert = builder.create();
alert.show();
    }

}

另一种选择是在第一次回按时显示一个Toast/Snackbar，要求再次返回退出，这比显示一个AlertDialog来确认用户是否想退出应用程序要少得多。

你可以使用DoubleBackPress Android库来实现这一点，只需几行代码。示例GIF显示类似的行为。

首先，将依赖项添加到应用程序中:

dependencies {
    implementation 'com.github.kaushikthedeveloper:double-back-press:0.0.1'
}

接下来，在您的活动中，实现所需的行为。

// set the Toast to be shown on FirstBackPress (ToastDisplay - builtin template)
// can be replaced by custom action (new FirstBackPressAction{...})
FirstBackPressAction firstBackPressAction = new ToastDisplay().standard(this);

// set the Action on DoubleBackPress
DoubleBackPressAction doubleBackPressAction = new DoubleBackPressAction() {
    @Override
    public void actionCall() {
        // TODO : Exit the application
        finish();
        System.exit(0);
    }
};

// setup DoubleBackPress behaviour : close the current Activity
DoubleBackPress doubleBackPress = new DoubleBackPress()
        .withDoublePressDuration(3000)     // msec - wait for second back press
        .withFirstBackPressAction(firstBackPressAction)
        .withDoubleBackPressAction(doubleBackPressAction);

最后，将此设置为背压的行为。

@Override
public void onBackPressed() {
    doubleBackPress.onBackPressed();
}

在中国，大多数App会通过“点击两次”确认退出:

boolean doubleBackToExitPressedOnce = false;

@Override
public void onBackPressed() {
    if (doubleBackToExitPressedOnce) {
        super.onBackPressed();
        return;
    }

    this.doubleBackToExitPressedOnce = true;
    Toast.makeText(this, "Please click BACK again to exit", Toast.LENGTH_SHORT).show();

    new Handler().postDelayed(new Runnable() {

        @Override
        public void run() {
            doubleBackToExitPressedOnce=false;                       
        }
    }, 2000);
}