java - 如果不创建抽象类，则无法实现 Google Map API

Question

从 Big Nerd Ranch 学习 Java 编程，我正在尝试利用 Google Map API 开发一个简单的应用程序，但遇到错误，提示“MapsActivity 不是抽象的，并且不会覆盖 ConnectionCallbacks 中的抽象方法 onConnectionSuspished(int)”。抽象类似乎不是可行的方法，因为组件无法实例化。

package com.example.test.empower;

import android.content.pm.PackageManager;
import android.graphics.Typeface;
import android.location.Location;
import android.location.LocationManager;
import android.support.v4.app.FragmentActivity;
import android.os.Bundle;
import android.widget.TextView;

import com.google.android.gms.common.api.GoogleApiClient;
import com.google.android.gms.location.FusedLocationProviderClient;
import com.google.android.gms.location.LocationServices;
import com.google.android.gms.maps.CameraUpdateFactory;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.OnMapReadyCallback;
import com.google.android.gms.maps.SupportMapFragment;
import com.google.android.gms.maps.model.LatLng;
import com.google.android.gms.maps.model.MarkerOptions;
import android.Manifest;

public class MapsActivity extends FragmentActivity implements OnMapReadyCallback, GoogleApiClient.ConnectionCallbacks, GoogleApiClient.OnConnectionFailedListener {

    private GoogleMap mMap;
    protected GoogleApiClient mGoogleApiClient;
    TextView textView_location;

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

        if (mGoogleApiClient == null) {
            mGoogleApiClient = new GoogleApiClient.Builder(this)
                    .addConnectionCallbacks((GoogleApiClient.ConnectionCallbacks) this)
                    .addOnConnectionFailedListener((GoogleApiClient.OnConnectionFailedListener) this)
                    .addApi(LocationServices.API)
                    .build();
        }
        mGoogleApiClient.connect();

        textView_location = (TextView) findViewById(R.id.textView_location);
        // Font Awesome for Location Icon
        Typeface font = Typeface.createFromAsset(getAssets(), "fa_regular_400.ttf" );
        TextView fa_map_marker = (TextView)findViewById(R.id.map_marker);
        fa_map_marker.setTypeface(font);

        // Obtain the SupportMapFragment and get notified when the map is ready to be used.
        SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager()
                .findFragmentById(R.id.map);
        mapFragment.getMapAsync(this);

        textView_location.setText("First");
    }

    /**
     * Manipulates the map once available.
     * This callback is triggered when the map is ready to be used.
     * This is where we can add markers or lines, add listeners or move the camera. In this case,
     * we just add a marker near Sydney, Australia.
     * If Google Play services is not installed on the device, the user will be prompted to install
     * it inside the SupportMapFragment. This method will only be triggered once the user has
     * installed Google Play services and returned to the app.
     */
    @Override
    public void onMapReady(GoogleMap map) {
        mMap = map;
    }

    public void onConnected(Bundle bundle)
    {
        // Create array of permission
        String mPerms[] = new String[]{Manifest.permission.ACCESS_FINE_LOCATION};
        // Checks for permission
        if(checkSelfPermission(mPerms[0]) == PackageManager.PERMISSION_DENIED) {
            requestPermissions(mPerms, 1);

        }
        else {
            // Enable Location
            mMap.setMyLocationEnabled(true);
            Location mLastLocation = LocationServices.FusedLocationApi.getLastLocation(
                    mGoogleApiClient);

            if(mLastLocation != null){
                double latitude = mLastLocation.getLatitude();
                double longitude = mLastLocation.getLongitude();

                // Add a marker in Sydney and move the camera
                LatLng myLocation = new LatLng(latitude, longitude);
                mMap.addMarker(new MarkerOptions().position(myLocation).title("You're here."));
                mMap.moveCamera(CameraUpdateFactory.newLatLng(myLocation));
                mMap.animateCamera( CameraUpdateFactory.zoomTo( 12.0f ) );
                textView_location.setText(String.valueOf(latitude));

            }
        }
    }
}

这是activity_maps.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
xmlns:map="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#ffffff">

    <fragment
        android:id="@+id/map"
        android:layout_width="match_parent"
        android:layout_height="500dp"
        class="com.google.android.gms.maps.SupportMapFragment"/>

    <Button
        android:id="@+id/round_alert_button"
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:layout_gravity="center"
        android:layout_marginTop="170dp"
        android:background="@drawable/round_alert_button"
        android:drawableStart="@drawable/handshake_100x66"
        android:paddingLeft="2dp"
        android:paddingTop="12dp" />

    <TextView
        android:id="@+id/textView_title"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:layout_marginTop="240dp"
        android:text="Get Assistance"
        android:textColor="#000000"
        android:fontFamily="Proxima Nova"
        android:textStyle="bold"
        android:textSize="16dp"/>

    <com.example.denise.empower.FontAwesome
        android:id="@+id/map_marker"
        android:text="@string/fa_map_marker"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:layout_marginTop="265dp"
        android:layout_marginRight="35dp"
        android:textSize="18sp"
        android:textColor="#000000"
        />

    <TextView
        android:id="@+id/textView_location"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:layout_marginTop="265dp"
        android:text="Singapore"
        android:textColor="#878787"
        android:fontFamily="Proxima Nova"
        android:textStyle=""
        android:textSize="12dp"/>

    <TextView
        android:id="@+id/textView_description"
        android:layout_width="250dp"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:layout_marginTop="290dp"
        android:text="Do not subject yourself to a silent victim, reach out to our nearest law-enforcer."
        android:textColor="#878787"
        android:fontFamily="Proxima Nova"
        android:textStyle=""
        android:textSize="12dp"
        android:textAlignment="center"/>

</FrameLayout>

Answer 1

GoogleMapsApiClient.ConnectionCallbacks 是 interface有两个抽象方法:onConnected() 和 onConnectionSuspended()。由于您正在实现接口(interface)，因此您必须实现这两个方法，或者声明您的类abstract以指示子类将执行该实现。

正如您所提到的，您正在执行实际的实现，因此抽象不是可行的方法。在类中的某个位置，您必须重写 onConnectionSuspished()。如果您不打算使用该回调，只需将方法主体留空即可，但声明必须包含在您的类中。