使用构造函数调用方法

【问题标题】：Using constructor to call a method使用构造函数调用方法
【发布时间】：2013-01-02 21:05:27
【问题描述】：

我想使用调用方法 points() 的构造函数。该方法计算3个点的经纬度。我需要另一个班级的这些价值观。因此，我试图在构造函数中调用该方法。 请帮我了解一下语法。

这是我的班级 FirstActivity

package com.example.gpsdistance;

import android.os.Bundle;
import android.os.Handler;
import android.app.Activity;
import android.content.Intent;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.TextView;
import android.widget.Toast;

public class FirstActivity extends Activity {

    Button btnShowLocation, btnMinDis;
    TextView display= null;
    public float check= 5; 

    //private final Context mctx;
    double latuser;
    double longuser;
    double dx1, dy1, delta_long1, delta_lat1, final_long1, final_lat1;
    double dx2, dy2, delta_long2, delta_lat2, final_long2, final_lat2;
    double dx3, dy3, delta_long3, delta_lat3, final_long3, final_lat3;
    public float latA, longA, latB, longB, latC, longC, lati1;
    // GPSTracker class
    GPSTracker gps;


    void points()
    {
        latuser = gps.getLatitude();
        longuser = gps.getLongitude();

        dx1 = 120*Math.cos(35); 
        dy1 = 120*Math.sin(35); 
        delta_long1 = dx1/(111320*Math.cos(latuser));   
        delta_lat1 = dy1/110540;   
        final_long1 = longuser + delta_long1;
        final_lat1 = latuser + delta_lat1;
        dx2 = 150*Math.cos(38); 
        dy2 = 150*Math.sin(38); 
        delta_long2 = dx2/(111320*Math.cos(latuser));   
        delta_lat2 = dy2/110540;   
        final_long2 = longuser + delta_long2;
        final_lat2 = latuser + delta_lat2;
        dx3 = 180*Math.cos(38); 
        dy3 = 180*Math.sin(38); 
        delta_long3 = dx3/(111320*Math.cos(latuser));   
        delta_lat3 = dy3/110540;   
        final_long3 = longuser + delta_long3;
        final_lat3 = latuser + delta_lat3;

        latA= (float)final_lat1;
        longA= (float)final_long1;
        latB= (float)final_lat2;
        longB= (float)final_long2;
        latC= (float)final_lat3;
        longC= (float)final_long3;
    }

    private float computeDistance(double lat1, double lon1,
            double lat2, double lon2) {
            // Based on http://www.ngs.noaa.gov/PUBS_LIB/inverse.pdf
            // using the "Inverse Formula" (section 4)

            int MAXITERS = 20;
            // Convert lat/long to radians
            lat1 *= Math.PI / 180.0;
            lat2 *= Math.PI / 180.0;
            lon1 *= Math.PI / 180.0;
            lon2 *= Math.PI / 180.0;

            double a = 6378137.0; // WGS84 major axis
            double b = 6356752.3142; // WGS84 semi-major axis
            double f = (a - b) / a;
            double aSqMinusBSqOverBSq = (a * a - b * b) / (b * b);

            double L = lon2 - lon1;
            double A = 0.0;
            double U1 = Math.atan((1.0 - f) * Math.tan(lat1));
            double U2 = Math.atan((1.0 - f) * Math.tan(lat2));

            double cosU1 = Math.cos(U1);
            double cosU2 = Math.cos(U2);
            double sinU1 = Math.sin(U1);
            double sinU2 = Math.sin(U2);
            double cosU1cosU2 = cosU1 * cosU2;
            double sinU1sinU2 = sinU1 * sinU2;

            double sigma = 0.0;
            double deltaSigma = 0.0;
            double cosSqAlpha = 0.0;
            double cos2SM = 0.0;
            double cosSigma = 0.0;
            double sinSigma = 0.0;
            double cosLambda = 0.0;
            double sinLambda = 0.0;

            double lambda = L; // initial guess
            for (int iter = 0; iter < MAXITERS; iter++) {
                double lambdaOrig = lambda;
                cosLambda = Math.cos(lambda);
                sinLambda = Math.sin(lambda);
                double t1 = cosU2 * sinLambda;
                double t2 = cosU1 * sinU2 - sinU1 * cosU2 * cosLambda;
                double sinSqSigma = t1 * t1 + t2 * t2; // (14)
                sinSigma = Math.sqrt(sinSqSigma);
                cosSigma = sinU1sinU2 + cosU1cosU2 * cosLambda; // (15)
                sigma = Math.atan2(sinSigma, cosSigma); // (16)
                double sinAlpha = (sinSigma == 0) ? 0.0 :
                    cosU1cosU2 * sinLambda / sinSigma; // (17)
                cosSqAlpha = 1.0 - sinAlpha * sinAlpha;
                cos2SM = (cosSqAlpha == 0) ? 0.0 :
                    cosSigma - 2.0 * sinU1sinU2 / cosSqAlpha; // (18)

                double uSquared = cosSqAlpha * aSqMinusBSqOverBSq; // defn
                A = 1 + (uSquared / 16384.0) * // (3)
                    (4096.0 + uSquared *
                     (-768 + uSquared * (320.0 - 175.0 * uSquared)));
                double B = (uSquared / 1024.0) * // (4)
                    (256.0 + uSquared *
                     (-128.0 + uSquared * (74.0 - 47.0 * uSquared)));
                double C = (f / 16.0) *
                    cosSqAlpha *
                    (4.0 + f * (4.0 - 3.0 * cosSqAlpha)); // (10)
                double cos2SMSq = cos2SM * cos2SM;
                deltaSigma = B * sinSigma * // (6)
                    (cos2SM + (B / 4.0) *
                     (cosSigma * (-1.0 + 2.0 * cos2SMSq) -
                      (B / 6.0) * cos2SM *
                      (-3.0 + 4.0 * sinSigma * sinSigma) *
                      (-3.0 + 4.0 * cos2SMSq)));

                lambda = L +
                    (1.0 - C) * f * sinAlpha *
                    (sigma + C * sinSigma *
                     (cos2SM + C * cosSigma *
                      (-1.0 + 2.0 * cos2SM * cos2SM))); // (11)

                double delta = (lambda - lambdaOrig) / lambda;
                if (Math.abs(delta) < 1.0e-12) {
                    break;
                }
            }

            float distance = (float) (b * A * (sigma - deltaSigma));

            return distance;
    }

    FirstActivity()
        {

            points();
        }


    @Override
    protected void onCreate(Bundle savedInstanceState)
        {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.firstactivity);
            final LinearLayout lView = new LinearLayout(this);  
            display= new TextView(this);
            btnMinDis=new Button(this);
            btnMinDis.setText("Nearest Point");
            final Handler mHandler = new Handler();
            btnShowLocation = (Button) findViewById(R.id.buttonLocation);

            btnShowLocation.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View arg0) 
                {
                    // TODO Auto-generated method stub

                    // create class object
                    gps = new GPSTracker(FirstActivity.this);

                    // check if GPS enabled
                    if(gps.canGetLocation())
                        {
                        points();
                            float initialdisA = computeDistance( latuser, longuser, latA, longA);
                            float initialdisB = computeDistance( latuser, longuser, latB, longB);
                            float initialdisC = computeDistance( latuser, longuser, latC, longC);         
                            Toast.makeText(getApplicationContext(),"\nLatitude: " + latA ,  Toast.LENGTH_LONG).show();
                            lView.addView(display);
                            setContentView(lView);
                            display.setText("Initial Location is - \nLatitude: " + latuser + " N" + "\nLongitude: " + longuser + " E"
                                        + "\n\n\nPoint A:\n" + "Latitude: " + latA + "N" + "\nLongitude: " + longA + "E" 
                                        + "\nPoint B:\n" + "Latitude: " + latB + "N" + "\nLongitude: " + longB + "E" 
                                        + "\nPoint C:\n" + "Latitude: " + latC + "N" + "\nLongitude: " + longC + "E"
                                        + "\n\n\nInitial Distance A: " + initialdisA + "m" + "\nInitial Distance B: " + initialdisB + "m" + "\nInitial Distance C: " + initialdisC + "m");


                            mHandler.postDelayed(mLaunchTask,10000);
                        }
                    else
                        {
                    // GPS or Network is not enabled
                    // Ask user to enable GPS/network in settings
                    gps.showSettingsAlert();
                        }   
              }  

            private Runnable mLaunchTask = new Runnable() 
            {
                public void run() 
                {
                    Intent i = new Intent(getApplicationContext(),NearestPoint.class);
                    startActivity(i);
                }
            };    
         });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.firstactivity, menu);
        return true;
    }

}

我尝试使用这些：

1) 

FirstActivity()
{
    points();
}

2)

FirstActivity(Context context)
{
this.mContext = context;
  points();
}

我都遇到了错误：“应用程序停止工作...”

这是我的日志：

01-19 10:12:40.488: D/dalvikvm(608): newInstance failed: Lcom/example/gpsdistance/FirstActivity;.<init>() not accessible to Landroid/app/Instrumentation;
01-19 10:12:40.538: D/AndroidRuntime(608): Shutting down VM
01-19 10:12:40.578: W/dalvikvm(608): threadid=1: thread exiting with uncaught exception (group=0x40015560)
01-19 10:12:40.688: E/AndroidRuntime(608): FATAL EXCEPTION: main
01-19 10:12:40.688: E/AndroidRuntime(608): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.gpsdistance/com.example.gpsdistance.FirstActivity}: java.lang.IllegalAccessException: access to constructor not allowed
01-19 10:12:40.688: E/AndroidRuntime(608):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1569)
01-19 10:12:40.688: E/AndroidRuntime(608):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
01-19 10:12:40.688: E/AndroidRuntime(608):  at android.app.ActivityThread.access$1500(ActivityThread.java:117)
01-19 10:12:40.688: E/AndroidRuntime(608):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
01-19 10:12:40.688: E/AndroidRuntime(608):  at android.os.Handler.dispatchMessage(Handler.java:99)
01-19 10:12:40.688: E/AndroidRuntime(608):  at android.os.Looper.loop(Looper.java:123)
01-19 10:12:40.688: E/AndroidRuntime(608):  at android.app.ActivityThread.main(ActivityThread.java:3683)
01-19 10:12:40.688: E/AndroidRuntime(608):  at java.lang.reflect.Method.invokeNative(Native Method)
01-19 10:12:40.688: E/AndroidRuntime(608):  at java.lang.reflect.Method.invoke(Method.java:507)
01-19 10:12:40.688: E/AndroidRuntime(608):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
01-19 10:12:40.688: E/AndroidRuntime(608):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
01-19 10:12:40.688: E/AndroidRuntime(608):  at dalvik.system.NativeStart.main(Native Method)
01-19 10:12:40.688: E/AndroidRuntime(608): Caused by: java.lang.IllegalAccessException: access to constructor not allowed
01-19 10:12:40.688: E/AndroidRuntime(608):  at java.lang.Class.newInstanceImpl(Native Method)
01-19 10:12:40.688: E/AndroidRuntime(608):  at java.lang.Class.newInstance(Class.java:1409)
01-19 10:12:40.688: E/AndroidRuntime(608):  at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
01-19 10:12:40.688: E/AndroidRuntime(608):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1561)
01-19 10:12:40.688: E/AndroidRuntime(608):  ... 11 more
01-19 10:12:43.669: I/Process(608): Sending signal. PID: 608 SIG: 9

【问题讨论】：

  • 不要通过构造函数初始化活动，正如你所见，它失败了。另外，您的继承是如何设置的？每个 Activity 都是一个“屏幕”，而不是一个数据对象（因此没有构造函数 init）。考虑将这些东西移到 Helper 类中。
  • 你做错了，因为 gps 实例很可能是 null 并且在所有崩溃时提供 logcat 堆栈跟踪
  • 我已经发布了我正在使用构造函数的类的代码。为了解决这个错误，目前我没有为变量“latA”等设置任何继承。只需在类中使用构造函数就会出错。当我删除构造函数时，一切正常。但我真的需要构造函数，因为我需要在新类中使用 points() 中的计算变量。
  • @phoenix : 如在日志中，您无法将一个活动的构造函数访问到另一个活动，那么您将需要使用 Intent 或 SharedPreferences 在两个活动之间共享价值
  • @ρяσѕρєя K：在发布的 logcat 中，我没有尝试在另一个活动中访问构造函数。目前我删除了甚至在另一个类中为 FirstActivity 创建对象的代码。我只是在 FirstActivity 中声明了构造函数。即使这样做也会出错。

标签： java android methods constructor


【解决方案1】：

您的积分函数包含此调用

latuser = gps.getLatitude();

但是，gps 变量尚未初始化。 gps 对象的创建仅在稍后且仅当用户按下您在onCreate 事件中添加到活动的按钮时才会创建。

早点创建 gps...例如在points()

或创建一个您将在整个课程中使用的函数，而不是简单地调用gps。

function  getGPS() {
  if (gps == null)
    gps = new GPSTracker(FirstActivity.this);
  return gps;
}

【讨论】：

    【解决方案2】：

    是的，有可能

      class AdapterVersion {

        AdapterVersion() {
            show();
        }

        public void show(){
            System.out.println("Hello");
        }
    }

   class AdapterVersionNew {
        AdapterVersionNew()
        {
            AdapterVersion a= new AdapterVersion();
        }
        public void show2(){
            System.out.println("Hello2");
        }
    }


    class test2 {
        public static void main(String[] args) {
            AdapterVersionNew q = new AdapterVersionNew();
        }
    }

    【讨论】：

    • 这段代码调用方法的方式与问题的方式完全相同......它解决了什么问题？
    • 作者没有正确解释这个问题，这个问题需要解释才能解决
    • @ManishNagar：我想我已经尝试过使用您的建议。但我遇到了错误。
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