Java静态非静态[重复]

Question

我的程序有问题。Eclipse写道：

Exception in thread "main" java.lang.Error: Unresolved compilation problems: 
Cannot make a static reference to the non-static method umfang() from the type GeoPolygon
Cannot make a static reference to the non-static method laenge() from the type GeoPolyline

我做错了什么？我的应用程序中有2个错误。班你能改进我的代码并解释一下吗？先谢谢你。

    import java.util.ArrayList; //import Array List (java.util)
import java.util.List;  //import List (java.util)

public class App {      //new public class App


    public static void main(String[] args) { //main methode

        GeoPoint p1 = new GeoPoint(10.681,-48.857);   // new Points (x,y)
        GeoPoint p2 = new GeoPoint(96.980,20.724);
        GeoPoint p3 = new GeoPoint(66.647,-66.558);
        GeoPoint p4 = new GeoPoint(-2.674,-58.571);
        GeoPoint p5 = new GeoPoint(40.11,-12.342);
        GeoPoint p6 = new GeoPoint(27.782,46.809);
        GeoPoint p7 = new GeoPoint(54.759,-46.709);
        GeoPoint p8 = new GeoPoint(-33.89,-90.787);
        GeoPoint p9 = new GeoPoint(15.84,-67.553);
        GeoPoint p10 = new GeoPoint(19.481,51.331);


        List<GeoPoint> list1 = new ArrayList<GeoPoint>(); // Create ArrayList and add Points
        list1.add(p1);
        list1.add(p2);
        list1.add(p3);
        list1.add(p4);
        list1.add(p5);
        list1.add(p6);
        list1.add(p7);
        list1.add(p8);
        list1.add(p9);
        list1.add(p10);



        BoundingBox bbox = new BoundingBox(list1);          // get the list from BoundingBox
        double c = GeoPolygon.umfang();     //new variable c = height
        double d = GeoPolyline.laenge();        //new variable d = width

        System.out.println("Das Array beinhaltet die Punkte: " + list1 ); // This prints: "Das Array beinhaltet die Punkte" and the list
        System.out.println("Der Gesamtlaenge des Polygons betraegt (" + d +")." );  // This prints the polygons circumference
        System.out.println("Der Umang des Polygons betraegt: (" + c +")." );    // This prints the polygons area


    }
} 

多边形

import java.util.List;

public class GeoPolygon implements Geometry {

    private List<GeoPoint> Polylinelist;
    private String name;
    private double umfang;

    public double umfang() {

        umfang = 0;

        for (int i = 0; i < Polylinelist.size() -1 ; i++) {
            GeoPoint p1 = Polylinelist.get(i);
            GeoPoint p2 = Polylinelist.get(i + 1);

            umfang += GeoUtil.distance(p1, p2);

        }
        umfang += GeoUtil.distance(Polylinelist.get(0), Polylinelist.get(Polylinelist.size() -1 ));

        return umfang;
    }

    public GeoPolygon(List<GeoPoint> punktliste, String name) {
        this.Polylinelist = punktliste;
        this.name = name;
    }

    public BoundingBox getBoundingBox() {
        BoundingBox bbox = new BoundingBox(Polylinelist);
        return bbox;
    }

    public String getName() {
        {
            return name;
        }
    }

    public void addGeoPoint(GeoPoint geoPoint) {
        Polylinelist.add(geoPoint);
    }

}

地聚线

import java.util.List;

public class GeoPolyline implements Geometry {

    private List<GeoPoint> Polylinelist;
    private String name;
    private double laenge;

    public double laenge() {

        laenge = 0;

        for (int i = 0; i < Polylinelist.size() - 1; i++) {
            GeoPoint p1 = Polylinelist.get(i);
            GeoPoint p2 = Polylinelist.get(i + 1);

            laenge += GeoUtil.distance(p1, p2);

        }

        return laenge;
    }

    public BoundingBox getBoundingBox() {
        BoundingBox bbox = new BoundingBox(Polylinelist);
        return bbox;
    }

    public GeoPolyline(List<GeoPoint> punktliste, String name) {    //Konstruktor
        this.Polylinelist = punktliste;
        this.name = name;
    }

    public String getName() {
        {
            return name;
        }
    }

    public List<GeoPoint> getPolylinelist() {
        {
            return Polylinelist;
        }

    }

    public void addGeoPoint(GeoPoint geoPoint) {
        Polylinelist.add(geoPoint);
    }

}

Answer 1

您需要将关键字static添加到GeoPolygon上的方法中

    public static double umfang() {
    ...
    }

Answer 2

在你的应用程序类中，你在publicstaticvoidmain中有以下行：

double c = GeoPolygon.umfang();

注意它所在的方法是静态的。因此，您不能调用umfang（），因为它不是静态的。这同样适用于laenge（）。

在这里阅读静电

可以直接访问类中的静态项，但不能直接访问类中的非静态项。若要访问非静态项（如方法或字段），必须创建该类的新实例。

Answer 3

double c = GeoPolygon.umfang();     
double d = GeoPolyline.laenge();

这些不是静态方法。如果希望它们是静态的，请将它们声明为静态的。

或者，您可以在调用类之前创建类的实例：

如：

GeoPolygon x = new GeoPolygon();
double c = x.umfang();