我调用方法时出现空指针异常

Question

当我调用我的两个方法时,我得到一个空指针异常.我不知道,为什么我明白了.当我检查代码时似乎是正确的

这是我的HentbestillingsordreHandler班

public class HentbestillingsordreHandler extends JPanel{

private Connection con = null;
private ResultSet rs = null;
private HentbestillingsordreRegistrer ho;
private ArrayList<HentbestillingsordreRegistrer> hbor = new ArrayList<HentbestillingsordreRegistrer>();

HentbestillingsordreHandler() {

}

public void Hentbestillingsordre(){
    KaldSQL ks = new KaldSQL();
    try {
        con = ks.connectNow();
    } catch (ClassNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (SQLException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    ResultSet rs = ks.Hentalleordreliste(con);
    try {
        while(rs.next()){
            String status = "";
            if(rs.getInt("BestillingsStatus") == 1){
                status = "Leveret";
            }else{
                status = "Ikke Leveret";
            }
            System.out.println(status);
            HentbestillingsordreRegistrer ho = new HentbestillingsordreRegistrer(
                    rs.getInt("BestillingsID"),
                    rs.getString("BestillingsStatus"),
                    rs.getInt("ModtagetAf"),
                    rs.getInt("Vare"),
                    rs.getInt("Antal"));

                    hbor.add(ho);
                    System.out.println(ho);

        }
    } catch (SQLException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    try {
        rs.close();
    } catch (SQLException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

}   
public void printBestillingsordre(){

        for(HentbestillingsordreRegistrer hentbestillingsordreRegistrer: hbor){
        String temp = "";
        temp += ho.getLeverandoerID()+" \n";
        System.out.println(temp);
    }

}

}

我的kaldsql

public ResultSet Hentalleordreliste(Connection con){

    ResultSet Hentalleordreliste = null;

    try {
        PreparedStatement statement = con.prepareStatement("select varebestillinger.BestillingsID, " +
                "varebestillinger.LeverandoerID, "+
                "varebestillinger.BestillingsStatus, varebestillinger.ModtagetAf, "+
                "varebestillingsliste.Vare, " +
                "varebestillingsliste.Antal from varebestillinger left outer join " +
                "varebestillingsliste on  ListeID = BestillingsID");
                ResultSet result = statement.executeQuery();
                Hentalleordreliste = result;
    } catch (Exception e) {
        e.printStackTrace();
    }
    return Hentalleordreliste;


}

这是我的班级HentbestillingsordreRegistrer

int LeverandoerID;
String BestillingsStatus;
int ModtagetAf;
int Vare;
int Antal;

public HentbestillingsordreRegistrer(int LeverandoerID, String BestillingsStatus, int ModtagetAf,int Vare,int Antal){
    this.LeverandoerID = LeverandoerID;
    this.BestillingsStatus = BestillingsStatus;
    this.ModtagetAf = ModtagetAf;
    this.Antal = Antal;
}

public int getLeverandoerID() {
    return LeverandoerID;
}

public void setLeverandoerID(int leverandoerID) {
    LeverandoerID = leverandoerID;
}

public String getBestillingsStatus() {
    return BestillingsStatus;
}

public void setBestillingsStatus(String bestillingsStatus) {
    BestillingsStatus = bestillingsStatus;
}

public int getModtagetAf() {
    return ModtagetAf;
}

public void setModtagetAf(int modtagetAf) {
    ModtagetAf = modtagetAf;
}

public int getVare() {
    return Vare;
}

public void setVare(int vare) {
    Vare = vare;
}

public int getAntal() {
    return Antal;
}

public void setAntal(int antal) {
    Antal = antal;
}

当我打电话给它时,我输入它

    HentbestillingsordreHandler hboh = null;
hboh.Hentbestillingsordre();
hboh.printBestillingsordre();

Answer 1

不要用

HentbestillingsordreHandler hboh = null;

相反,试试吧

HentbestillingsordreHandler hboh = new HentbestillingsordreHandler();

当你打电话时new HentbestillingsordreHandler(),你正在创建一个新的HentbestillingsordreHandler对象,它将被存储在计算机内存的某个地方; 然后,你要hboh指出在你访问时使用的那段特定内存hboh.

然而,第一个陈述只是指向hboh计算机内存中无用(且无法访问)的点; 所以在运行时,当你尝试访问时hboh,你会得到一个Null Pointer Exception,因为变量hboh是"指向"一个不存在的内存.