我调用方法时出现空指针异常

Question

i'm getting a null pointer exception when i call my two methods. And i have no idea, why i get it. when i check the code it seems right

this is my HentbestillingsordreHandler class

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);
    }

}

}

my 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;


}

and this is my class 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;
}

and when i call it i type this

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

Answer 1

Don't use

HentbestillingsordreHandler hboh = null;

Instead, try

HentbestillingsordreHandler hboh = new HentbestillingsordreHandler();

When you call new HentbestillingsordreHandler() , you're creating a new HentbestillingsordreHandler object that will be stored somewhere in the computer's memory; you're then telling hboh to point to that particular piece of memory to be used whenever you access hboh .

The first statement, however, simply points hboh to a useless (and unreachable) point of the computer's memory; so at runtime, when you try to access hboh , you get a Null Pointer Exception, since the variable hboh is "Pointing" to a non-existent piece of memory.

Answer 2

You are declaring the name of the method as well as a variale inside it with the same name ::

public ResultSet Hentalleordreliste(Connection con){
ResultSet result = 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");
                result = statement.executeQuery();

    } catch (Exception e) {
        e.printStackTrace();
    }
    return result;


}

Answer 3

Java as opposed to Objective-C for example, doesn't allow method invocations on null references. So in general you have to initialize your reference with an object instance before trying to call a method on it.

Answer 4

NullPointerExceptions are usually pretty easy to figure out. In the stacktrace you get the class and line number where the exception occurred. In your case it should be the line hboh.Hentbestillingsordre() . Using that information you can check where a value was assigned to the variable. Your assignment is HentbestillingsordreHandler hboh = null . Since you cannot make any method calls on null you get the exception.

In some cases you do not need any attributes of the objects for the method. In that case you can make the method static . You would call it with HentbestillingsordreHandler.Hentbestillingsordre() then, without the need for an instance of that class.