填充通用列表时出现 java.lang.NullPointerException

Question

所以，我在学校用 Bluej 编程，我们的目标是使用通用列表以图形方式设计一个图鉴。我在设计界面时没有任何问题，但在填写列表时，我遇到了问题。

--列表(自制，所以我提供完整的内容，但重点关注老师告诉我们需要在其中的一些内容)--

import basis.*;
public class List<ContentType>
{
    private Node<ContentType> head;
    private Node<ContentType>  tail;
    private Node<ContentType> current;

    private int Listlenght;

    private boolean isEmpty;
    private boolean hasAccess;

    public List(){
        head = null;
        tail = null;
        current = null;
    }

    public int Listlenght(){
        int counter= 0;
        toFirst();
        while(hasAccess()){
            next();
            counter++;
        }

        return counter;
    }

    public boolean isEmpty(){
        return head== null;
    }    

    public boolean hasAccess(){
        return current!=null;
    }

    public void next(){
        if(isEmpty())
            System.out.println("Liste ist leer");
        else{
            if(!hasAccess())
                System.out.println("Kein momentanes Current");
            else{
                current= current.getNachfolger();    
            }
        }

    }

    public void toFirst(){
        if(isEmpty())
            System.out.println("Liste ist leer");
        else
            current=head;
    }

    public void toLast(){
        if(isEmpty())
            System.out.println("Liste ist leer");
        else
            current=tail;
    }

    public Node<ContentType> getCurrent(){
        if(hasAccess)
            return current;

        else{
            System.out.println("Kein momentanes Current");
            return null;
        }
    }

    public void setCurrent(Node<ContentType> pCurrent){
        if(hasAccess){
            if(pCurrent!= null)
                current.setInhalt(pCurrent.getInhalt());
        }
        else
            System.out.println("Kein momentanes Current");

    }


    public void append(ContentType pContent){

        if(pContent != null){

            Node<ContentType> N;

            N =new Node<ContentType>();
            N.setNachfolger(null);
            N.setInhalt(pContent);
            if(isEmpty){
                head =N;

            }
            else
               tail.setNachfolger(N);

            tail = N;
        }
    }

    public void insert(ContentType pContent){
        if(pContent != null){
            if(hasAccess()){
                Node<ContentType> N;

                N =new Node<ContentType>();
                N.setInhalt(pContent);
                N.setNachfolger(current);
                getPrevious(current).setNachfolger(N);

            }
            else
                System.out.println("Kein momentanes Current");
            if(isEmpty())
                append(pContent);
        }
        else{
            System.out.println("Kein Inhalt");

        }
    }

    public void concat(List<ContentType> pList){
        if (pList != this && pList != null && !pList.isEmpty()) {
            pList.toFirst();
            int i;
            for(i=0;i< pList.Listlenght();i++){

                //append(pList.getCurrent().getInhalt());
                pList.next();    

            }
        }
    }

    public void remove(){
        if(hasAccess ){
            getPrevious(current).setNachfolger(current.getNachfolger());
            next();
        }
    }

    private Node<ContentType> getPrevious(Node<ContentType> pNode) {
        if (pNode != null && pNode != head && !this.isEmpty()) {
            Node<ContentType> temp = head;
            while (temp != null && temp.getNachfolger() != pNode) {
                temp = temp.getNachfolger();
            }
            return temp;
        } else {
            return null;
        }
    }

}

import basis.*;
public class Node <ContentTypeN> {

    private ContentTypeN inhalt;
    private Node<ContentTypeN> Nachfolger;

    public Node(){

        Nachfolger = null;

    }



    public void setInhalt(ContentTypeN ninhalt){
        this.inhalt= ninhalt;
    }

    public void setNachfolger(Node nNachfolger){
        this.Nachfolger= nNachfolger;
    }

    public ContentTypeN getInhalt(){

        return inhalt;
    }

    public Node<ContentTypeN> getNachfolger(){

        return Nachfolger;
    }

}

-- 我的对象类 --

import basis.*;
public class Pokemon{
    private int Index;

    private String Name;
    private String Typ;


    public Pokemon(int pIndex,String pName, String pTyp){
        Index = pIndex;
        Name = pName;
        Typ  = pTyp;



    }

    public int Index(int X){

        return X;
    }
}

主类

import basis.*;
public class Anwendung
{
    private Fenster Window;
    private Farbe Color;

    private Maus mouse;

    private List<Pokemon> Pokedex;

    private TextFeld TFStatus;
    private TextFeld TFName;
    private TextFeld TFTyp;

    private ZahlenFeld ZFNummer;

    private Knopf Knext;
    private Knopf KtoFirst;
    private Knopf ktoLast;
    public Anwendung(){


        WindowDesign();

        gottacatchthemall();
        IhaveNoIdea();
    }

    public void IhaveNoIdea(){
    Pokedex.toFirst();

    Pokedex.getCurrent().getInhalt();


    }
    public void WindowDesign(){
        Window= new Fenster(700,700);
        //Window.setzeHintergrundFarbe(Color.GRUEN);
        Window.setzeTitel("Pokedex");
        Pokedex = new List<Pokemon>();
        mouse = new Maus();

        TFZFGeneration();
        KnopfGeneration();

    }

    public void KnopfGeneration(){
        Knext = new Knopf();
        Knext.setzePosition(550, 300);
        Knext.setzeText("next");

        KtoFirst = new Knopf();
        KtoFirst.setzePosition(550, 350);
        KtoFirst.setzeText("toFirst");

        ktoLast = new Knopf();
        ktoLast.setzePosition(550, 400);
        ktoLast.setzeText("toLast");
    }

    public void TFZFGeneration(){
        TFStatus = new TextFeld();
        TFStatus.setzeGroesse(120,20);
        TFStatus.setzePosition(0,0);
        TFStatus.setzeEditierbar(false);
        TFStatus.setzeText("Methode");

        TFName = new TextFeld();
        TFName.setzeGroesse(120,20);
        TFName.setzePosition(400,300);
        TFName.setzeEditierbar(false);
        TFName.setzeText("PokeName");

        TFTyp = new TextFeld();
        TFTyp.setzeGroesse(120,20);
        TFTyp.setzePosition(400,350);
        TFTyp.setzeEditierbar(false);
        TFTyp.setzeText("PokeTyp");

    }

    public void gottacatchthemall(){
        TFStatus.setzeText("gottacatchthemall");

        Pokemon P;

        P = new Pokemon(1,"Bisasam","Pflanze");
        Pokedex.append(P);

        P = new Pokemon(2,"Bisaknosp","Pflanze");
        Pokedex.append(P);

        P = new Pokemon(3,"Bisaflor","Pflanze");
        Pokedex.append(P);

        P = new Pokemon(4,"Glumanda","Pflanze");
        Pokedex.append(P);

        P = new Pokemon(5,"Glutexo","Feuer");
        Pokedex.append(P);

        P = new Pokemon(6,"Glurak","Feuer");
        Pokedex.append(P);

        P = new Pokemon(7,"Schiggy","Wasser");
        Pokedex.append(P);

        P = new Pokemon(8,"Schillok","Wasser");
        Pokedex.append(P);

        P = new Pokemon(9,"Turtok","Wasser");
        Pokedex.append(P);

    }
}

现在的问题是:每当我启动“Anwendung”类(class)时，都会发生错误。它说:

java.lang.NullPointerException
    at List.append(List.java:100)
    at Anwendung.gottacatchthemall(Anwendung.java:88)
    at Anwendung.<init>(Anwendung.java:25)
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:490)
    at bluej.runtime.ExecServer$3.lambda$run$1(ExecServer.java:834)
    at bluej.runtime.ExecServer.runOnTargetThread(ExecServer.java:930)
    at bluej.runtime.ExecServer$3.run(ExecServer.java:832)

这条线显然有问题

}
            else
               tail.setNachfolger(N);

看起来尾部是未定义的，即使它在下面几行被定义，并且这一行不是 Activity 的，因为当列表为空时，有一个 if 条件阻止 else 条件消失。该列表一开始是空的，我检查过。当我删除此代码时，列表似乎根本没有被填充，因为方法 goFirst 根本不起作用。

The Interface有什么建议么？我有点卡住了，我的老师也无法向我解释，所以如果我有点不具体，我很抱歉。

Answer 1

你说“尾部似乎未定义，尽管它在下面几行被定义了” - 你这是什么意思？在 Java 中，您必须首先分配值 - “上面几行”。

if(isEmpty){
    head =N;
}
else
    tail.setNachfolger(N); // Use brackets

tail = N; 

在上面的代码中，您似乎试图在初始化 tail 之前调用 tail.setNachfolger(N)，因此 tail 为 null，因此出现 NullPointerException。

此外，请始终使用括号(靠近“else”)，因为现在尚不清楚您的意图。

很难理解你的代码，但我认为应该是:

else {
    tail = N;
    tail.setNachfolger(N);
}