Java初学者CSDN第4问！

[nisus_lee]

class Link
{
public long dData;
public Link next;

public Link(long dd)
{
dData=dd;
}

public void displayLink()
{
System.out.print(dData+" ");
}
}

class LinkList
{
private Link first;

public LinkList()
{
first=null;
}

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

public void insertFirst(long dd)
{
Link newLink=new Link(dd);
newLink.next=first;
first=newLink;
}

public long deleteFirst()
{
Link temp=first;
first=first.next;
return temp.dData;
}

public void displayList()
{
Link current=first;
while(current!=null)
{
current.displayLink();
current=current.next;
}
System.out.println("");
}
}

class LinkStack
{
private LinkList theList;

public void push(long j)
{
theList.insertFirst(j);
}

public long pop()
{
return theList.deleteFirst();
}

public boolean isEmpty()
{
return (theList.isEmpty());
}

public void displayStack()
{
System.out.print("Stack(top-->bottom):");
theList.displayList();
}
}

class LinkStackApp
{
public static void main(String[] args)
{
LinkStack theStack=new LinkStack();

theStack.push(20);
theStack.push(40);

theStack.displayStack();

theStack.push(60);
theStack.push(80);

theStack.displayStack();

theStack.pop();
theStack.pop();

theStack.displayStack();
}
}

编译通过，可是无法运行，显视的是：
Exception in thread "main" java.lang.NullPointerExcep
at LinkStack.push(LinkStackApp.java:66)
at LinkStackApp.main(LinkStackApp.java:92)
望各位指点下 ！

[classjava]

class LinkStack
{
private LinkList theList;//这个改成private LinkList theList=new LinkList();

public void push(long j)
{
theList.insertFirst(j);
}

public long pop()
{
return theList.deleteFirst();
}

public boolean isEmpty()
{
return (theList.isEmpty());
}

public void displayStack()
{
System.out.print("Stack(top-->bottom):");
theList.displayList();
}
}

[ianok]

up

[zhaoguoqingluntan]

up

[yanxiazhiqiu]

你就是问题终结者 ，求倒！

[nisus_lee]

问题解决 ..谢谢 ……

[superslash]

wo dao