java抽象类的定义(java抽象类必须有抽象方法吗)

抽象类实现线性表增删等操作：

public class Sample {
 public static void main(String[] args) {
  Node n  =new Node(10);
  System.out.println(n);
 }
}
//定义一个线性表list类，声明了所有操作
abstract  class  List{
 //在指定角标index处添加元素e
 public abstract void add(int index ,int e) throws IllegalAccessException;
 //删除指定角标index处的元素，并返回元素的值
 public abstract int delete(int index) throws IllegalAccessException;
 //返回指定角标index处的元素
 public abstract int get(int index) throws IllegalAccessException;
//修改指定角标处的index元素为新元素e，并返回原先的值
 public abstract  int set(int index , int e) throws IllegalAccessException;
 //返回对象的字符串表现形式，可以不用定义为抽象函数，有共同的类object
 public abstract String toString ();
//返回线性表有效元素的个数
 public abstract int size();
 //清空线性表
 public abstract  void clear();
 //判断线性表是否为空
 public  abstract boolean isEmpty();
}

class  Node {
 int data;//数据域
 Node next;//指针域

 public Node() {

 }

 public Node(int data) {
  this.data = data;
 }

 public Node(int data, Node next) {
  this.data = data;
  this.next = next;
 }

 public String toString() {
  return "(" + data + ")";
 }


 //linkedList
  class linkedList extends List {
  private Node head;
  private int size;
  public linkedList() {
   head = new Node();
   size = 0;
  }
  public void add(int index, int e) throws IllegalAccessException {
   if(index < 0 ||index > size){
    throw  new IllegalAccessException("get() index out of range");
   }
    Node pre = getNode(index -1 );
    Node n  =  new Node(e);
    n.next = pre.next;
    pre.next = n;
    size++;
  }
//删除指定角标index处的元素，并返回元素的值
   public int delete(int index) throws IllegalAccessException {
    if(index < 0 ||index >= size){
     throw  new IllegalAccessException("get() index out of range");
    }
    Node pre =getNode(index - 1);
    Node p = pre.next;
    pre.next = p.next;
    p.next = null;
    size--;
    return p.data;
   }

   //返回指定角标index处的元素
   public int get(int index) throws IllegalAccessException {
    if(index < 0 ||index >= size){
    throw  new IllegalAccessException("get() index out of range");
   }
   return getNode(index).data;
  }

  //返回指定角标index处的节点对象
  private Node getNode(int index) {
  Node p = head;
   for (int i = 0; i <= index ; i++) {
    p = p.next;
   }
   return p;
  }

  //修改指定角标处的index元素为新元素e，并返回原先的值
  public int set(int index, int e) throws IllegalAccessException {
    if(index < 0 || index >= size){
     throw new IllegalAccessException("set() index out of range");
    }
    Node p = getNode(index);
    int ret = p.data;
    p.data = e;
    return ret;
  }

   @Override
   public String toString() {
    return null;
   }

   //返回线性表有效元素的个数
  public int size() {
   return size;
  }

  //清空线性表
  public void clear() {
   head.next = null;
   size = 0;
  }

  //判断线性表是否为空
  public boolean isEmpty() {
   return size == 0 && head.next == null;
  }
  }
}