【发布时间】：2014-10-04 19:03:30
【问题描述】：
在下面的程序中，DList1 是一个列表抽象。
我想在 DList1 类的 main() 函数中运行一些单元测试用例。
具体来说，
在Line 105、l.head.item != 9 的main() 方法中，出现编译错误：Incompatible operand types Object and int。
在编译时，值9 是基本类型int，l.head.item 是class Object 类型。
在运行时，l.head.item 的类型为 class Integer。不确定9的值类型。
/* DList1.java */
/**
* A DList1 is a mutable doubly-linked list. (No sentinel, not
* circularly linked.)
*/
public class DList1 {
/**
* head references the first node.
* tail references the last node.
*
* DO NOT CHANGE THE FOLLOWING FIELD DECLARATIONS.
*/
protected DListNode1 head;
protected DListNode1 tail;
protected long size;
/* DList1 invariants:
* 1) head.prev == null.
* 2) tail.next == null.
* 3) For any DListNode1 x in a DList, if x.next == y and x.next != null,
* then y.prev == x.
* 4) For any DListNode1 x in a DList, if x.prev == y and x.prev != null,
* then y.next == x.
* 5) The tail can be accessed from the head by a sequence of "next"
* references.
* 6) size is the number of DListNode1s that can be accessed from the
* head by a sequence of "next" references.
*/
/**
* DList1() constructor for an empty DList1.
*/
public DList1() {
this.head = null;
this.tail = null;
this.size = 0;
}
/**
* insertFront() inserts an item at the front of a DList1.
*/
public void insertFront(Object item) {
if(this.head == null){
this.head = new DListNode1(item);
this.tail = this.head;
}else{
DListNode1 newNode = new DListNode1(item);
newNode.next = this.head;
this.head.prev = newNode;
this.head = newNode;
}
this.size++;
}
/**
* removeFront() removes the first item (and node) from a DList1. If the
* list is empty, do nothing.
*/
public void removeFront() {
if(this.size == 0){
return;
}else if(size ==1){
this.head = null;
this.tail = null;
}else{
this.head.next.prev = null;
this.head = this.head.next;
}
}
/**
* toString() returns a String representation of this DList.
*
* DO NOT CHANGE THIS METHOD.
*
* @return a String representation of this DList.
*/
public String toString() {
String result = "[ ";
DListNode1 current = head;
while (current != null) {
result = result + current.item + " ";
current = current.next;
}
return result + "]";
}
public static void main(String[] args) {
// DO NOT CHANGE THE FOLLOWING CODE.
DList1 l = new DList1();
System.out.println("### TESTING insertFront ###\nEmpty list is " + l);
l.insertFront(9);
System.out.println("\nInserting 9 at front.\nList with 9 is " + l);
if (l.head == null) {
System.out.println("head is null.");
} else {
if (l.head.item != 9) { //Line 105
System.out.println("head.item is wrong.");
}
if (l.head.prev != null) {
System.out.println("head.prev is wrong.");
}
}
if (l.tail == null) {
System.out.println("tail is null.");
} else {
/*if (l.tail.item != 9) {
System.out.println("tail.item is wrong.");
}
if (l.tail.next != null) {
System.out.println("tail.next is wrong.");
}*/
}
if (l.size != 1) {
System.out.println("size is wrong.");
}
l.insertFront(8);
System.out.println("\nInserting 8 at front.\nList with 8 and 9 is " + l);
if (l.head == null) {
System.out.println("head is null.");
} else {
/*if (l.head.item != 8) {
System.out.println("head.item is wrong.");
}*/
if (l.head.prev != null) {
System.out.println("head.prev is wrong.");
}
if (l.head.next != l.tail) {
System.out.println("head.next is wrong.");
}
}
if (l.tail == null) {
System.out.println("tail is null.");
} else {
if (l.tail.next != null) {
System.out.println("tail.next is wrong.");
}
if (l.tail.prev != l.head) {
System.out.println("tail.prev is wrong.");
}
}
if (l.size != 2) {
System.out.println("size is wrong.");
}
}
}
/* DListNode1.java */
/**
* A DListNode1 is a node in a DList1 (doubly-linked list).
*/
class DListNode1 {
/**
* item references the item stored in the current node.
* prev references the previous node in the DList.
* next references the next node in the DList.
*
* DO NOT CHANGE THE FOLLOWING FIELD DECLARATIONS.
*/
Object item;
DListNode1 prev;
DListNode1 next;
/**
* DListNode1() constructor.
*/
DListNode1() {
this.item = null;
this.prev = null;
this.next = null;
}
DListNode1(Object item) {
this.item = item;
this.prev = null;
this.next = null;
}
}
我的问题：
如何使类型在编译时和运行时都兼容，并让值在Line 105 中进行比较？ 9 的值的运行时类型是什么？
【问题讨论】：
-
转换为整数，然后 !l.item.head.equals(9)
-
@sscnapoli1926 更新中的小错字
head.item。我的第二部分查询呢？ -
如果你指的是“！”这不是错字，而是否定运算符！无论如何，如果您需要在运行时决定项目的类型，唯一的方法就是用泛型替换 Object 字段。如果“item”可以是你的类的一个实例，你必须提供一个 equals() 覆盖，游戏就完成了
-
item.head是一个错字 -
@sscnapoli1926 - 泛型不允许您“在运行时决定项目的类型”。相反，它们允许您对不同的参数类型（即，在代码中的不同位置）使用相同的类，而无需进行显式转换。 （转换由编译器隐式执行。）
标签： java