This question already has answers here:
What is a NullPointerException, and how do I fix it?
(12个答案)
2年前关闭。
在BlueJ和Eclipse中,当检查String-equality时,我得到了nullpointer异常。
这是我的代码:
在方法
奇怪的是,
因此,在方法
这打破了。
若要更正此问题,请在实例化
您会后悔
而是创建方法
就像是
编辑
最后一件事,您要做的测试
应该给你相同的结果
但是更容易理解(
(12个答案)
2年前关闭。
在BlueJ和Eclipse中,当检查String-equality时,我得到了nullpointer异常。
这是我的代码:
import java.util.Set;
import java.util.HashMap;
import java.util.ArrayList;
import java.util.Iterator;
/**
* Class Room - a room in an adventure game.
*
* This class is part of the "World of Zuul" application.
* "World of Zuul" is a very simple, text based adventure game.
*
* A "Room" represents one location in the scenery of the game. It is
* connected to other rooms via exits. For each existing exit, the room
* stores a reference to the neighboring room.
*
* @author Michael Kölling and David J. Barnes
* @version 2016.02.29
*/
public class Room
{
private String description;
private String itemDescription;
private HashMap<String, Room> exits; // stores exits of this room.
private ArrayList<Item> items;
/**
* Create a room described "description". Initially, it has
* no exits. "description" is something like "a kitchen" or
* "an open court yard".
* @param description The room's description.
*/
public Room(String description)
{
this.description = description;
this.itemDescription = "This room contains: ";
exits = new HashMap<>();
items = new ArrayList<Item>();
}
public Room(String description, Item item)
{
items = new ArrayList<Item>();
this.description = description;
addItem(item);
this.itemDescription = "This room contains: " + item.getLongDescription() ;
exits = new HashMap<>();
}
/**
* Define an exit from this room.
* @param direction The direction of the exit.
* @param neighbor The room to which the exit leads.
*/
public void setExit(String direction, Room neighbor)
{
exits.put(direction, neighbor);
}
public void addItem(Item item)
{
items.add(item);
System.out.println(itemDescription.equals("This room contains: "));
this.itemDescription = this.itemDescription + /*(this.itemDescription.equals("This room contains: ") ?
"" : ", ")*/ ", " + item.getLongDescription();
}
public void removeItem(String shortItemDescription){
int i= 0;
Iterator<Item> it = items.iterator();
while(it.hasNext()){
if (it.next().getShortDescription().equals(shortItemDescription)){
it.remove();
i++;
}
else
i++;
}
}
public ArrayList<Item> getItems(){
return items;
}
/**
* @return The short description of the room
* (the one that was defined in the constructor).
*/
public String getShortDescription()
{
return description;
}
/**
* Return a description of the room in the form:
* You are in the kitchen.
* Exits: north west
* @return A long description of this room
*/
public String getLongDescription()
{
return "You are " + description + ".\n" + (itemDescription.equals("This room contains: ") ?
"This room is empty" : itemDescription) + ".\n" + getExitString();
}
/**
* Return a string describing the room's exits, for example
* "Exits: north west".
* @return Details of the room's exits.
*/
private String getExitString()
{
String returnString = "Exits:";
Set<String> keys = exits.keySet();
for(String exit : keys) {
returnString += " " + exit;
}
return returnString;
}
/**
* Return the room that is reached if we go from this room in direction
* "direction". If there is no room in that direction, return null.
* @param direction The exit's direction.
* @return The room in the given direction.
*/
public Room getExit(String direction)
{
return exits.get(direction);
}
}
在方法
addItem(Item item)
中,当检查字符串比较方法itemDescription.equals("This room contains: ")
时以及在System.out.println()
语句中以及在对itemDescription.equals("This room contains: ")
的三元运算符测试中,都得到了nullpointer异常错误。奇怪的是,
getLongDescription()
中的三元运算符可以正常工作。 最佳答案
如果使用Room
构建Item
实例。您在实例化addItem
变量之前调用itemDescription
。
public Room(String description, Item item)
{
// ...
addItem(item);
this.itemDescription = "This room contains: " + item.getLongDescription() ;
// ...
}
因此,在方法
addItem
中,this.itemDescription
仍然为空。public void addItem(Item item)
{
// ...
System.out.println(itemDescription.equals("This room contains: "));
// ...
}
这打破了。
若要更正此问题,请在实例化
addItem
后致电itemDescription
。请注意,您不需要添加item
描述,因为这已经在addItem
方法中完成了。public Room(String description, Item item)
{
// ...
this.itemDescription = "This room contains: ";
addItem(item);
// ...
}
您会后悔
String itemDescription
真的很快!而是创建方法
getRoomDescription
,该方法将迭代items
列表并生成该String itemDescription
(使用StringBuilder
)!就像是
public String getRoomDescription(){
StringBuilder sb = new StringBuilder("Rooms contains :");
for(Item i : items){
sb.append("\n".append(i.getLongDescription());
}
return sb.toString();
}
编辑
itemDescription
变量很麻烦,并且容易出现问题(就像您遇到的一样)。最后一件事,您要做的测试
itemDescription.equals("This room contains: ")
应该给你相同的结果
items.size() == 0
但是更容易理解(
items
列表中没有值),并且如果您要编辑打印描述的方式,也不太容易出错。07-24 20:53