我正在学习Java，我遇到ArrayList和RandomGenerator的问题。
我有一个名为catalogue的对象，它有一个从另一个名为item的类创建的对象数组列表。
我需要catalogue中的一个方法，它返回列表中item个对象之一的所有信息。
item需要随机选择。
import java.util.ArrayList;
import java.util.Random;
public class Catalogue
{
private Random randomGenerator;
private ArrayList<Item> catalogue;
public Catalogue ()
{
catalogue = new ArrayList<Item>();
}
public Item anyItem()
{
int index = randomGenerator.nextInt(catalogue.size());
return catalogue.get(index);
System.out.println("Managers choice this week" + anyItem + "our recommendation to you");
}
当我尝试编译时，我收到一个错误，指向System.out.println行说..
'找不到符号变量anyItem'
答案 0 :(得分：107)
anyItem是一个方法，System.out.println调用在return语句之后，因此无法编译，因为它无法访问。
可能想重写它：
import java.util.ArrayList;
import java.util.Random;
public class Catalogue
{
private Random randomGenerator;
private ArrayList<Item> catalogue;
public Catalogue()
{
catalogue = new ArrayList<Item>();
randomGenerator = new Random();
}
public Item anyItem()
{
int index = randomGenerator.nextInt(catalogue.size());
Item item = catalogue.get(index);
System.out.println("Managers choice this week" + item + "our recommendation to you");
return item;
}
}
答案 1 :(得分：15)
public static Item getRandomChestItem(List<Item> items) {
return items.get(new Random().nextInt(items.size()));
}
答案 2 :(得分：4)
你回来后的印刷品 - 你永远无法达到这个说法。此外，您从未将anyItem声明为变量。你可能想要
public Item anyItem()
{
int index = randomGenerator.nextInt(catalogue.size());
Item randomItem = catalogue.get(index);
System.out.println("Managers choice this week" + randomItem.toString() + "our recommendation to you");
return randomItem;
}
toString部分只是一个快速的部分 - 你可能想要添加一个方法'getItemDescription'，它为此目的返回一个有用的字符串......
答案 3 :(得分：4)
您必须从system.out.println下方删除return邮件，如下所示：
public Item anyItem()
{
randomGenerator = new Random();
int index = randomGenerator.nextInt(catalogue.size());
Item it = catalogue.get(index);
System.out.println("Managers choice this week" + it + "our recommendation to you");
return it;
}
return语句基本上表示该函数现在将结束。除return语句之外的任何内容都包含在其范围内，这将导致您遇到的行为
答案 4 :(得分：2)
在这里，您可以使用Generics：
private <T> T getRandomItem(List<T> list)
{
Random random = new Random();
int listSize = list.size();
int randomIndex = random.nextInt(listSize);
return list.get(randomIndex);
}
答案 5 :(得分：1)
试试这个
public Item anyItem()
{
int index = randomGenerator.nextInt(catalogue.size());
System.out.println("Managers choice this week" + catalogue.get(index) + "our recommendation to you");
return catalogue.get(index);
}
我强烈建议你买一本书，比如Ivor Horton的Beginning Java 2
答案 6 :(得分：0)
anyItem从未被声明为变量，因此它会导致错误。但更重要的是，您在返回语句后面有代码，这将导致无法访问的代码错误。
答案 7 :(得分：0)
System.out.println（“本周经理选择”+ anyItem +“我们向您推荐”）;
你没有变量anyItem初始化甚至声明。
此代码： + anyItem +
表示获取Object anyItem
的toString方法的值为什么这不起作用的第二件事。在return语句之后有System.out.print。计划永远不会到达那条线。
您可能需要以下内容：
public Item anyItem() {
int index = randomGenerator.nextInt(catalogue.size());
System.out.println("Managers choice this week" + catalogue.get(index) + "our recommendation to you");
return catalogue.get(index);
}
btw：在Java中，它将对象括号放在与函数声明相同的行上。
答案 8 :(得分：0)
我可以看到代码
System.out.println("Managers choice this week" + anyItem + "our recommendation to you");
无法到达。
答案 9 :(得分：-1)
解决方案并不好，即使您修复了该打印的命名和无法访问的声明。
你应该注意的事情 1.随机性种子和大数据，该项目的数量是如此之大的那个随机的返回数量＆lt; itemlist.size（）。
答案 10 :(得分：-1)
请参阅https://gist.github.com/nathanosoares/6234e9b06608595e018ca56c7b3d5a57
public static void main(String[] args) {
RandomList<String> set = new RandomList<>();
set.add("a", 10);
set.add("b", 10);
set.add("c", 30);
set.add("d", 300);
set.forEach((t) -> {
System.out.println(t.getChance());
});
HashMap<String, Integer> count = new HashMap<>();
IntStream.range(0, 100).forEach((value) -> {
String str = set.raffle();
count.put(str, count.getOrDefault(str, 0) + 1);
});
count.entrySet().stream().forEach(entry -> {
System.out.println(String.format("%s: %s", entry.getKey(), entry.getValue()));
});
}
输出：
2.857142857142857
2.857142857142857
8.571428571428571
85.71428571428571
a：2
b：1
c：9
d：88
答案 11 :(得分：-5)
这是一种更好的做事方式：
import java.util.ArrayList;
import java.util.Random;
public class facultyquotes
{
private ArrayList<String> quotes;
private String quote1;
private String quote2;
private String quote3;
private String quote4;
private String quote5;
private String quote6;
private String quote7;
private String quote8;
private String quote9;
private String quote10;
private String quote11;
private String quote12;
private String quote13;
private String quote14;
private String quote15;
private String quote16;
private String quote17;
private String quote18;
private String quote19;
private String quote20;
private String quote21;
private String quote22;
private String quote23;
private String quote24;
private String quote25;
private String quote26;
private String quote27;
private String quote28;
private String quote29;
private String quote30;
private int n;
Random random;
String teacher;
facultyquotes()
{
quotes=new ArrayList<>();
random=new Random();
n=random.nextInt(3) + 0;
quote1="life is hard";
quote2="trouble shall come to an end";
quote3="never give lose and never get lose";
quote4="gamble with the devil and win";
quote5="If you don’t build your dream, someone else will hire you to help them build theirs.";
quote6="The first step toward success is taken when you refuse to be a captive of the environment in which you first find yourself.";
quote7="When I dare to be powerful – to use my strength in the service of my vision, then it becomes less and less important whether I am afraid.";
quote8="Whenever you find yourself on the side of the majority, it is time to pause and reflect";
quote9="Great minds discuss ideas; average minds discuss events; small minds discuss people.";
quote10="I have not failed. I’ve just found 10,000 ways that won’t work.";
quote11="If you don’t value your time, neither will others. Stop giving away your time and talents. Value what you know & start charging for it.";
quote12="A successful man is one who can lay a firm foundation with the bricks others have thrown at him.";
quote13="No one can make you feel inferior without your consent.";
quote14="Let him who would enjoy a good future waste none of his present.";
quote15="Live as if you were to die tomorrow. Learn as if you were to live forever.";
quote16="Twenty years from now you will be more disappointed by the things that you didn’t do than by the ones you did do.";
quote17="The difference between a successful person and others is not a lack of strength, not a lack of knowledge, but rather a lack of will.";
quote18="Success is about creating benefit for all and enjoying the process. If you focus on this & adopt this definition, success is yours.";
quote19="I used to want the words ‘She tried’ on my tombstone. Now I want ‘She did it.";
quote20="It is our choices, that show what we truly are, far more than our abilities.";
quote21="You have to learn the rules of the game. And then you have to play better than anyone else.";
quote22="The successful warrior is the average man, with laser-like focus.";
quote23="Develop success from failures. Discouragement and failure are two of the surest stepping stones to success.";
quote24="If you don’t design your own life plan, chances are you’ll fall into someone else’s plan. And guess what they have planned for you? Not much.";
quote25="The question isn’t who is going to let me; it’s who is going to stop me.";
quote26="If you genuinely want something, don’t wait for it – teach yourself to be impatient.";
quote27="Don’t let the fear of losing be greater than the excitement of winning.";
quote28="But man is not made for defeat. A man can be destroyed but not defeated.";
quote29="There is nothing permanent except change.";
quote30="You cannot shake hands with a clenched fist.";
quotes.add(quote1);
quotes.add(quote2);
quotes.add(quote3);
quotes.add(quote4);
quotes.add(quote5);
quotes.add(quote6);
quotes.add(quote7);
quotes.add(quote8);
quotes.add(quote9);
quotes.add(quote10);
quotes.add(quote11);
quotes.add(quote12);
quotes.add(quote13);
quotes.add(quote14);
quotes.add(quote15);
quotes.add(quote16);
quotes.add(quote17);
quotes.add(quote18);
quotes.add(quote19);
quotes.add(quote20);
quotes.add(quote21);
quotes.add(quote22);
quotes.add(quote23);
quotes.add(quote24);
quotes.add(quote25);
quotes.add(quote26);
quotes.add(quote27);
quotes.add(quote28);
quotes.add(quote29);
quotes.add(quote30);
}
public void setTeacherandQuote(String teacher)
{
this.teacher=teacher;
}
public void printRandomQuotes()
{
System.out.println(quotes.get(n++)+" ~ "+ teacher);
}
public void printAllQuotes()
{
for (String i : quotes)
{
System.out.println(i.toString());
}
}
}