Java 检查地图中的关键点

我有一个方法，它应该检查一个键是否在映射中，然后增加一个值

  papers.add(paper); //add a paper to the paper collection
    papersOrdered.get(papers); //returns the papers
    if(!(papersOrdered.containsKey(paper))) {
        paperNo = 1;
        String noPaper = Integer.toString(paperNo); //convert the paperNo integer to a String
        papersOrdered.put(papers,noPaper); //put the rooNo and the paper collection into the HashMap
        System.out.println("'" + paper + "'" + " has been added to the paper order. " + noPaper + " copy ordered" ); //a message to say a paper has been added
    }
    else{
        paperNo = paperNo ++;
        String noPaper = Integer.toString(paperNo); //convert the paperNo integer to a String
        papersOrdered.put(papers,noPaper); //put the rooNo and the paper collection into the HashMap
        System.out.println("'" + paper + "'" + " has been added to the paper order." + noPaper + " copies ordered" ); //a message to say a paper has been added
    }
    System.out.println("======================================================="); ; //a line to seperate text 
    System.out.println(" "); //a blank line

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然而，它不工作，我不知道为什么

基于原始代码，但将整数而不是字符串放入HashMap值中，并在为该类型的纸张添加新顺序时增加键的当前值

int paperNo;
String paper;
HashMap papersOrdered;
...
papers.add(paper); //add a paper to the paper collection
papersOrdered.get(papers); //returns the papers
if(!(papersOrdered.containsKey(paper))) {
    paperNo = 1;
    papersOrdered.put(papers, new Integer(paperNo)); //put the rooNo and the paper collection into the HashMap
    System.out.println("'" + paper + "'" + " has been added to the paper order. " + paperNo + " copy ordered" ); //a message to say a paper has been added
} else{
    paperNo = papersOrdered.get(paper) + 1; //get the current value, and add 1 to it
    papersOrdered.put(papers, new Integer(paperNo)); //put the rooNo and the paper collection into the HashMap
    System.out.println("'" + paper + "'" + " has been added to the paper order." + paperNo + " copies ordered" ); //a message to say a paper has been added
}
System.out.println("======================================================="); ; //a line to seperate text 
System.out.println(" "); //a blank line

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看看Google Guava的Multiset，它就是这样做的。

它以什么方式不起作用？你为什么要做paperNo=paperNo++；？您可以简单地执行paperNo++；变量将递增；永远不会对变量进行正确的增量吗？@JesseLukeOrange-您使用了后缀增量运算符。此运算符使变量递增，但返回其递增前的值。所以，具有讽刺意味的是，虽然一方面更新变量，但另一方面立即将其设置回原始值。阅读更多细节。@JesseLukeOrange关于paperNo=paperNo++的内容可能会引起您的兴趣。我第一次尝试了类似的代码，但遇到了一个.put错误，但平心而论，我认为这也行。找不到适合putjava.util.Array、java.lang.String的方法，int发现：必须使用整数而不是int。更新答案