如何在Java中将对象添加到ArrayList中

Question

这里有两个类，查询语句和Stackov。arraylist用于存储查询语句类的对象。但是最近添加的对象覆盖了前一个对象。如何添加对象以使它们不被覆盖？

QuerySentence.java

public class QuerySentence {

    public static String query;
    public static String label;
    public QuerySentence(){

    }
    public QuerySentence(String query,String label){
        this.query = query;
        this.label = label;
    }
}  

Stackov.java

package QueryClassifier;

import java.util.ArrayList;

public class stackov {

    public static void main(String args[])
    {
        QuerySentence qs1 = new QuerySentence("What state produces the best lobster to eat","LOCATION");
        QuerySentence qs2 = new QuerySentence("What is Dick Clark's birthday","DATE");
        ArrayList<Object> doclist = new ArrayList<Object>();

            doclist.add(0,qs1);
            doclist.add(1,qs2);

            int size = doclist.size();
               while(size>0)
               {
                    QuerySentence qs3 = (QuerySentence) doclist.get(size-1);
                    System.out.println("\nin loop : " + qs3.label + qs3.query);
                    size--;
                }

    }
}   

Answer 1

问题不是来自您的循环，而是来自您的类querystenting。您正在创建静态对象，这意味着您不会为类的不同实例创建不同的字段，但类将只有它们的唯一副本。这里所发生的事情是，您将这些值赋给您的类QueryStenting:

QuerySentence qs1 = new QuerySentence("What state produces the best lobster to eat","LOCATION");

然后抹去这个：

QuerySentence qs2 = new QuerySentence("What is Dick Clark's birthday","DATE");

当您将两个对象添加到arraylist中时，当然在循环中您将打印两个结果。而仅限于QS2中的结果。从字段声明中删除static，它将运行良好：

public String query;
public String label;

在添加时不需要索引查询语句。

doclist.add(qs1);
doclist.add(qs2);

您还可以改进您的循环。你可以这样做：

ArrayList<QuerySentence> doclist = new ArrayList<QuerySentence>();

// some code...

for(QuerySentence q : doclist)
{
    System.out.println("In loop : " + q.label + q.query);
}

还请记住Java惯例：您的类应该以大写(stackov)开头。如果您还研究封装，那么最好声明字段private，并为它们创建getters和setters。这样，您的for-statement应该如下所示：

for(QuerySentence q : doclist)
{
    System.out.println("In loop : " + q.getLabel() + q.getQuery());
}