java为什么main（）很好地显示“Lukaku”并包含_names（）返回null？

Question

我已经创建了一个通常可以在方法“main（）”和“contains_names（）”中使用的hashmap，我在main的方法的hashmap中添加了一些元素，但是如果我尝试在“contains_names”中显示它们，它会返回“null”，我不明白为什么

例如： 如果我试着做这个系统。出来println（names.get（“Lukaku”））在“main”中返回0

如果我对“contains_names（）”执行相同的操作，它将返回null

import java.util.HashMap;
import java.util.ArrayList;

public class devideV2 {

    static int no_element=8;

    public static HashMap<String,Integer> names =  newHashMap<String,Integer>();


    /**
     *
     * @param arrays is an arraylist of String which contains some names (ArrayList)
     * @return the names who figure out more than an half or "null" if there's not (String)
     */
    public static String contains_names(ArrayList<String> arrays){

        //will return null but why ?
        System.out.println(names.get("lukaku"));

        //increment the value of the key that is represented by the first element of arrays
        names.put(arrays.get(0),names.get(arrays.get(0)+1));

        //will return True
        System.out.println(names.containsKey("Lukaku"));

        //At line 20 I've a null pointer Exception who has been thrown

        //if the name figure out more than an half then return this name
        if(names.get(arrays.get(0)) > (no_element/2)){
            return arrays.get(0);
        }


        //remove the name
        arrays.remove(0);

        //making recursion
        if(arrays.size()>0) {
            return contains_names(arrays);
        }

        //if no names figure out more than an half then return "null"
        return "null";
    }

    public static void main(String[] args){

        String[] names_array={"Lukaku", "Lukaku", "Lukaku", "Hazard","Hazard","Hazard","Hazard","Hazard"};
        ArrayList<String> Names_arrayList= new ArrayList<String>();

        //put every names in the Hashmap and their occurences to 0
        for(int i=0;i<names_array.length;i++){
            Names_arrayList.add(names_array[i]);
            names.put(names_array[i],0);
        }

        //will return 0
        System.out.println(names.get("Lukaku"));

        System.out.println(contains_names(Names_arrayList));
    }

}

Answer 1

括号中有一个拼写错误：

    names.put(arrays.get(0),names.get(arrays.get(0)+1));

这是用“卢卡库1”的计数覆盖“卢卡库”，该计数不存在，因此返回null

您需要的是（具有适当的格式）：

    names.put(arrays.get(0), names.get(arrays.get(0)) + 1);