java - java 类中带有静态方法的私有(private)构造函数

Question

我正在尝试创建一个类，以标记的降序对数组列表进行排序。由于我所有的方法都是静态的，我想编写一个构造函数来防止类实例化，但不知道如何去做。我读到可以使用私有(private)构造函数，但不确定如何对其进行编码。

这是我的代码:

import java.util.*;
import java.io.*;

public class ProcessDegreeMark{

public static ArrayList<Finalist> finalistsInList(String s) throws Exception{
    ArrayList<Finalist> finalists = new ArrayList<Finalist>();
    String id;
    double mark;
    Scanner in = null;
    try
    {
        in = new Scanner(new FileReader(s));
        try
        {
            while(in.hasNextLine())
            {
                id =in.nextLine();
                mark = Double.parseDouble(in.nextLine());
                finalists.add(new Finalist(id,mark));
            }
        }
        finally
        {
            in.close();
        }
    }
    catch(IOException e)
    {
        System.out.println(s+" not found");
    }
    return finalists;
 }


public static void displayFinalists(ArrayList<Finalist> finalists){
  for (int i = 0; i < finalists.size(); i++)
  {
       System.out.println(finalists.get(i));
  }
}


public static void findFinalistID(ArrayList<Finalist> a, String s){
    int count =0;
    for (int i=1;i<a.size();i++)
    {
        if (((a.get(i))).getId().equals(s))
        {
            System.out.println(a.get(i));
            count++;
        }
    }
    if(count==0)
    {
        System.out.println("No candidate found with ID number "+s);
    }
}

public static void findFinalistClass(ArrayList<Finalist> a, String s){
    int count =0;
    for (int i=1;i<a.size();i++)
    {
        if (((a.get(i))).getdegreeClass().equals(s))
        {
            System.out.println(a.get(i));
            count++;
        }
    }
    if(count==0)
    {
        System.out.println("No candidate found with degree class "+s);
    }
}

public static ArrayList<Finalist> sortDegreeMark(ArrayList<Finalist> a){
    ArrayList<Finalist> sortedFinalists = new ArrayList<Finalist>();
    sortedFinalists.addAll(a);
    Collections.sort(sortedFinalists, new FinalistComparator());
    return sortedFinalists;


}

public static void finalistsToFile2(ArrayList<Finalist> finalists, String s) {
    PrintStream out = null;
    try
    {
        out = new PrintStream(new FileOutputStream(s));
        try
        {
            for(int i = 0; i < finalists.size(); i++)
            {
                out.println(finalists.get(i));
            }
        }
        finally
        {
            out.close();
        }
    }
    catch (FileNotFoundException e)
    {
        e.printStackTrace();
    }

}

public static void findAndSaveFinalistClass(ArrayList<Finalist> a, String s){
    ArrayList<Finalist> searchFinalists = new ArrayList<Finalist>();
    int count =0;
    for (int i=1;i<a.size();i++)
    {
        if (((a.get(i))).getdegreeClass().equals(s))
        {
            System.out.println(a.get(i));
            searchFinalists.add(a.get(i));
            finalistsToFile2(searchFinalists,"testSorted.txt");
            count++;
        }
    }
    if(count==0)
    {
        System.out.println("No candidate found with degree class "+s);
    }
}

    public static void main(String[] args) throws Exception{
 System.out.println("/****************************************************/");
         System.out.println("/*******finalistsInList with invalid file name*******/");
         System.out.println();
         ArrayList<Finalist> testList = finalistsInList("file***.txt");
         System.out.println();
         System.out.println("/****************************************************/");
         System.out.println("/********finalistsInList with valid file name********/");
         System.out.println("/********display to check arraylist populated********/");
         System.out.println();
         ArrayList<Finalist> finalists = finalistsInList("finalMark.txt");
         displayFinalists(finalists);
         System.out.println();
         System.out.println("/****************************************************/");
         System.out.println("/*testing findFinalistID with valid and invalid data*/");
         System.out.println();
         findFinalistID(finalists, "75021");  
         findFinalistID(finalists, "21050");
         System.out.println();
         System.out.println("/****************************************************/");
         System.out.println("/*test findFinalistClass with valid and invalid data*/");
         System.out.println();
         findFinalistClass(finalists, "FIRST"); 
         findFinalistClass(finalists, "THIRD"); 
         System.out.println();
         System.out.println("/****************************************************/");
         System.out.println("/*****run sortedFinalists then test with display*****/");
         System.out.println();
         ArrayList<Finalist> sortedFinalists = sortDegreeMark(finalists);
         displayFinalists(sortedFinalists);
         System.out.println();
         System.out.println("/****************************************************/");
         System.out.println("/*****test finalistsToFile2 with sorted arraylist*****/");
         System.out.println("/**************check file testSorted.txt**************/");
         System.out.println();
         finalistsToFile2(sortedFinalists, "testSorted.txt"); //save the sorted arraylist to a new file, check by opening file
         System.out.println();
         System.out.println("/****************************************************/");
         System.out.println("/*test findAndSaveFinalistClass with valid and invalid data*/");
         System.out.println();
         findAndSaveFinalistClass(finalists, "FIRST"); //test method finds
         findAndSaveFinalistClass(finalists, "THRID"); //check appropriate error message when nothing found, open new text file
         System.out.println();
         System.out.println("/*********************THE END************************/");
     }
}

Answer 1

静态方法属于类。我真的不明白你为什么担心有/没有类实例化。无论他们创建一个实例还是多个实例，调用者都将拥有相同的静态方法副本。话虽如此，您仍然可以通过添加如下所示的默认私有(private)构造函数来限制类外部的实例化

private ProcessDegreeMark(){}