使用Java反射覆蓋另一個類的方法

Question

我有一個Java項目，我有2個類。我需要使用Java反射來首先打印由構造函數ProjectAccount.java從mywork.java類設置的默認值，然後我需要重寫toString()方法以傳遞mywork.java類的值並打印它們。

我能夠使用java反射來打印默認的構造函數集值。但是在嘗試覆蓋並使用參數打印toString()方法時出現錯誤。

ProjectAccount.java

package relive; 

public class ProjectAccount { 
    private String projectAccountID; 
    private double budget; 
    public int noOfProjects; 

    public ProjectAccount(){ 
     this.projectAccountID = "MARTINDAWS-BillAnalyzer-001"; 
     this.budget = 120000.00; 
     this.noOfProjects = 10; 
    } 

    @Override 
    public String toString(){ 
     return "Project Accoutn ID = " + projectAccountID + " , Project Budget = "+budget + " , No of Projects = "+ noOfProjects; 
    } 
} 

mywork.java

package relive; 

import java.lang.reflect.Constructor; 
import java.lang.reflect.InvocationTargetException; 
import java.lang.reflect.Method; 

public class mywork { 

    public static void main(String[] args) { 
     try{ 
      Class<?> clazz = Class.forName("relive.ProjectAccount"); 
      Constructor<?> constr = clazz.getDeclaredConstructor(null); 
      Method mets = clazz.getDeclaredMethod("toString", null); 
      Object obj = constr.newInstance(new Object[] {}); 

      //Print Default values 
      System.out.println(mets.invoke(obj, null)); 

      String mod_projectAccountID="ORPT-BT-EMP-DEV"; 
      double mod_budget = 2200000.0; 
      int mod_noOfProjects = 20; 

      //Print new passed values using the overridden method 
      System.out.println(mets.invoke(obj, mod_projectAccountID,mod_budget, mod_noOfProjects));   
     } 

     catch(ClassNotFoundException | InstantiationException | IllegalAccessException | IllegalArgumentException | InvocationTargetException | NoSuchMethodException | SecurityException e){ 
      e.printStackTrace(); 
     } 
    } 
    public static String toString(String mod_projectAccountID, double mod_budget, int mod_noOfProjects){ 
     return "Project Accoutn ID = " + mod_projectAccountID + " , Project Budget = "+mod_budget + " , No of Projects = "+ mod_noOfProjects; 
    } 

} 

輸出和錯誤

我想我遇到了這個錯誤，因爲我還沒有重寫toString()方法。任何關於如何覆蓋和打印此建議將受到高度讚賞。

Answer 1

這裏有幾個問題。

1. 覆蓋的toString是通過定義一個實例方法（你是靜態的）
2. 你的反射訪問是mets，它引用ProjectAccount類
3. mywork不是ProjectAccount
4. 子類中，你可以」 t添加參數，

子類覆蓋上午的能力ethod允許類從行爲足夠「接近」的超類繼承，然後根據需要修改行爲。重寫方法具有相同的名稱，參數的數量和類型，以及返回類型與其覆蓋的方法相同。重寫方法也可以返回重寫方法返回的類型的子類型。這個子類型稱爲協變返回類型。

Oracle Docs

package de.me.example; 

public class SuperType { 

    @Override 
    public String toString() { 

     return "SuperType"; 
    } 

} 


package de.me.example; 

public class SubType extends SuperType { 

    // this annatotion is syntactic sugar nothing more, 
    //it behaves the same without it 
    @Override 
    public String toString() { 
     return "SubType"; 
    } 

} 

Answer 2

我只是修改了mywork.java類直接訪問ProjectAccount.java類變量和設置。

我訪問了聲明的字段，並根據變量的名稱或類型修改了聲明的變量，並調用了相同的toString()方法，而不用重寫toString()方法。

mywork。java的

package relive; 

import java.lang.reflect.Constructor; 
import java.lang.reflect.Field; 
import java.lang.reflect.InvocationTargetException; 
import java.lang.reflect.Method; 

public class answer_a { 

    public static void main(String[] args) { 
     try{ 
      Class<?> clazz = Class.forName("relive.ProjectAccount"); 
      Constructor<?> constr = clazz.getDeclaredConstructor(null); 
      Method mets = clazz.getDeclaredMethod("toString", null); 
      Object obj = constr.newInstance(new Object[] {}); 

      //Print Default values 
      System.out.println(mets.invoke(obj, null)); 

      String mod_projectAccountID= "ORPT-BT-EMP-DEV"; 
      double mod_budget = 2200000.0; 
      int mod_noOfProjects = 20; 

      //Method that accesses the Fields and modifies them with the new variables 

      Field [] fields = clazz.getDeclaredFields(); 
      for(int y=0; y<fields.length; y++){ 
       fields[y].setAccessible(true); 

       if(fields[y].getName() == "projectAccountID"){ 
        fields[y].set(obj, mod_projectAccountID); 
       } 
       else if(fields[y].getType().getSimpleName() == "double"){ 
        fields[y].set(obj, mod_budget); 
       } 
       else if (fields[y].getType().getSimpleName() == "int"){ 
        fields[y].set(obj, mod_noOfProjects); 
       } 
      } 

      System.out.println(mets.invoke(obj, null)); 

     } 
     catch(ClassNotFoundException | InstantiationException | IllegalAccessException | IllegalArgumentException | InvocationTargetException | NoSuchMethodException | SecurityException e){ 
      e.printStackTrace(); 
     } 
    } 

} 

輸出