使用Java反射覆盖另一个类的方法

Question

我有一个Java项目，我有2个类。我需要使用Java反射来首先打印由构造函数ProjectAccount.java从mywork.java类设置的默认值，然后我需要重写toString()方法以传递mywork.java类的值并打印它们。

我能够使用java反射来打印默认的构造函数集值。但是在尝试覆盖并使用参数打印toString()方法时出现错误。

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; 
    } 

} 

输出和错误

我想我遇到了这个错误，因为我还没有重写toString()方法。任何关于如何覆盖和打印此建议将受到高度赞赏。

Answer 1

这里有几个问题。

1. 覆盖的toString是通过定义一个实例方法（你是静态的）
2. 你的反射访问是mets，它引用ProjectAccount类
3. mywork不是ProjectAccount
4. 子类中，你可以” t添加参数，

子类覆盖上午的能力ethod允许类从行为足够“接近”的超类继承，然后根据需要修改行为。重写方法具有相同的名称，参数的数量和类型，以及返回类型与其覆盖的方法相同。重写方法也可以返回重写方法返回的类型的子类型。这个子类型称为协变返回类型。

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类直接访问ProjectAccount.java类变量和设置。

我访问了声明的字段，并根据变量的名称或类型修改了声明的变量，并调用了相同的toString()方法，而不用重写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(); 
     } 
    } 

} 

输出