选择getConstructor方法的条件

Question

我有一个类，在这个类中我初始化了静态块中的hashmap。通过传递键，我已经检索到了一个类的值。以便为这个类创建对象。我使用了构造函数类来获取构造函数，并传递了参数和创建了对象。

我在hashmap中有两个类。要为EchoExpression创建对象，我需要传递两个参数，而对于OutExpression类，我只需要传递一个参数(字符串)。

问题：

根据键返回的类，我需要执行哪个构造函数来获取和实现，无论是具有一个参数还是两个参数的构造函数。

public class ExampleFactory {

  private static HashMap<String,Class<?>> hmap = new HashMap<String,Class<?>>();

  static
  {                   
      hmap.put("echo", EchoExpression.class);         
      hmap.put("Out", OutExpression.class);                       
  }

  public void getExpo(String key,String expression)
  {
    Class aClass =map.get(key);

    //Constructor implementation for OutExpression where only one argument string is passed

    Constructor constructor = aClass.getConstructor(new Class[]{String.class});

    Object object= constructor.newInstance(expression);

    //constructor for passing two arguments string for EchoExpression

    Constructor constructor = aClass.getConstructor(new Class[]{String.class,Class.class});

    Object object= constructor.newInstance(expression, Boolean.class);


    return null;        
  }                
}

如何在不使用if else的情况下从值(类)中选择要实现的类？

Answer 1

使用枚举并打开它。这里是一个可执行的存根，没有深入了解你的例子的反射和语法：

package com.trip.test;

import java.util.HashMap;
import java.util.Map;

public class ExampleFactory {

    private static Map<String, Class<?>> hmap = new HashMap<String, Class<?>>();

    static {
        hmap.put("echo", EchoExpression.class);
        hmap.put("Out", OutExpression.class);
    }

    public static void getExpo(String key, String expression) {

        Class aClass = hmap.get(key);

        ClassMappingEnum myType = ClassMappingEnum.getClassMappingEnum(aClass);

        switch (myType) {
        case ECHO_EXPRESSION:{
            System.out.println(aClass.getName());
            // do something
            break;
        }
        case OUT_EXPRESSION:{
            System.out.println(aClass.getName());
            // do something
            break;          
        }
        case UNKNOWN:
        default:
            System.out.println("Bummer: " + aClass.getName());          
        }

    }

    public static void main(String[] args) {
        getExpo("echo", "B");
        getExpo("Out", "B");
    }   
}

enum ClassMappingEnum {
    ECHO_EXPRESSION(EchoExpression.class), OUT_EXPRESSION(OutExpression.class), UNKNOWN(null);

    private Class typeDes;

    private ClassMappingEnum(Class typeDes) {
        this.typeDes = typeDes;
    }

    public static ClassMappingEnum getClassMappingEnum(Class compare) {
        for (ClassMappingEnum cme : ClassMappingEnum.values()) {
            if (cme.typeDes.equals(compare)) {
                return cme;
            }
        }
        return UNKNOWN;
    }


}

class EchoExpression<T> {
    private String someString;
    private Class<T> someClass;
    public EchoExpression(String someString, Class<T> someClass) {
        super();
        this.someString = someString;
        this.someClass = someClass;
    }
    public String getSomeString() {
        return someString;
    }
    public void setSomeString(String someString) {
        this.someString = someString;
    }
    public Class<T> getSomeClass() {
        return someClass;
    }
    public void setSomeClass(Class<T> someClass) {
        this.someClass = someClass;
    }


}

class OutExpression {
    private String someString;

    public OutExpression(String someString) {
        super();
        this.someString = someString;
    }

    public String getSomeString() {
        return someString;
    }

    public void setSomeString(String someString) {
        this.someString = someString;
    }

}

Answer 2

如果您可以修改类，使两个构造函数具有相同的签名(以相同的顺序接受相同数量/类型的参数)，您可以这样做

Constructor constructor = aClass.getConstructor(new Class[]{String.class,Class.class});
Object object= constructor.newInstance(expression, Boolean.class);

两个班级都有。

当然，这意味着现在不需要额外参数的类在更改后将不得不忽略之前未使用的传入参数

更新:以下是使用工厂类实现该想法的一种可能方式：

public interface ObjectFactory
{
  Object create(String expr, Class cls);
}

public class EchoExpressionFactory implements ObjectFactory
{
  public EchoExpression create(String expr, Class cls)
  {
    return new EchoExpression(expr, cls);
  }
}

public class OutExpressionFactory implements ObjectFactory
{
  public OutExpression create(String expr, Class cls)
  {
    return new OutExpression(expr);
  }
}

public class ExampleFactory { 

  private static HashMap<String,ObjectFactory> hmap = new HashMap<String,ObjectFactory>(); 

  static 
  {                    
      hmap.put("echo", new EchoExpressionFactory());          
      hmap.put("Out", new OutExpressionFactory());                        
  } 

  public void getExpo(String key,String expression) 
  { 
    ObjectFactory factory = map.get(key); 

    //Constructor implementation for Expression
    Object object = factory.create(expression); 

    Object object= constructor.newInstance(expression, Boolean.class); 

    return;
  }                 
}