进阶java大师之路，泛型在hashmap上的灵活运用

由于，公司的试用的框架上，存在大量的hashmap的创建，产生大量的I/O 以及内存的消耗，所以，抽了点时间，看书 effective java 这本书。

同时，也从项目的作者 framework 上学习 ，吸取 ，最终写出了，符合生产的方案。

存在的问题是，一直困惑与hashmap的清除。

package com.framework.entity.factory;

import java.util.HashMap;

import org.apache.poi.ss.formula.functions.T;
import org.apache.tools.ant.types.resources.selectors.InstanceOf;

import com.framework.entity.CompanyFormMap;
import com.framework.entity.base.FormMap;

public class FrommapFactory {
	private static HashMap<Class<?>, ? extends FormMap<String, Object> > hashMap=new  HashMap<>();
	
	
	@SuppressWarnings({ "hiding", "unchecked" })
	public static <T> T getFromBeanMap(Class<? extends FormMap<String, Object>> type) {
		// TODO Auto-generated method stub
		if (hashMap.containsKey(type)) {
			FormMap<String, Object> t=hashMap.get(type);
			t.clear();
			return (T)t;
		}else {
			
			Object object = null;
			try {
				System.out.println(type.getSimpleName());
				object = Class.forName(type.getName());
			} catch (ClassNotFoundException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
			return (T) object;
		}
	}
	
	private static HashMap<T,  Object> hashMap2=new  HashMap<>();
	@SuppressWarnings("hiding")
	public static <T extends FormMap<String, Object>> T getFromBeanMap2(T t) {
		t.clear();
		FormMap<String, Object> formMap = (FormMap<String, Object>) t;
		
		
		
		return t;
	}
	
	
	
	private static HashMap<T,  Object> hashMap3=new  HashMap<>();
	@SuppressWarnings({ "unchecked", "hiding" })
	public static <T> T getBeanFormMap(Class<T> clazz) throws InstantiationException, IllegalAccessException{
		if (hashMap3.containsKey(clazz)) {
			FormMap<String, Object> formMap = (FormMap<String, Object>) hashMap3.get(clazz);
			T t=(T) hashMap3.get(clazz);
			formMap.clear();
			return t;
		}else {
			T t=clazz.newInstance();
			return t;
		}
	}
	
	
	
	public static void main(String[] args) throws InstantiationException, IllegalAccessException {
		CompanyFormMap companyFormMap=getBeanFormMap(CompanyFormMap.class);
		System.out.println(companyFormMap);
	}
}

前面第一个，是看书之后的结果。

有错误。

估计就是class 与 map ，不能有效的相互转换，或者转换太多。

最后一个就是符合方案的产品了。

其实还是可以优化的，

就是利用 T extends ？

不过，我没有测试。觉得麻烦。

大道至简。

2.0补充

// Json生成Map,2.0 ,破解了复杂，list，树
@SuppressWarnings("unchecked")
public static Map<String, Object> jsonToMap(String jsonString) throws JSONException {
// JSONObject必须以"{"开头
JSONObject jsonObject = JSONObject.fromObject(jsonString);
Map<String, Object> resultMap = new HashMap<String, Object>();
Iterator<String> iter = jsonObject.keys();
String key = null;
Object value = null;
while (iter.hasNext()) {
key = iter.next();
value = jsonObject.get(key);
if (value.getClass() == JSONArray.class) {
JSONArray jsonArray = JSONArray.fromObject(value);
for (Object list : jsonArray) {
Map<String, Object> resultMap2 = jsonToMap(list.toString());
resultMap.put(key, resultMap2);
}
} else {
resultMap.put(key, value);
}
}
// System.out.println(resultMap);
/*
* 打印结果{map={"sex":"female","age":"23","name":"Alexia"},
* employ={"sex":"female","age":"24","name":"wjl"}}
*/
return resultMap;
}


// Json生成Map,2.0 ,破解了复杂，list，树
@SuppressWarnings("unchecked")
public static <T extends FormMap<String, Object>> T  jsonToMap(String jsonString,Class<T> clazz) throws JSONException, InstantiationException, IllegalAccessException {
// JSONObject必须以"{"开头
JSONObject jsonObject = JSONObject.fromObject(jsonString);
T t=clazz.newInstance();
Iterator<String> iter = jsonObject.keys();
String key = null;
Object value = null;
while (iter.hasNext()) {
key = iter.next();
value = jsonObject.get(key);
if (value.getClass() == JSONArray.class) {
JSONArray jsonArray = JSONArray.fromObject(value);
for (Object list : jsonArray) {
Map<String, Object> resultMap2 = jsonToMap(list.toString());
t.put(key, resultMap2);
}
} else {
t.put(key, value);
}
}
// System.out.println(resultMap);
/*
* 打印结果{map={"sex":"female","age":"23","name":"Alexia"},
* employ={"sex":"female","age":"24","name":"wjl"}}
*/
return t;
}

————————————————

普通类的使用

 // 定义一个泛型类
    private class GenericClass<T> {
        // 在此处定义泛型类的属性和方法

        private List<T> getCsvToBean (Class<? extends T> obj, String pathname) {
            CSVReader reader = null;
            try {
                reader = new CSVReader(new FileReader(pathname));
                String[] nextLine = reader.readNext();
                // 读取每行数据
                int columnCount = nextLine.length;

                ColumnPositionMappingStrategy<T> strategy = new ColumnPositionMappingStrategy<>();
                strategy.setType(obj);
                String[] columnMapping = new String[columnCount];
                for (int i = 0; i < columnCount; i++) {
                    columnMapping[i] = String.valueOf(i);
                }
                strategy.setColumnMapping(columnMapping);

                CsvToBean<T> csvToBean = new CsvToBeanBuilder<T>(reader)
                        .withMappingStrategy(strategy)
                        .build();

                List<T> parse = csvToBean.parse();
                return parse;
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        }

    }
    
    
    
       GenericClass<AcsCmOrderDto> genericClass = new GenericClass();
                    List<AcsCmOrderDto> csvToBean = genericClass.getCsvToBean(AcsCmOrderDto.class, pathname);
                    cmOrderDtoList = csvToBean;