java - 使用 getOrDefault() 时，我没有获得默认值

Question

如果我在文本框页面中提供一些值，那么我会得到正确的输出，但如果我没有在文本框中提供任何值，我希望它将打印默认值。

为此，我使用 Map 类的 getOrDefault() 方法，但仍然没有获得默认值。

StudentAdmissionController.java

package com.diwakar;

import java.util.Map;

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.servlet.ModelAndView;

@Controller
public class StudentAdmissionController {

    @RequestMapping(value="/admission.html", method = RequestMethod.GET)
    public ModelAndView getAdmissionForm() {

        ModelAndView model =  new ModelAndView("AdmissionForm");
        return model;
    }

    @RequestMapping(value="/submitForm", method =  RequestMethod.POST)
    public ModelAndView submitAdmissionForm(@RequestParam Map<String, String> reqParm) {

        String name  = reqParm. ("studentName", "Default Name");
        String hobby = reqParm.getOrDefault("studentHobby", "Default Hobby");

        ModelAndView model = new ModelAndView("AdmissionSuccess");
        model.addObject("msg", "Details submitted by you, Name : " + name + " and hobby is : " + hobby);
        return model;
    }
}

spring-dispatcher-servlet.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:mvc="http://www.springframework.org/schema/mvc"
    xmlns:p="http://www.springframework.org/schema/p"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
                        http://www.springframework.org/schema/beans/spring-beans.xsd
                        http://www.springframework.org/schema/context
                        http://www.springframework.org/schema/context/spring-context.xsd
                        http://www.springframework.org/schema/mvc
                        http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd">



<!--      <bean id="HandlerMapping" class="org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping" />
          <bean name="/welcome.html" class="com.diwakar.HelloController" />  -->

     <context:component-scan base-package="com.diwakar" />
     <mvc:annotation-driven />

     <bean id="viewResolver" class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="prefix" value="/WEB-INF/" />
        <property name="suffix" value=".jsp"></property>
     </bean>                    
</beans>

输出是

恭喜!详细信息已提交。

您提交的详细信息，姓名:和爱好:

而不是

Answer 1

getOrDefault仅当没有映射时才返回默认值，您可能有一个到空字符串的映射，您应该检查自定义代码，也许构建一个方法:

private static String getOrDefaultIfEmpty(Map<String,String> map, String key, String def)
    {
        String v = map.get(key);
        return v==null || v.length()==0 ? def : v;
    }

您需要执行此检查，因为空 HTML 文本框会发送一个创建映射的空字符串作为参数，因此您不会获得默认值，而是会获得一个空字符串。