報錯：Type mismatch: cannot convert from Object to Car

問題描述：

一個非常簡單的spring項目，用靜態工廠方法配置bean實例。項目的目錄結構如下：

代碼如下：

Car.java

package com.tt.spring.beans.factory;

public class Car {

    private String brand;
    private double price;
    
    public String getBrand() {
        return brand;
    }
    public void setBrand(String brand) {
        this.brand = brand;
    }
    public double getPrice() {
        return price;
    }
    public void setPrice(double price) {
        this.price = price;
    }
    
    public Car(){
        System.out.println("Car's Constructor...");
    }
    
    
    
    public Car(String brand, double price) {
        super();
        this.brand = brand;
        this.price = price;
    }
    
    @Override
    public String toString() {
        return "Car [brand=" + brand + ", price=" + price + "]";
    }
    
    
}

StaticCarFactory.java

package com.tt.spring.beans.factory;

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

/**
 * 靜態工廠方法：直接調用某一個類的靜態方法就可以返回Bean的實例
 */
public class StaticCarFactory {

    
    private static Map<String,Car> cars = new HashMap<String, Car>();
    
    static{
        cars.put("Audi",new Car("Audi",300000));
        cars.put("Ford", new Car("Ford",40000));
    }
    
    //靜態工廠方法
    public static Car getCar(String name){
        return cars.get(name);
        
    }
    
}

beans-factory.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:context="http://www.springframework.org/schema/context"
    xmlns:p="http://www.springframework.org/schema/p"
    xmlns:util="http://www.springframework.org/schema/util"
    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-4.0.xsd
        http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-4.0.xsd">

    <!-- 通過靜態方法來配置bean.注意不是配置靜態工廠方法實例，而是配置bean實例 -->
    <bean id="car1" 
          class="com.tt.spring.beans.factory.StaticCarFactory"
          factory-method="getCar">
          <constructor-arg value="Audi"></constructor-arg>
    </bean>
    
    
</beans>

Main.java:

package com.tt.spring.beans.factory;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Main {

    public static void main(String[] args){
        
        ApplicationContext ctx = new ClassPathXmlApplicationContext("beans-factory.xml");
        
        Car car1 = (Car) ctx.getBean("car1");
        System.out.println(car1);
    }
}

運行Main.java程序，控制台報錯如下：

Exception in thread "main" java.lang.Error: Unresolved compilation problem:
Type mismatch: cannot convert from Object to Car

at com.tt.spring.beans.factory.Main.main(Main.java:12)

 

原因分析：

第一, 需要裝 jre1.5.0及以上的版本

第二, 在eclipse的'Window' 'Preference' 'Java'里,
'Install JREs'里設置你裝的jre

第三,在eclipse的'Window' 'Preference' 'Java'里,
'Compiler'里設'Compiler compliance level'為5.0以上
（關鍵是第三步, 兼容級別）

 

問題解決：

由於我安裝的jdk版本是1.8.0的，所以問題應該出在Compiler compliance level上。

進入Window->Preference->Java->Compiler,發現Compiler compliance level=1.5

 

將Compiler compliance level改成1.8：

 

再運行Main.java代碼，運行正常，控制台顯示如下信息：