Java编译能通过，但是运行的时候提示有错误

Question

Java编译能通过，但是运行的时候提示有错误。

提示出现这样的错误：

 

Exception in thread "main" java.lang.ExceptionInInitializerError
    at org.idea.ok.Person.main(Person.java:32)
Caused by: java.lang.NullPointerException
    at org.idea.ok.Gender1.<init>(Gender1.java:28)
    at org.idea.ok.Gender1.<clinit>(Gender1.java:17)
    ... 1 more

 原程序如下：

package org.idea.ok;

/*
 *枚举类
 *既可以通过public访问权限的静态常量来访问实例；
 *也可以通过Gender1类的getInstance静态方法来访问实例变量。 
 */
 
import java.io.Serializable;
import java.util.Collection;
import java.util.Collections;
import java.util.NoSuchElementException;
import java.util.Map;
import java.util.HashMap;
 
public class Gender1 implements Serializable {
    public static final Gender1 FEMALE = new Gender1(new Character('F'), "FEMALE");
    public static final Gender1 MALE = new Gender1(new Character('M'), "MALE");
    private final Character sex;
    private final transient String description;
    private static Map<Character, Gender1> instancesBySex = 
        new HashMap<Character, Gender1>();

    
    private Gender1(Character sex, String description) {
        this.sex = sex;
        this.description = description;
        instancesBySex.put(sex, this);
    }
    
    public Character getSex() {
        return sex;
    }
    
    public String getDescription() {
        return description;
    }
    
    public static Collection getAllValues() {
        return Collections.unmodifiableCollection(instancesBySex.values());
    }
    
    public static Gender1 getInstance(Character sex) {
        
        /*按照参数指定的性别缩写查找Gender1实例*/
        Gender1 result = (Gender1)instancesBySex.get(sex);
        if (result == null) {
            throw new NoSuchElementException(sex.toString());
        }
        return result;
    }
    
    public String toString() {
        return description;
    }
    
    private Object readResolve() {
        
        /*保证反序列化时直接返回Gender1类包含的静态实例*/
        return getInstance(sex);
    }
}

package org.idea.ok;

/*
 */
 
public class Person {
    private String name;
    private Gender1 gender;
    
    public Person(String name, Gender1 gender) {
        this.name = name;
        this.gender = gender;
    }
    
    public String getName() {
        return name;
    }
    
    public void setName(String name) {
        this.name = name;
    }
    
    public Gender1 getGender() {
        return gender;
    }
    
    public void setGender(Gender1 gender) {
        this.gender = gender;
    }
    
    public static void main(String[] args) {
        Person mary = new Person("Mary", Gender1.FEMALE);
        System.out.println("name = " + mary.getName() + "\t" + "gender = " + mary.getGender());
    }
}

 程序异常提示是：初始化的时候错误；但是造成这种异常的根源是Gender1中的代码的问题。这个我还真是找不出来了，麻烦谁能找出来的，小弟不胜感谢。

Answer 1

private static Map instancesBySex =

new HashMap();

放到Gender1的开头就可以了，

是加载顺序的问题，当运行到
Person mary = new Person("Mary", Gender1.FEMALE);

这行时，系统加载Gender1类，当加载到
public static final Gender1 FEMALE = new Gender1(new Character('F'), "FEMALE");
这行时，由于还没有初始化instancesBySex ，此时instancesBySex 为null，而上面用到了instancesBySex 的put方法，出现空指针。

使用调试模式，一步一步执行，查看对象的属性就可以发现问题所在了。

自认为instancesBySex 没有必要存在，
public static Collection getAllValues() {

return Collections.unmodifiableCollection(instancesBySex.values());

}

public static Gender1 getInstance(Character sex) {   

    /*按照参数指定的性别缩写查找Gender1实例*/   
    Gender1 result = (Gender1)instancesBySex.get(sex);   
    if (result == null) {   
        throw new NoSuchElementException(sex.toString());   
    }   
    return result;   
}   

可修改为
public static Gender1[] getAllValues() {
return new Gender1[]{Gender1.FEMALE,Gender1.MALE};
}
public static Gender1 getInstance(Character sex) {

    if(set.equals(new Character("F")) {
         return Gender1.FEMALE;
    } else {
         return Gender1.MALE;
    }
}