java - 获取错误“线程“主”java.lang.ExceptionInInitializerError中的异常”
问题描述
我正在尝试使用 Enum Month 初始化 HashMap。但是,收到如下错误:“线程“主”java.lang.ExceptionInInitializerError 中的异常”。请让我知道为什么我会收到此错误。我在静态主类方法和行中得到这个ChatGroups.initializeMap();
package static_initialization;
import java.lang.reflect.Type;
import java.util.HashMap;
import java.util.Map;
public class ChatGroups {
public static final Map<String, Month> months = new HashMap<String, Month>() {{
months.put("Jan", Month.January);
months.put("Feb", Month.February);
months.put("Mar", Month.March);
months.put("Apr", Month.April);
months.put("Jun", Month.June);
months.put("Jul", Month.July);
months.put("Aug", Month.August);
months.put("Sep", Month.September);
months.put("Oct", Month.October);
months.put("Nov", Month.November);
months.put("Dec", Month.December);
}};
public static void initializeMap() {
System.out.println();
}
}
package static_initialization;
public enum Month {
January(0),
February(1),
March(2),
April(3),
May(4),
June(5),
July(6),
August(7),
September(8),
October(9),
November(10),
December(11);
private int month;
Month(int i) {
month = i;
}
}
package static_initialization;
public class Main {
public static void main(String[] args) {
initializeMaps();
}
public static void initializeMaps() {
try {
ChatGroups.initializeMap();
} catch (Exception ex) {
System.out.println("Neelabh exception = " + ex);
}
}
}
请找到堆栈跟踪
Exception in thread "main" java.lang.ExceptionInInitializerError
at static_initialization.Main.initializeMaps(Main.java:10)
at static_initialization.Main.main(Main.java:5)
Caused by: java.lang.NullPointerException
at static_initialization.ChatGroups$1.<init>(ChatGroups.java:9)
at static_initialization.ChatGroups.<clinit>(ChatGroups.java:8)
解决方案
它应该是:
public static final Map<String, Month> months = new HashMap<String, Month>() {{
put("Jan", Month.January);
put("Feb", Month.February);
put("Mar", Month.March);
put("Apr", Month.April);
put("Jun", Month.June);
put("Jul", Month.July);
put("Aug", Month.August);
put("Sep", Month.September);
put("Oct", Month.October);
put("Nov", Month.November);
put("Dec", Month.December);
}};
months.put()
导致NullPointerException
,因为执行语句时static
变量months
仍然存在。null
put
作为替代方案,避免创建匿名类实例,而是编写:
public static final Map<String, Month> months = new HashMap<String, Month>();
static {
months.put("Jan", Month.January);
months.put("Feb", Month.February);
months.put("Mar", Month.March);
months.put("Apr", Month.April);
months.put("Jun", Month.June);
months.put("Jul", Month.July);
months.put("Aug", Month.August);
months.put("Sep", Month.September);
months.put("Oct", Month.October);
months.put("Nov", Month.November);
months.put("Dec", Month.December);
}
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