在java8以后，接口中可以添加使用default或者static修饰的方法，在这里我们只讨论default方法，default修饰方法只能在接口中使用，在接口中被default标记的方法为普通方法，可以直接写方法体。

 

实现类会继承接口中的default方法
如果接口A中有default方法：

public interface A {
	public default void a(){
		System.out.println("这是A");
	}
}

Test类实现接口A：

public class Test implements A{
	
}

那么Test类将会继承接口A中的a方法：

public class Test2 {
	public static void main(String[] args) {
		Test t = new Test();
		t.a();
	}
}

2.如果一个类同时实现接口A和B，接口A和B中有相同的default方法，这时，该类必须重写接口中的default方法

 


为什么要重写呢？是因为，类在继承接口中的default方法时，不知道应该继承哪一个接口中的default方法。


接口A：

public interface A {
	public default void a(){
		System.out.println("这是A");
	}
}

接口B：

public interface B {
	public default void a(){
		System.out.println("这是B");
	}
}

Test类：



 

 

 

3.如果子类继承父类，父类中有b方法，该子类同时实现的接口中也有b方法（被default修饰），那么子类会继承父类的b方法而不是继承接口中的b方法

接口A：

public interface A {
	public default void b(){
		System.out.println("AAA");
	}
}

类C：

public class C {
	public void b(){
		System.out.println("CCC");
	}
}

子类：

public class Test extends C implements A{
	
}

测试类：



说明子类继承的b方法为父类C中的b方法，不是接口中的default b(){}方法。

 

     前些天发现了一个风趣幽默的人工智能学习网站，通俗易懂，忍不住分享一下给大家。点击跳转到教程  

    今天在训练模型的时候无意间遇到了一个错误，这个可能是自己之前没有多去了解过python的参数相关的知识，这个错误还是第一次遇到。所以查了一下资料，在网上找到几个可以参考的链接明白了怎么回事：

参考：

http://stackoverflow.com/questions/17893820/python-default-argument-syntax-error

http://stackoverflow.com/questions/16932825/why-non-default-arguments-cant-follows-default-argument

http://stackoverflow.com/questions/24719368/syntaxerror-non-default-argument-follows-default-argument

    终于明白了为什么报错：就是说我把含有默认值的参数放在了不含默认值的参数的前面，这样问题就好解决了，调换一下参数的位置就好了

    我原始的函数为：

    

def random_predict(model_file='model/svm_model.pkl', X_train, y_train, X_test, y_test)
    clf = joblib.load("model/svm_model.pkl")
    result = []
    for i in range(10):
        lin = random.randint(0, 150)
        prediction_train = clf.predict(X_train[lin])
        prediction_test = clf.predict(X_test[lin])
        print prediction_train
        print '----------------------------------------------------------------'
        print prediction_test
        for i in prediction_train:
            result.append(i)
        for j in prediction_test:
            result.append(j)
    print result

修改为：

 

 

def random_predict(X_train, y_train, X_test, y_test, model_file='model/svm_model.pkl'):
    model = joblib.load(model_file)
    clf = joblib.load("model/svm_model.pkl")
    result = []
    for i in range(10):
        lin = random.randint(0, 150)
        prediction_train = clf.predict(X_train[lin])
        prediction_test = clf.predict(X_test[lin])
        print prediction_train
        print '----------------------------------------------------------------'
        print prediction_test
        for i in prediction_train:
            result.append(i)
        for j in prediction_test:
            result.append(j)
    print result

这样修改之后就没有错误了



 

 

一、新增记录出现异常

1.问题的出现，我在新增一个用户信息时，报的错误，不能添加记录到数据库；



 

2.详细异常信息

Hibernate: insert into t_user (uname, userAddress, userBirth, userEmail, userGender, userName, userPassword, userPhone) values (?, ?, ?, ?, ?, ?, ?, ?)
org.springframework.jdbc.UncategorizedSQLException: Hibernate operation: could not insert: [biz.entity.main.User]; uncategorized SQLException for SQL [insert into t_user (uname, userAddress, userBirth, userEmail, userGender, userName, userPassword, userPhone) values (?, ?, ?, ?, ?, ?, ?, ?)]; SQL state [HY000]; error code [1364]; Field 'userID' doesn't have a default value; nested exception is java.sql.SQLException: Field 'userID' doesn't have a default value
	at org.springframework.jdbc.support.AbstractFallbackSQLExceptionTranslator.translate(AbstractFallbackSQLExceptionTranslator.java:83)
	at org.springframework.jdbc.support.AbstractFallbackSQLExceptionTranslator.translate(AbstractFallbackSQLExceptionTranslator.java:80)
	at org.springframework.jdbc.support.AbstractFallbackSQLExceptionTranslator.translate(AbstractFallbackSQLExceptionTranslator.java:80)
	at org.springframework.orm.hibernate3.HibernateAccessor.convertJdbcAccessException(HibernateAccessor.java:424)
	at org.springframework.orm.hibernate3.HibernateAccessor.convertHibernateAccessException(HibernateAccessor.java:410)
	at org.springframework.orm.hibernate3.HibernateTemplate.doExecute(HibernateTemplate.java:411)
	at org.springframework.orm.hibernate3.HibernateTemplate.executeWithNativeSession(HibernateTemplate.java:374)
	at org.springframework.orm.hibernate3.HibernateTemplate.save(HibernateTemplate.java:683)
	at common.dao.hibernate.BaseHibernateDao.save(BaseHibernateDao.java:57)
	at biz.web.service.impl.BizService.addSimpleUser(BizService.java:44)
	at biz.web.action.sys.SimpleUserAction.add(SimpleUserAction.java:80)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:446)
	at com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:285)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
	at org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:256)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
	at com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:176)
	at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
	at com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:265)
	at org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68)
	at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
	at com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:138)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
	at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:238)
	at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
	at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:238)
	at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
	at com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:191)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
	at org.apache.struts2.interceptor.MultiselectInterceptor.intercept(MultiselectInterceptor.java:73)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
	at org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:91)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
	at org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:325)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
	at com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:100)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
	at com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:141)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
	at com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:145)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
	at com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:171)
	at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
	at com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:176)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
	at org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
	at com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:193)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
	at com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:187)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
	at org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:54)
	at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:544)
	at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77)
	at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:91)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at common.filter.SessionFilter.doFilter(SessionFilter.java:69)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:230)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:104)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:261)
	at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
	at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:581)
	at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
	at java.lang.Thread.run(Thread.java:744)
Caused by: java.sql.SQLException: Field 'userID' doesn't have a default value
	at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1074)
	at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:4074)
	at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:4006)
	at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2468)
	at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2629)
	at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2719)
	at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:2155)
	at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:2450)
	at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:2371)
	at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:2355)
	at com.mchange.v2.c3p0.impl.NewProxyPreparedStatement.executeUpdate(NewProxyPreparedStatement.java:139)
	at org.hibernate.id.IdentityGenerator$GetGeneratedKeysDelegate.executeAndExtract(IdentityGenerator.java:93)
	at org.hibernate.id.insert.AbstractReturningDelegate.performInsert(AbstractReturningDelegate.java:56)
	at org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:2346)
	at org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:2853)
	at org.hibernate.action.EntityIdentityInsertAction.execute(EntityIdentityInsertAction.java:71)
	at org.hibernate.engine.ActionQueue.execute(ActionQueue.java:273)
	at org.hibernate.event.def.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:320)
	at org.hibernate.event.def.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:203)
	at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:129)
	at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:210)
	at org.hibernate.event.def.DefaultSaveEventListener.saveWithGeneratedOrRequestedId(DefaultSaveEventListener.java:56)
	at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:195)
	at org.hibernate.event.def.DefaultSaveEventListener.performSaveOrUpdate(DefaultSaveEventListener.java:50)
	at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:93)
	at org.hibernate.impl.SessionImpl.fireSave(SessionImpl.java:713)
	at org.hibernate.impl.SessionImpl.save(SessionImpl.java:701)
	at org.hibernate.impl.SessionImpl.save(SessionImpl.java:697)
	at org.springframework.orm.hibernate3.HibernateTemplate$12.doInHibernate(HibernateTemplate.java:686)
	at org.springframework.orm.hibernate3.HibernateTemplate$12.doInHibernate(HibernateTemplate.java:1)
	at org.springframework.orm.hibernate3.HibernateTemplate.doExecute(HibernateTemplate.java:406)
	... 73 more

二、异常信息分析

1.insert语句执行失败

    异常显示就是insert插入sql语句执行错误导致添加记录到数据库失败；

    根本原因就是Field 'userID' doesn't have a default value；

    字段userID没有设置默认值；

    下面该表的create语句中确实没有给userID设置默认值；

CREATE TABLE `t_user` (
  `userID` int(11) NOT NULL,
  `uname` varchar(20) default NULL,
  `userAddress` varchar(100) default NULL,
  `userBirth` varchar(10) default NULL,
  `userEmail` varchar(50) default NULL,
  `userGender` int(11) default NULL,
  `userName` varchar(20) default NULL,
  `userPassword` varchar(150) default NULL,
  `userPhone` varchar(255) default NULL,
  PRIMARY KEY  (`userID`)
);

    导致insert语句对应的userID字段没有插入数据，而该字段不允许为null导致的；

Hibernate: insert into t_user (uname, userAddress, userBirth, userEmail, userGender, userName, userPassword, userPhone) values (?, ?, ?, ?, ?, ?, ?, ?)

三、解决异常

3-1.给字段设置默认值

    这种问题简单的解决办法就是设置默认值,但是当前字段userID是主键不能设置默认值；

`userAddress` varchar(100) default '阿拉加斯加'

3-2.Hibernate配置检查

    那可能是Hibernate配置出现了问题；然后找到当前表对应的Entity实体类

    字段userID的配置

	@Id
	@GeneratedValue(strategy = IDENTITY)
	@Column(name = "userID", unique = true, nullable = false)
	public Integer getUserId() {
		return this.userId;
	}

    该字段设置了自增策略，指定不可为空，且是唯一的；

    感觉没啥问题；再次测试新增一条记录还是失败；

3-3.数据库中表t_user的修改

    然后就从数据表这里入手；

    将当t_user的userID修改为自增

alter table t_user modify userID int auto_increment;

    因为该字段有外键约束不允许修改；

    索性就把表删了；新建表t_user；直接在设置字段自增且不为空；然后新增一条记录成功了；

CREATE TABLE `t_user` (
  `userID` int(11) NOT NULL auto_increment,
  `uname` varchar(20) default NULL,
  `userAddress` varchar(100) default NULL,
  `userBirth` varchar(10) default NULL,
  `userEmail` varchar(50) default NULL,
  `userGender` int(11) default NULL,
  `userName` varchar(20) default NULL,
  `userPassword` varchar(150) default NULL,
  `userPhone` varchar(255) default NULL,
  PRIMARY KEY  (`userID`)
);

 



四、其他解决方法

1.修改mysql配置。打开my.ini，查找sql-mode="STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION"修改为sql-mode="NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION"然后重启MYSQL

2.设置默认值即可，当然这个字段不是主键的情况下；

3.MySQL 5 uses a strict mode which needs to be disabled. In Windows, Goto Start-->Programs-->MySQL->MySQL Instance Config Wizard. Follow through the Reconfigure Instance option-->Detailed Configuration-->Continue Next a few screens. At the bottom under Enable TCP/IP option there is 'Enable Strict Mode'. Deslect this option (no tick). Save changes and MySQL will restart.

 


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最近在项目中遇到一个问题，就是以前用android studio能正常打开的工程，今天突然报default activity not found错误，该错误导致app那块显示一个红叉，无法直接launch app，可以build工程产生apk文件，但是不能用studio直接launch app，这样对开发来讲还是很不方便的。正常情况下如下图



遇见该错误的时候如下图



进到Edit Configurations，如下图



default activity not found这个错误产生的正常原因就是你的AndroidManifest.xml文件没有声明app默认启动的activity，如下：

<activity
        android:name=".MainActivity">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
</activity>

所以正常情况下就是补上这种activity的声明就行了。

非正常情况

估计很多同志们遇到的都是非正常情况，上述方法解决不了的时候，可以尝试下下面几种：

1、默认启动的activity的类名不要使用缩略形式，可是加上完整包名试试；

2、清除Android Studio的缓存，如下图所示：



3、检查你的module是否有要启动的activity，如果没有，需要设置成Nothing，如下图：



4、以上办法都试过之后，还不行，只能重新克隆代码，重新创建工程了；

5、以上方法还不行，只能考虑重置android studio了，方法就是删除c盘下的配置文件， 路径为C:\Users\用户名\.AndroidStudio1.2和C:\Users\用户名\.android两个文件夹。不过这样就相当于重装android studio，所依赖的谷歌原生包，比如每个api level对应的source源码，和第三方库都需要重新下载一遍，项目第一次编译需要花费点时间。重置android studio请参考：https://blog.csdn.net/mp624183768/article/details/79029722

我遇到的问题最终的解决办法就是方法5，前面4个方法都试过都无效，感觉代码没有问题，而是android studio自己出了问题，AndroidManifest.xml文件无法设识别出activity，项目里的哪一个activity都无法识别，最后还是用方法5，重新创建项目之后就好了，这个问题折腾了大半天，所以这里总结一下，希望能够为其他开发者节省一些时间。