Java 基础汇总（补）
1、Switch语句中表达式类型：
Unlike if-then and if-then-else statements, the switch statement can have a number of possible execution paths. A switch works with the byte,short,char, and int primitive data types. It also works with enumerated types(discussed in Enum Types), the String class, and a few special classes that wrap certain primitive types:Character,Byte,Short, and Integer (discussed in Numbers and Strings).
译自：http://docs.oracle.com/javase/tutorial/java/nutsandbolts/switch.html
表达式可以是 byte , short , char 和 int 类型；也可以是枚举类型，String类；
还可以是 byte 、short 、 char 、 int 对应的包装类。
2、格式化输出 函数 format() ， println()
示例展示：
3、判别字符类型总结
4、转义字符
\f 换页
5、泛型编程中的注意事项：否则会报编译错误。
if (list instanceof ArrayList) { // compile-time error }
if (list instanceof ArrayList) { // OK; instanceof requires a reifiable type }
Typically, you cannot cast to a parameterized type unless it is parameterized by unbounded wildcards. For example:
List li = new ArrayList<>();
List ln = (List) li; // compile-time error
However, in some cases the compiler knows that a type parameter is always valid and allows the cast. For example:
List l1 = ...;
ArrayList l2 = (ArrayList)l1; // OK
6、注解
（1）JavaSE API中预定义的注解
a. java 语言自己使用的注解：
@Deprecated
被@Deprecated标记过期的接口、类、方法、域 ，应该在Javadoc 注释中用过期注解使用 @deprecated 解释过期的原因及用什么替代了（注意：是小写的 'd'）。
@Override
通知编译器，被Mark的类、接口、方法 将会被重写
@SuppressWarnings
@SafeVarargs :
when applied to a method or constructor, asserts that the code does not perform potentially unsafe operations on itsvarargsparameter. When this annotation type is used, unchecked warnings relating tovarargsusage are suppressed.
@FunctionalInterface : JavaSE 8 中引入的，该注解表示 类型声明 是个函数接口（functional interface ）
b.用在其他注解上的注解（Annotations that Apply to Other Annotations）
这类注解也称为： meta-annotation，在java.lang.annotation 中，定义了好几个这样的注解：
@Retention: 用于指明被标记的注解的存储方式：
RetentionPolicy.SOURCE– The marked annotation is retained only in the source level and is ignored by the compiler.
源码级存储：在源码级保留，但是会被编译器忽略。
RetentionPolicy.CLASS– The marked annotation is retained by the compiler at compile time, but is ignored by the Java Virtual Machine (JVM).
类级存储：编译时会存储，但是会被JVM忽略。
RetentionPolicy.RUNTIME– The marked annotation is retained by the JVM so it can be used by the runtime environment.
运行时存储：会被JVM存储，将来在运行时会被用到。
@Documented
indicates that whenever the specified annotation is used those elements should be documented using the Javadoc tool. (By default, annotations are not included in Javadoc.)
表示被标记的元素会被用于Javadoc工具 。默认情况，注解是不会被Javadoc的。
@Target
用于标记限制其他注解该应用于什么样的Java element
ElementType.ANNOTATION_TYPEcan be applied to an annotation type.
ElementType.CONSTRUCTORcan be applied to a constructor.
ElementType.FIELDcan be applied to a field or property.
ElementType.LOCAL_VARIABLEcan be applied to a local variable.
ElementType.METHODcan be applied to a method-level annotation.
ElementType.PACKAGEcan be applied to a package declaration.
ElementType.PARAMETERcan be applied to the parameters of a method.
ElementType.TYPEcan be applied to any element of a class.
@Inherited：继承
用于指明该注解可以继承超类的注解（默认是不可以的，为false）。当用户查询注解类型时，若这个类没有定义这个类型的注解，那么这个类的超类会被查询。这个注解仅适用于类声明（class declaration）。
@Repeatable
Java SE 8 引入的，用于指明被标记的注解可以多次应用于同一个声明或同一种类型。
can be applied to the same declaration or type use.
（2）Type Annotations and Pluggable Type Systems
Before the Java SE 8 release, annotations could only be applied to declarations. As of the Java SE 8 release, annotations can also be applied to any type use. This means that annotations can be used anywhere you use a type.
这样的插件式注解可以自己实现也可以只用第三方已经实现的框架，推荐一个Checker Framework
