Generics
1, the concept of generics
Generic Java is a new feature introduced J2 SE1.5, which is essentially a parameterized type, that is to say the operation of the data type is specified as a parameter (type parameter) parameter type may be used in this class, interface, and create a method, called generic class, generic interfaces, generic method.
2, the use of generics in the collection, summary:
①, or a set of classes in the set of interfaces jdk5 belt structure is modified to have generic
②, Note: The generic type must be a class, not the basic data types.
③, if instantiated, generic type not specified. The default type is java.lang.Object type
3, the use of generics should be noted :
①, static methods can not be used like generic (generic must first create an object re-use)
②, an exception can not be used in a generic class
4, generic re-used in the method are as follows :
//泛型方法：在方法中出现了泛型的结构，泛型参数与类的泛型参数没有任何关系
public <E> list<E> copyFromArrayToList(E[] arr){
ArrayList<E> list = new ArrayList<>();
for(E e : arr){
list.add(e);
}
return list;
}
5, reflected in the succession of generics
- While class C is the parent class of class D, but G <C> and G <D> do not have both parent child relationships, both are juxtaposed relationship, as follows:
List<Object> list1 = null;
List<String> list2 = null;
//此时的list1和list2的类型不具有子父类关系
list1 = list2;(报错)
反证法：
假设list1 = list2
list.add(123);//导致混入非String的数据，出错
NOTE: C is the parent class of class D, C <G> is D <G> parent class, as follows:
AbstarctList<String> list1 = null;
List<String> list2 = null;
ArrayList<String> list3 = null;
list1 = list3;
list2 = list3;
6, using wildcards
-
Wildcards:?
A class is a superclass of class B, G <C> and G <D> is not related, both common parent class is: G <?>Add : <?> The List will not be able to add to its internal data, in addition to adding null
Acquires (reads) : allowed to read data, the read data type Object
-
There are limitations to the use of wildcards
the extends C? :
G <? C the extends> as G <C> and G <D> of the parent class, where D is a subclass of C? Super C :
G <Super C?> As G <C> and G <D> of the parent class, where D is the parent of C
Use the File class:
1, an object of the File class represents a file or a directory (commonly known as: Folder)
2, File class declaration in the package java.io
①, how to create an instance of the File class
File(String filePath);
File(String parentPath, String childPath);
File(File parentFile, String childPath);
②, the relative path : Compared to a particular path, specify the path to
absolute path path to the file or directory that contains the letter, including:
③, path separator
windows : \\
unix : /
3, File class acquisition function
public String getAbsolutePath(): 获取绝对路径
public String getPath(): 获取路径
public String getName()：获取名称
public String getParent()：获取上层文件目录路径。若无，返回null
public long length()：获取文件长度（字节数）。不能获取目录的长度
public long lastModified()：获取最后一次的修改时间，毫秒值
public String[] list()：获取指定目录下的所有文件或者文件目录的名称数组
public File[] listFiles()：获取指定目录下的所有文件或者文件目录的File数组
public boolean renameTo(File dest)：吧文件重命名为指定的文件路径比如：file1.renameTo(file2)为例：要想保证返回true，需要file1在硬盘中是存在的，且file2不能在硬盘中存在
public boolean isDirectory()：判断是否是文件目录
public boolean isFile()：判断是否是文件
public boolean exists()：判断是否存在
public boolean canRead()：判读是否可读
public boolean canWrite()：判断是否可写
public boolean isHidden()：判断是否隐藏
pulic boolean createNewFile()：创建文件。若文件存在，则不创建，返回false
public boolean mkdir()：创建文件目录。如果此文件目录存在，就不创建了。如果此文件目录的上层目录不存在，也不创建
public boolean mkdirs()：创建文件目录。如果上层文件目录不存在，一并创建
public boolean delete()：删除文件或文件夹（注意：Java中的删除不走回收站)
