Java API的基本使用方法

Java API地址

JAVA API
http://docs.oracle.com/javase/8/docs/api/

一. 什么是Java API

Java API, Application Programming Interface, 应用程序编程接口.
是一些预先定义的函数, 目的是提供应用程序与开发人员基于某软件或硬件的以访问一组例程的能力, 而又无需访问源码, 或理解内部工作机制的细节.

二.如何使用Java API

假设目前你想深入的了解File这个类,我们知道File类在Java.io包下,所以在JAVA API
中先找到java.io,点击进入,再找到File类,可以看到对File类的解释

An abstract representation of file and directory pathnames.

然后点击File可以查看API中对File类的详细讲解

Class File

java.lang.Object
java.io.File
All Implemented Interfaces:
Serializable, Comparable

public class File
extends Object
implements Serializable, Comparable
An abstract representation of file and directory pathnames.
User interfaces and operating systems use system-dependent pathname strings to name files and >directories. This class presents an abstract, system-independent view of hierarchical pathnames. An >abstract pathname has two components:

An optional system-dependent prefix string, such as a disk-drive specifier, “/” for the UNIX root directory, or “\\” for a Microsoft Windows UNC pathname, and
A sequence of zero or more string names.
The first name in an abstract pathname may be a directory name or, in the case of Microsoft Windows UNC pathnames, a hostname. Each subsequent name in an abstract pathname denotes a directory; the last name may denote either a directory or a file. The empty abstract pathname has no prefix and an empty name sequence.
The conversion of a pathname string to or from an abstract pathname is inherently system-dependent. When an abstract pathname is converted into a pathname string, each name is separated from the next by a single copy of the default separator character. The default name-separator character is defined by the system property file.separator, and is made available in the public static fields separator and separatorChar of this class. When a pathname string is converted into an abstract pathname, the names within it may be separated by the default name-separator character or by any other name-separator character that is supported by the underlying system.

A pathname, whether abstract or in string form, may be either absolute or relative. An absolute pathname is complete in that no other information is required in order to locate the file that it denotes. A relative pathname, in contrast, must be interpreted in terms of information taken from some other pathname. By default the classes in the java.io package always resolve relative pathnames against the current user directory. This directory is named by the system property user.dir, and is typically the directory in which the Java virtual machine was invoked.

The parent of an abstract pathname may be obtained by invoking the getParent() method of this class and consists of the pathname’s prefix and each name in the pathname’s name sequence except for the last. Each directory’s absolute pathname is an ancestor of any File object with an absolute abstract pathname which begins with the directory’s absolute pathname. For example, the directory denoted by the abstract pathname “/usr” is an ancestor of the directory denoted by the pathname “/usr/local/bin”.

The prefix concept is used to handle root directories on UNIX platforms, and drive specifiers, root directories and UNC pathnames on Microsoft Windows platforms, as follows:

For UNIX platforms, the prefix of an absolute pathname is always “/”. Relative pathnames have no prefix. The abstract pathname denoting the root directory has the prefix “/” and an empty name sequence.
For Microsoft Windows platforms, the prefix of a pathname that contains a drive specifier consists of the drive letter followed by “:” and possibly followed by “\” if the pathname is absolute. The prefix of a UNC pathname is “\\”; the hostname and the share name are the first two names in the name sequence. A relative pathname that does not specify a drive has no prefix.
Instances of this class may or may not denote an actual file-system object such as a file or a directory. If it does denote such an object then that object resides in a partition. A partition is an operating system-specific portion of storage for a file system. A single storage device (e.g. a physical disk-drive, flash memory, CD-ROM) may contain multiple partitions. The object, if any, will reside on the partition named by some ancestor of the absolute form of this pathname.

A file system may implement restrictions to certain operations on the actual file-system object, such as reading, writing, and executing. These restrictions are collectively known as access permissions. The file system may have multiple sets of access permissions on a single object. For example, one set may apply to the object’s owner, and another may apply to all other users. The access permissions on an object may cause some methods in this class to fail.

Instances of the File class are immutable; that is, once created, the abstract pathname represented by a File object will never change.

Interoperability with java.nio.file package

The java.nio.file package defines interfaces and classes for the Java virtual machine to access files, file attributes, and file systems. This API may be used to overcome many of the limitations of the java.io.File class. The toPath method may be used to obtain a Path that uses the abstract path represented by a File object to locate a file. The resulting Path may be used with the Files class to provide more efficient and extensive access to additional file operations, file attributes, and I/O exceptions to help diagnose errors when an operation on a file fails.

Since:
JDK1.0
See Also:
Serialized Form