Java核心类库（下）

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这篇博客深入探讨了Java异常处理机制，包括Throwable类、Error和Exception的区别，以及异常捕获和finally块的执行规则。此外，还详细介绍了File类的使用，如创建、删除文件，以及文件的读写权限设置等。最后提到了IO流的基本概念和分类。

Java核心类库（下）

参加拉勾教育大数据训练营课程笔记

Java异常机制

`Throwable`类

java.lang.Object
- java.lang.Throwable

All Implemented Interfaces:

Serializable

Direct Known Subclasses:

Error, Exception

是所有Error，Exception的父类（超类）。

Error是JVM遇到的无法解决的严重错误
Exception是程序错误，或者偶然外在因素导致的错误，可以通过修改代码解决

`Exception`分类

RuntimeException，运行时异常，也称非检测性异常
IOException和其他Exception，也叫检测性异常，是指编译阶段能被检测出来

异常处理

多个异常捕获时，小的放前，大的放后

try {}
catch (ExceptionSmall e) {}
catch (ExceptionBig e) {}
finally {}

finally考点

public class Test {
    private static int t(int a, int b) {
        try {
            return a / b;
        } catch (ArithmeticException e) {
            System.out.println(e.getMessage());
            return 1;
        } finally {
            return 2;
        }
    }
    public static void main(String[] args) {
        int a = 1;
        int b = 0;
        System.out.println(t(a, b));
    }
}

输出结果：

/ by zero
2

catch中的return得不到执行，而是进入了finally，会绕过return，continue，break。下面的代码块，在catch执行return前，finally代码块得到执行，然后回到catch，继续执行return 1返回1。

package phase01.module4.code;

public class Test {
    private static int t(int a, int b) {
        try {
            return a / b;
        } catch (ArithmeticException e) {
            System.out.println(e.getMessage());
            return 1;
        } finally {
            System.out.println("finally");
        }
    }
    public static void main(String[] args) {
        int a = 1;
        int b = 0;
        System.out.println(t(a, b));
    }
}

运行结果：

/ by zero
finally
1

Notes

The finally block always executes when the try block exits. This ensures that the finally block is executed even if an unexpected exception occurs. But finally is useful for more than just exception handling — it allows the programmer to avoid having cleanup code accidentally bypassed by a return, continue, or break. Putting cleanup code in a finally block is always a good practice, even when no exceptions are anticipated.

方法重写回顾：

参数相同
访问权限可以不变或更大
返回类型相同或父类
方法重写不能抛出更大的异常，或平级的不同异常，可以抛出相同或更小的异常，可以不抛出异常

抛出或捕获：

父类没有抛出异常时，重写方法时只能捕获处理
多层调用时，最后一次调用捕获处理异常

自定义异常类

继承Exception或其子类。
提供两个构造方法：无参，字符串参数

public class AgeException extends Exception {
    public AgeException() {
        super();
    }
    public AgeException(String message) {
        supper(message);
    }
}

`File`类

java.lang.Object
- java.io.File

All Implemented Interfaces:

Serializable, Comparable<File>

public class File
extends Object
implements Serializable, Comparable<File>

字段

Modifier and Type	Field	Description
`static String`	`pathSeparator`	The system-dependent path-separator character, represented as a string for convenience.
`static char`	`pathSeparatorChar`	The system-dependent path-separator character.
`static String`	`separator`	The system-dependent default name-separator character, represented as a string for convenience.
`static char`	`separatorChar`	The system-dependent default name-separator character.

构造方法

Constructor	Description
`File(String pathname)`	Creates a new `File` instance by converting the given pathname string into an abstract pathname.
`File(File parent, String child)`	Creates a new `File` instance from a parent abstract pathname and a child pathname string.
`File(String parent, String child)`	Creates a new `File` instance from a parent pathname string and a child pathname string.
`File(URI uri)`	Creates a new `File` instance by converting the given `file:` URI into an abstract pathname.

常用方法

Modifier and Type	Method	Description
`boolean`	`canExecute()`	Tests whether the application can execute the file denoted by this abstract pathname.
`boolean`	`canRead()`	Tests whether the application can read the file denoted by this abstract pathname.
`boolean`	`canWrite()`	Tests whether the application can modify the file denoted by this abstract pathname.
`int`	`compareTo(File pathname)`	Compares two abstract pathnames lexicographically.
`boolean`	`createNewFile()`	Atomically creates a new, empty file named by this abstract pathname if and only if a file with this name does not yet exist.
`boolean`	`delete()`	Deletes the file or directory denoted by this abstract pathname.
`void`	`deleteOnExit()`	Requests that the file or directory denoted by this abstract pathname be deleted when the virtual machine terminates.
`boolean`	`equals(Object obj)`	Tests this abstract pathname for equality with the given object.
`boolean`	`exists()`	Tests whether the file or directory denoted by this abstract pathname exists.
`File`	`getAbsoluteFile()`	Returns the absolute form of this abstract pathname.
`String`	`getAbsolutePath()`	Returns the absolute pathname string of this abstract pathname.
`String`	`getName()`	Returns the name of the file or directory denoted by this abstract pathname.
`String`	`getParent()`	Returns the pathname string of this abstract pathname’s parent, or `null` if this pathname does not name a parent directory.
`File`	`getParentFile()`	Returns the abstract pathname of this abstract pathname’s parent, or `null` if this pathname does not name a parent directory.
`boolean`	`isDirectory()`	Tests whether the file denoted by this abstract pathname is a directory.
`boolean`	`isFile()`	Tests whether the file denoted by this abstract pathname is a normal file.
`long`	`lastModified()`	Returns the time that the file denoted by this abstract pathname was last modified.
`long`	`length()`	Returns the length of the file denoted by this abstract pathname.
`File[]`	`listFiles()`	Returns an array of abstract pathnames denoting the files in the directory denoted by this abstract pathname.
`boolean`	`mkdir()`	Creates the directory named by this abstract pathname.
`boolean`	`mkdirs()`	Creates the directory named by this abstract pathname, including any necessary but nonexistent parent directories.
`boolean`	`setExecutable(boolean executable)`	A convenience method to set the owner’s execute permission for this abstract pathname.
`boolean`	`setExecutable(boolean executable, boolean ownerOnly)`	Sets the owner’s or everybody’s execute permission for this abstract pathname.
`boolean`	`setLastModified(long time)`	Sets the last-modified time of the file or directory named by this abstract pathname.
`boolean`	`setReadable(boolean readable)`	A convenience method to set the owner’s read permission for this abstract pathname.
`boolean`	`setReadable(boolean readable, boolean ownerOnly)`	Sets the owner’s or everybody’s read permission for this abstract pathname.
`boolean`	`setReadOnly()`	Marks the file or directory named by this abstract pathname so that only read operations are allowed.
`boolean`	`setWritable(boolean writable)`	A convenience method to set the owner’s write permission for this abstract pathname.
`boolean`	`setWritable(boolean writable, boolean ownerOnly)`	Sets the owner’s or everybody’s write permission for this abstract pathname.

IO流

按读写数据类型分：

字节流，可以读写任意类型的文件
字符流（2个字节），只能读取文本文件

以运行程序视角，按方向分为：

输入流
输出流

按流的角色分为：

节点流，直接与文件（输入输出流）直接关联
处理流，与其他流关联

体系结构表

分类	字节输入流	字节输出流	字符输入流	字符输出流
抽象基类	InputStream	OutputStream	Reader	Writer
访问文件	FileInputStream	FileOutputStream	FileReader	FileWriter
访问数组	ByteArrayInputStream	ByteArrayOutputStream	CharArrayReader	CharArrayWriter
访问管道	PipedInputStream	PipedOutputStream	PipedReader	PipedWriter
访问字符串	-	-	StringReader	StringWriter
缓冲流	BufferedInputStream	BufferedOutputStream	BufferedReader	BufferedWriter
转换流	-	-	InputStreamReader	OutputStreamWriter
对象流	ObjectInputStream	ObjectOutputStream	-	-
	FilterInputStream	FilterOutputStream	FilterReader	FilterWriter
打印流	-	PrintStream	-	PrintWriter
推回输入流	PushbackInputStream	-	PushbackReader	-
特殊流	DataInputStream	DataOutputStream	-	-