The Java™ Tutorials——（1）Essential Classes——Exception一丶异常（Exception）

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																/*
使用电脑版看下面的文字"示意图"，更好哦。			
			
----------------------------- 
|Method where error occurred|←←↖
|-------------------------- |	↑		
								↑  Method Call
-----------------------------   ↑
|Method without an exception|→→↗
|		  handler           |
|-------------------------- |←←↖	
						        ↑  Method Call
-----------------------------   ↑
|Method  with an  exception |→→↗
|		  handler           |
|---------------------------|←←↖
		               		    ↑  Method Call
		main →→→→→→→→→→→→→→→→→→↗   

	
	      The call stack.	                                      */

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																	   /*		
使用电脑版看下面的文字"示意图"，更好哦。	
			
                    ----------------------------- 
  Throws exception  |Method where error occurred|→→↘
                    |---------------------------|	↓  looking for 	
                                                    ↓  appropriate
                    -----------------------------   ↓  handler
 Forward exception  |Method without an exception|←←↙
                    |		  handler           |
                    |---------------------------|→→↘	
                                                    ↓  looking for 	
                                                    ↓  appropriate
                    -----------------------------   ↓  handler
 Catches some other |Method  with an  exception |←←↙
     exception      |		  handler           |
                    |---------------------------|
                    
                              main                                    
                                                                   
         Searching the call stack for the exception handler.                                                                                                                                    												 */

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import java.io.*;
import java.util.List;
import java.util.ArrayList;

public class ListOfNumbers {

    private List<Integer> list;
    private static final int SIZE = 10;

    public ListOfNumbers () {
        list = new ArrayList<Integer>(SIZE);
        for (int i = 0; i < SIZE; i++) {
            list.add(new Integer(i));
        }
    }

    public void writeList() {
		// The FileWriter constructor throws IOException, which must be caught.
        PrintWriter out = new PrintWriter(new FileWriter("OutFile.txt"));
        for (int i = 0; i < SIZE; i++) {
            out.println("Value at: " + i + " = " + list.get(i));
        }
        out.close();
    }
}

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// 必须捕获或者声明抛出IOException异常，否则会报错
PrintWriter out = new PrintWriter(new FileWriter("OutFile.txt"));

// get方法可能会抛出IndexOutOfBoundsException指针越界异常
// 它是RuntimeException，unchecked exception，我们可以选择不抛出异常。
int i = list.get(i);

// 现在我们查看下面的代码。
for (int i = -1; i <= SIZE; i++) {
    try{
        System.out.println("Value at: " + i + " = " + list.get(i));
    } catch (IndexOutOfBoundsException e){
        System.out.printf("i=%d数组越界了！！%n", i);
        e.printStackTrace();
    }
}

/* 程序输出为：

i=-1数组越界了！！
java.lang.ArrayIndexOutOfBoundsException: -1
	at java.util.ArrayList.elementData(ArrayList.java:422)
	at java.util.ArrayList.get(ArrayList.java:435)
	at basisLang.essentialclasses.ListOfNumbers.writeList(ListOfNumbers.java:27)
	at basisLang.essentialclasses.ListOfNumbers.main(ListOfNumbers.java:37)
Value at: 0 = 0
Value at: 1 = 1
Value at: 2 = 2
Value at: 3 = 3
Value at: 4 = 4
Value at: 5 = 5
Value at: 6 = 6
Value at: 7 = 7
Value at: 8 = 8
Value at: 9 = 9
i=10数组越界了！！
java.lang.IndexOutOfBoundsException: Index: 10, Size: 10
	at java.util.ArrayList.rangeCheck(ArrayList.java:657)
	at java.util.ArrayList.get(ArrayList.java:433)
	at basisLang.essentialclasses.ListOfNumbers.writeList(ListOfNumbers.java:27)
	at basisLang.essentialclasses.ListOfNumbers.main(ListOfNumbers.java:37)      */

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try {
    //可能会抛出异常的，一行或者多行代码
}
catch //try后面必须要跟catch
  and
finally blocks . . .

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try {
	//code
} catch (ExceptionType name) {

} catch (ExceptionType name) {

}

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// ExceptionType必须是一个继承自Throwable类的类
public class Throwable extends Object implements Serializable

// 我们对writeList函数进行改造，能够得到下面的代码
public void writeList() {
	int i = -1;
    try {
    	PrintWriter out = new PrintWriter(new FileWriter("OutFile.txt"));
        for (; i <= SIZE; i++) 
        	System.out.println("Value at: " + i + " = " + list.get(i));
    } catch (IndexOutOfBoundsException e){
    	System.out.printf("i=%d数组越界了！！%n", i);
    	e.printStackTrace();
    } catch (IOException e){
    	out.println("IOException！！");
       	e.printStackTrace();
    }
}

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catch (IOException | SQLException ex) {
    logger.log(ex);
    throw ex;
}

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finally {
    if (out != null) { 
        System.out.println("Closing PrintWriter");
        out.close(); 
    } else { 
        System.out.println("PrintWriter not open");
    } 
} 

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// java.lang.AutoCloseable
public interface AutoCloseable {
	void close() throws Exception;
}

// java.io.Closeable
public interface Closeable extends AutoCloseable {
    public void close() throws IOException;
}

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static String readFirstLineFromFile(String path) throws IOException {
    try (BufferedReader br =
                   new BufferedReader(new FileReader(path))) {
        return br.readLine();
    }
}

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static String readFirstLineFromFileWithFinallyBlock(String path)
                                                     throws IOException {
    BufferedReader br = new BufferedReader(new FileReader(path));
    try {
        return br.readLine();
    } finally {
        if (br != null) br.close();
    }
}

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public void writeList() throws IOException, IndexOutOfBoundsException {
    //...
}

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// 语法
throw someThrowableObject;

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public class Throwable
	extends Object
	implements Serializable

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public Object pop() {
    Object obj;

    if (size == 0) {
        throw new EmptyStackException();
    }

    obj = objectAt(size - 1);
    setObjectAt(size - 1, null);
    size--;
    return obj;
}

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															/*
使用电脑版看下面的文字"示意图"，更好哦。	

														
    Object											
      ↑										
   Throwable	
   ↑       ↑
Error	   Exception		
           ↑       ↑
        . . .     RuntimeException
      ↗  ↑  ↖       ↗  ↑  ↖
     ..   ..  ..     ..   ..  ..
															
															*/

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Throwable getCause()
Throwable initCause(Throwable)
Throwable(String, Throwable)
Throwable(Throwable)

// 举个例子：
try {
	// ...
} catch (IOException e) {
    throw new SampleException("Other IOException", e);
}

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try{
    // ...
}
catch (Exception cause) {
    StackTraceElement elements[] = cause.getStackTrace();
    for (int i = 0, n = elements.length; i < n; i++) {       
        System.err.println(elements[i].getFileName()//类名
            + ":" + elements[i].getLineNumber() //多少行
            + ">> "
            + elements[i].getMethodName() + "()");//方法名
    }
}

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// 优点1，错误处理与业务性代码分离

// 程序流程
readFile {
    open the file;
    determine its size;
    allocate that much memory;
    read the file into memory;
    close the file;
}

// 通过状态码来管理/处理错误
errorCodeType readFile {
    initialize errorCode = 0;
    
    open the file;
    if (theFileIsOpen) {
        determine the length of the file;
        if (gotTheFileLength) {
            allocate that much memory;
            if (gotEnoughMemory) {
                read the file into memory;
                if (readFailed) {
                    errorCode = -1;
                }
            } else {
                errorCode = -2;
            }
        } else {
            errorCode = -3;
        }
        close the file;
        if (theFileDidntClose && errorCode == 0) {
            errorCode = -4;
        } else {
            errorCode = errorCode and -4;
        }
    } else {
        errorCode = -5;
    }
    return errorCode;
}

// 通过异常来管理/处理错误
readFile {
    try {
        open the file;
        determine its size;
        allocate that much memory;
        read the file into memory;
        close the file;
    } catch (fileOpenFailed) { // -5
       doSomething;
    } catch (sizeDeterminationFailed) { // -3
        doSomething;
    } catch (memoryAllocationFailed) { // -2
        doSomething;
    } catch (readFailed) { // 0
        doSomething;
    } catch (fileCloseFailed) { -4
        doSomething;
    }
    // -1
}

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// 优点2，错误传播
method1 {
    call method2;
}

method2 {
    call method3;
}

method3 {
    call readFile;
}

// 状态码
method1 {
    errorCodeType error;
    error = call method2;
    if (error)
        doErrorProcessing;
    else
        proceed;
}

errorCodeType method2 {
    errorCodeType error;
    error = call method3;
    if (error)
        return error;
    else
        proceed;
}

errorCodeType method3 {
    errorCodeType error;
    error = call readFile;
    if (error)
        return error;
    else
        proceed;
}

// 异常处理
method1 {
    try {
        call method2;
    } catch (exception e) {
        doErrorProcessing;
    }
}

method2 throws exception {
    call method3;
}

method3 throws exception {
    call readFile;
}

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// 优点3. 异常类型分类

// File相关
catch (FileNotFoundException e) {
    ...
}

// I/O相关
catch (IOException e) {
    ...
}