try_Keyword

迷南。 2021-11-04 10:40 196阅读 0赞

## Java try/catch Block ##

Java Exceptions Tutorial  
In this article, we will learn in depth about try/catch block and how to use try/catch block to handle exceptions.  
What will we learn in this articles?

*  try Block
 *  The syntax of java try-catch
 *  The syntax of a try-finally block
 *  Nested try block
 *  catch Block
 *  try/catch Block Examples
 *  Multi-catch Block
 *  Catching More Than One Type of Exception with One Exception Handler

### try Block ###

Enclose the code that might throw an exception within a try block. If an exception occurs within the try block, that exception is handled by an exception handler associated with it. The try block contains at least one catch block or finally block.

### The syntax of java try-catch ###

try\{  
//code that may throw exception  
\}catch(Exception\_class\_Name ref)\{\}

### The syntax of a try-finally block ###

try\{  
//code that may throw exception  
\}finally\{\}

### Nested try block ###

The try block within a try block is known as nested try block in java.  
Why use nested try block?  
Sometimes a situation may arise where a part of a block may cause one error and the entire block itself may cause another error. In such cases, exception handlers have to be nested.

public class NestedTryBlock {
     
             public static void main(String args[]) {
                     try {
                              try {
                                       System.out.println(" This gives divide by zero error");
                                       int b = 39 / 0;
                              } catch (ArithmeticException e) {
                                       System.out.println(e);
                              }
     
                              try {
                                       System.out.println(" This gives Array index out of bound exception");
                                       int a[] = new int[5];
                                       a[5] = 4;
                              } catch (ArrayIndexOutOfBoundsException e) {
                                       System.out.println(e);
                              }
     
                              System.out.println("other statement");
                     } catch (Exception e) {
                              System.out.println("handeled");
                     }
     
                     System.out.println("normal flow..");
             }
    }

### catch Block ###

Java catch block is used to handle the Exception. It must be used after the try block only. You can use multiple catch block with a single try.  
Syntax:

try
    {
          //code that cause exception;
    }
    catch(Exception_type  e)
    {
          //exception handling code
    }

try/catch Block Examples  
Let’s demonstrate the usage of catch block using ArithmeticException type.

#### Example 1: ArithmeticException exception type example. ####

public class Arithmetic {
    
     public static void main(String[] args) {
    
      try {
       int result = 30 / 0; // Trying to divide by zero
      } catch (ArithmeticException e) {
       System.out.println("ArithmeticException caught!");
      }
      System.out.println("rest of the code executes");
     }
    }

Output:

ArithmeticException caught!
    rest of the code executes

#### Example 2: ArrayIndexOutOfBoundsException exception type example. ####

public class ArrayIndexOutOfBounds {
    
        public static void main(String[] args) {
    
            int[] nums = new int[] { 1, 2, 3 };
    
            try {
                int numFromNegativeIndex = nums[-1]; // Trying to access at negative index
                int numFromGreaterIndex = nums[4]; // Trying to access at greater index
                int numFromLengthIndex = nums[3]; // Trying to access at index equal to size of the array
            } catch (ArrayIndexOutOfBoundsException e) {
                System.out.println("ArrayIndexOutOfBoundsException caught");
            }
    
            System.out.println("rest of the code executes");
        }
    
    }

If you have to perform different tasks at the occurrence of different Exceptions, use java multi-catch block.

#### Multi-catch Block ####

In some cases, more than one exception could be raised by a single piece of code. To handle this type of situation, you can specify two or more catch clauses, each catching a different type of exception. When an exception is thrown, each catch statement is inspected in order, and the first one whose type matches that of the exception is executed.  
After one catch statement executes, the others are bypassed, and execution continues after the try/catch block.

public class TestMultipleCatchBlock {
    public static void main(String args[]) {
     try {
      int a = args.length;
      System.out.println("a = " + a);
      int b = 42 / a;
      int c[] = { 1 };
      c[42] = 99;
     } catch (ArithmeticException e) {
      System.out.println("Divide by 0: " + e);
     } catch (ArrayIndexOutOfBoundsException e) {
      System.out.println("Array index oob: " + e);
     }
     System.out.println("After try/catch blocks.");
    }
    }

This program will cause a division-by-zero exception if it is started with no command line arguments, since a will equal zero. It will survive the division if you provide a command-line argument, setting a to something larger than zero.  
But it will cause an ArrayIndexOutOfBoundsException, since the int array c has a length of 1, yet the program attempts to assign a value to c\[42\].  
Here is the output generated by running it both ways.  
Output:

C:\>java MultipleCatches
    a = 0
    Divide by 0: java.lang.ArithmeticException: / by zero
    After try/catch blocks.
    C:\>java MultipleCatches TestArg
    a = 1
    Array index oob: java.lang.ArrayIndexOutOfBoundsException:42
    After try/catch blocks.

When you use multiple catch statements, it is important to remember that exception subclasses must come before any of their superclasses. This is because a catch statement that uses a superclass will catch exceptions of that type plus any of its subclasses. Thus, a subclass would never be reached if it came after its superclass.  
Catching More Than One Type of Exception with One Exception Handler  
In Java SE 7 and later, a single catch block can handle more than one type of exception. This feature can reduce code duplication and lessen the temptation to catch an overly broad exception.  
In the catch clause, specify the types of exceptions that block can handle, and separate each exception type with a vertical bar (|):

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

try_Keyword

发表评论取消回复

还没有评论，来说两句吧...

相关阅读