What's going on with the Java Java Builder?

Tag: The constructor is checked for exception capture

The code is as follows:

public class Example040 {private Example040 E40 = new Example040 ();p ublic Example040 () throws Exception {throw new Excepti On ("Here is exception, not error");} public void output () {System.out.println ("A new Class");} public static void Main (string[] args) throws Exception {Example040 example040 = new Example040 (); Example040.output ();}}

Code Analysis:

Run the above code and throw a Stackoverflowerror exception. Like most programs that throw Stackoverflowerror exceptions, this program also contains an infinite recursion. When you invoke a constructor, the initialization of the instance variable runs before the constructor's body. in the above code, the initialization of the E40 variable invokes the constructor recursively, and the constructor initializes the variable's own E40 domain by invoking the constructor again, so that the recursion goes on indefinitely. These recursive calls throw a Stackoverflowerror exception before the constructor program body obtains an execution opportunity, because Stackoverflowerror is a subtype of Error and not a subtype of Exception, so a catch clause cannot capture it. When you add a static modifier to E40, the stack overflow exception disappears.

Make the following modifications to the above code, add two new variables, the new variable initialization will produce an exception:

public class example040 {// private example040  e40 = new example040 ();p rivate static example040 e40 = new  Example040 ();p rivate static class internal = e40.new internalclazz (). GetClass (); private internalclazz clazz =  (Internalclazz)  internal.newinstance ();p ublic  Example040 ()  /* throws exception */{// throw new exception ("Here is Exception, Not error ");} Public void output ()  {system.out.println ("A new class");} Class internalclazz {public internalclazz ()  {}}public static void main ( String[] args)  throws Exception {Example040 example040 = new  Example040 (); Example040.output ();}} 

The above program will not compile correctly because the constructor must declare all of the checked exceptions thrown by its instance initialization operation. Although its constructor does not have any program body, it throws two checked exceptions, instantiationexception and illegalaccessexception. They are class.newinstance thrown, and this method is called when the Clazz domain is initialized.

An improved approach to the above procedure is to create a private, static helper method that calculates the initial value of the domain and handles the exception appropriately, as follows:

Private Internalclazz Clazz = Newclazz ();p rivate static Internalclazz Newclazz () {try {return (Internalclazz) internal.ne Winstance ();} catch (Instantiationexception | Illegalaccessexception e) {e.printstacktrace ();} return null;}

In summary, the instance initialization operation is run prior to the constructor's program body. Any exception thrown by the instance initialization operation is propagated to the constructor. If the initialization operation throws a check exception, the constructor must declare that the exception is thrown, but this should be avoided because it can cause confusion. Finally, for the class we are designing, if its instance contains other instances of the same class, be careful with this infinite recursion.

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Source code Address Https://github.com/rocwinger/java-disabuse

This article from "Winger" blog, declined reprint!

What's going on with the Java Java Builder?