尝试使用if语句捕获块

Question

So I got some problems implementing a try catch block in my program. It's quite simple, all I want is to throw an exception whenever the user enters a 0 or less into my dialog window. Here is my code:

try {
    if (this.sides <= 0);
} catch (NegativeSidesException exception) {
    System.out.println(exception + "You entered 0 or less");
}

The NegativeSidesException is my own defined exception.

When I make 0 the input the try catch block doesn't catch it and the compiler throws a normal exception and terminates the program.

Answer 1

Change if (this.sides <= 0);

To if (this.sides <= 0) throw new Exception ("some error message");

And every thing will work as you want

Answer 2

Create a new object for the exception and throw it explicitly.

try{
    if (this.sides <= 0)
        throw new NegativeSidesException();
}
catch (NegativeSidesException exception)
{
    System.out.println(exception + "You entered 0 or less");        
}

Answer 3

You have so bad syntax :)

1) if statement as it is doesn't throw exception of it's own

Lets repeat :)

If:

if(condition){
 //commands while true
}

if(condition)
 //if there is just 1 command, you dont need brackets

if(condition){
 //cmd if true
}else{
 //cmd for false
}

 //theoretically (without brackets)
    if(condition)
     //true command;
    else
     //false command;

Try-Catch:

try{
 //danger code
}catch(ExceptionClass e){
 //work with exception 'e'
}

2) There are more ways how to make that :

try{
    if (this.sides <= 0){
          throw new NegativeSidesException();
    }else{
           //make code which you want- entered value is above zero
           //in fact, you dont need else there- once exception is thrown, it goes into catch automatically
    }
}catch(NegativeSidesException  e){
   System.out.println(e + "You entered 0 or less");
}

Or maybe better:

try{
    if (this.sides > 0){
         //do what you want
    }else{
           throw new NegativeSidesException();
    }
 }catch(NegativeSidesException  e){
    System.out.println(e + "You entered 0 or less");
 }

Btw You can use java default Exception (that message is better to specify as constant in above of class):

throw new Exception("You entered 0 or less); 
//in catch block
System.out.println(e.getMessage());