如何在Java中将try-with-resources语句与接口对象一起使用

Question

I would like to use a try-with-resources statement to define an interface object as a concrete class. Here is some sample code loosely defining my interface and classes.

interface IFoo extends AutoCloseable
{
    ...
}

class Bar1 implements IFoo
{
    ...
}

class Bar2 implements IFoo
{
    ...
}

class Bar3 implements IFoo
{
    ...
}

// More Bar classes.........

I now need to define an IFoo object, but the concrete class is conditional on another variable of my code. The logic is the same for all concrete classes. So I would like to use a try-with-resources statement to define the interface object, but I need to use a conditional statement to see which concrete class I need define the interface object as.

Logically, this is what I am looking to do:

public void doLogic(int x)
    try (
        IFoo obj;
        if (x > 0) { obj = new Bar1(); }
        else if (x == 0) { obj = new Bar2(); }
        else { obj = new Bar3(); }
    )
    {
        // Logic with obj
    }
}

The only resource I have found relating to this is @Denis's question here: How to use Try-with-resources with if statement? However, the solution given there would require nested ternary statements for my scenario, and that gets messy real fast.

Does anyone know of an elegant solution for this issue?

Answer 1

Define a factory method to create the IFoo instance:

IFoo createInstance(int x) {
    if (x > 0) { return new Bar1(); }
    else if (x == 0) { return new Bar2(); }
    else { return new Bar3(); }
}

then invoke that in your try-with-resources initializer:

public void doLogic(int x) {
  try (IFoo ifoo = createInstance(x)) {
    // Logic with obj
  }
}

Answer 2

I agree that the best solution is to write a helper method like in this answer.

However, I also want to point out that nested ternary operators are not messy. You do not need brackets at all, and with good formatting it can be made to look like a switch statement:

try (IFoo foo = x > 20     ? new Bar1() :
                x < 0      ? new Bar2() :
                x == 10    ? new Bar3() :
                x % 2 == 0 ? new Bar4() : 
                             new Bar5()) {
        // do stuff
}