What does two consecutive blocks of code {}{} do? [duplicate]

Question

Possible Duplicate:
Anonymous code blocks in Java

I just came across the following.

if ( test ) {
    {
        // statements 1
    }
    {
        // statements 2
    }
}

It is the first time I've seen this. How does it work?

Answer 1

It's is just writing two different blocks of code in order to hide local variables.

From the answer to the question "Anonymous code blocks in Java":

Blocks restrict variable scope.

public void foo()
{
    {
        int i = 10;
    }
    System.out.println(i); // Won't compile.
}

In practice, though, if you find yourself using such a code block then it's probably a sign that you want to refactor that block out to a method.

Answer 2

Two examples of where this can be (slightly) useful - in unit tests, and in GUIs, both of which often involve repetitive code:

It's useful in GUI building, where it is incredibly easy to cut-and-paste lines relating to one component and forget to update them for the new component, leading to hard-to-find bugs, e.g.:

    JButton button1 = new JButton("OK");
    button1.setEnabled(false);
    button1.setAlignmentX(-1);

    JButton button2 = new JButton("Apply");
    button1.setEnabled(false);
    button1.setAlignmentX(-1);

Oops, I just configured button1 twice. If you put each button in its own block, then this mistake is picked up by the compiler. Again, you could create each button in a separate method, but that may make it hard to see what is going on (especially given the lack of keywords parameters in Java):

JButton button_ok = makeButton("OK", false, -1);
JButton button_apply = makeButton("Apply", true, 1);
// what exactly is being set here?

...

// much later:
private static JButton makeButton(String name, boolean enabled,
        int alignment)
{
    JButton button = new JButton(name);
    button.setEnabled(enabled);
    button.setAlignmentX(alignment);
    return button;
}

...and you may end up with numerous methods, each handling different variations of parameters, and each only being used maybe twice.

Answer 3

The two blocks are independent. So, whatever variables you may have in the first block won't be accessible in the second block - or anywhere outside the first block. It's called code isolation or scoping.