java只是花括号

Question

I was reading a book and there were a few example with programs that has just curly braces

for example

 public static void main(String args[]){
     //what is the uses of curly braces here.
     {
          //some code
     }
 }

Answer 1

It's a code block. The variables declared in there are not visible in the upper block (method body outside of these curlies), ie they have a more limited scope.

Answer 2

Be careful, it is NOT ALWAYS an initialisation block as others have suggested. In your case it is a variable scoping mechanism called a Code Block or block.

If it is outside of a method, then it is!

Example

public class MyClass {

   {
      // this is an initialisation block
   }

}

However, if it is inside a method, it is NOT! In this case (which is the case in your example), it is a code block. Anything initialised inside the curly braces is not visible outside

Example

public static void main(String args[]){

     {
          String myString = "you can't see me!";
     }
     System.out.println(myString); // this will not compile because myString is not visible.
 }

Answer 3

This idea of how to use curly braces as a coding construct is a debated issue in the Java world . There are several explanations people come up with when they see curly braces by themselves. So Im going to try to answer your question from a practical perspective.

The implied question in your post here is, really - when/why are these used ? Practically speaking, the following cases might result in a lone code block :

1) The programmer wanted additionally scoping to reuse variable names without fear of collisions for clarity (ie making several objects of the same type in a unit test or database insertion block).

other possible reasons :

2) Forgotten if/else/for/while loop code that is under development.

3) Remaining artifact of a removed if/else/for/while clause.

4) Autogenerated code uses scoping to simplify the creation of several similar components with identical variable names (ie consider a gui generator that needed to make code for 100 radio buttons - rather than incrementing variable names per button, it could use scoping).

5) As a tiny, reusable, pastable logical block with minimal side effects : the programmer felt like a block of code in a method was so obscure, its variables and internal side effects should have minimal visibility to the outside world. That is, the programmer has used a code block as a poor-man's anonymous lambda function (albeit, one without a return value). In this pattern one might do something akin to the below :

//lets say I want to make a primary key for a dogs name in a database. 
String dogNameKey=null;
{
    long time = System.currentTimeInMilliseconds();
    String theName = "spot";
    dogName=theName+"_"+time;
}

Clearly, the simple strategy for naming this record (dogNameKey) is not worthy of an external method - its too simple. But at the same time, the "time" variable should have no bearing or accessibility outside the logic for making this name up - ie it shouldn't even be relevant to the method which contains this tiny key generating block. So, by using braces, I've scoped it out . If a labmda were possible, than all of this scoping could be wrapped in a single, anonymous function.

Now - I could paste several of these blocks, and the variable names would be identical, so it would be easy to scan them by eye.

* Thus, when you see curly braces by themselves - they usually are pretty important - either they implement a specific custom-scoping, or they are an artifact of an error or potentially of autogenerated code. Scoping can also be used to "start" the refactoring of a method without actually writing a new method, by separating out its independant parts ... although IDEs are much better at this than humans. *

Answer 4

It is called Block

A block is a sequence of statements, local class declarations and local variable declaration statements within braces.

Also See:

• Documents

Answer 5

You can logically separate your code by this in some cases, and in fact there's one use case I apply very often: demo data. Eg, you have some demo data generation class that creates demo data entries and inserts into your database. You place each single item in such a block, and can do copy-paste without changing variable names.