Can anyone tell me that what is the use of a static block in a class?

Question

public class myclass{ 

    static{ 

        //some statements here 

    } 

    //some variables declared here 
    //some functions defined here 
} 

Answer 1

It's a static initializer. Analogously to the instance initializer (§8.6), you can use it to initialize your class when it's loaded. It is NOT "invoked" explicitly; it is executed automatically when the class is loaded, in textual order (static initializer that occurs later in the text is guaranteed to be performed later during the initialization).

You can use a static initializer to:

• Initialize some static fields
• Perform some other one-time calculations, perhaps something requiring a try-catch block, logging events related to the loading of the class, making sure that Java's assertion is enabled, etc.

There are some caveats, e.g. a class may be reloaded, and usually there are alternatives of writing it (e.g. refactoring into a private static named method), but using a static initializer is an option.

References

• JLS 8.7 Static initializers
• JLS 12.4 Initialization of Classes and Interfaces

Related questions

These questions discuss various aspects of static and instance initializers usage:

• Static initializer in Java
• How to handle a static final field initializer that throws checked exception
• Why is the order of declarations important for static initializers?
• Use of Initializers vs Constructors in Java
• Are Java static initializers thread safe?

---

An example usage: requiring that assertions are enabled

The following is the prescribed idiom in the Java Language Guide for Programming With Assertions to ensure that assertion is enabled at run-time:

Requiring that Assertions are Enabled

Programmers of certain critical systems might wish to ensure that assertions are not disabled in the field. The following static initialization idiom prevents a class from being initialized if its assertions have been disabled:

static {
    boolean assertsEnabled = false;
    assert assertsEnabled = true; // Intentional side effect!!!
    if (!assertsEnabled)
        throw new RuntimeException("Asserts must be enabled!!!");
}

Put this static-initializer at the top of your class.

By putting this snippet in a static initializer for the class, the code will be one of the first thing executed when the class is loaded, before any instances of the class can be created. The code itself checks if Java assertion is enabled, and throws a RuntimeException if it's not.

See also

• Wikipedia/Assertion (computing)

Related questions

• Do you use assertions?

Answer 2

Static blocks are executed when the class is loaded, so they are a good place to initialize static variables. Especially constants that require more logic than a one-liner may be initialized comfortably using a static block:

private static final String MY_CONSTANT;

static{
    // read properties from file
    MY_CONSTANT = properties.get("my.property.name");
}

Answer 3

Static blocs are used to initialize static values which might need real code to be initialized.

for example you want to do this

static boolean firstBoolean = staticMethodThatGetBoolean();
static boolean secondBoolean = staticMethodThatGetAnotherBoolean();
static int i = //if firstBoolean is true i = 1, if secondBoolean is true i = 2, if firstBoolean and secondBoolean are true i = 3;

Either you do something like this :

static boolean firstBoolean = staticMethodThatGetBoolean();
static boolean secondBoolean = staticMethodThatGetAnotherBoolean();
static int i = (firstBoolean && secondBoolean)?3:(secondBoolean)?2:(firstBoolean)?1:0;

or you create a static bloc to handle all of this.

Another example would be to handle exceptions thrown by staticMethodThatGetBoolean() or staticMethodThatGetAnotherBoolean()

Be careful, static bloc are meant to be initializer blocs in a static context. You must use them to initialize your static variables only. Even if you can invoke real code in here, this is a really really bad practice.

Answer 4

Static loop is executed, when the class is loaded. So without creating the object we can execute this loop.

Answer 5

public class Test {
    static int a=3; 
       static int b; 
    static void meth(int x) { 
       System.out.println("x = " + x); 
       System.out.println("a = " + a); 
       System.out.println("b = " + b); 
    } 

    static { 
      System.out.println("Static block initialized."); 
      b = a * 4; System.out.println("a = " + a); 
       System.out.println("b = " + b); 

    } 
    public static void main(String args[]) { 
      meth(42); 
     } 

}

output willl be--

Static block initialized.
a = 3
b = 12
x = 42
a = 3
b = 12