Static Blocks in Java
In simpler language whenever we use a static keyword and associate it to a block then that block is referred to as a static block. Unlike C++, Java supports a special block, called a static block (also called static clause) that can be used for static initialization of a class. This code inside the static block is executed only once: the first time the class is loaded into memory.
Calling of static block in java?
Now comes the point of how to call this static block. So in order to call any static block, there is no specified way as static block executes automatically when the class is loaded in memory. Refer to the below illustration for understanding how static block is called.
Illustration:
class GFG {
// Constructor of this class
GFG() {}
// Method of this class
public static void print() { }
static{}
public static void main(String[] args) {
// Calling of method inside main()
GFG geeks = new GFG();
// Calling of constructor inside main()
new GFG();
// Calling of static block
// Nothing to do here as it is called
// automatically as class is loaded in memory
}
}
Note: From the above illustration we can perceive that static blocks are automatically called as soon as class is loaded in memory and there is nothing to do as we have to in case of calling methods and constructors inside main().
Can we print something on the console without creating main() method?
It is very important question from the interview's perceptive point. The answer is yes we can print if we are using JDK version 1.6 or previous and if after that it will throw an. error.
Example 1-A: Running on JDK version 1.6 of Previous
// Java Program Running on JDK version 1.6 of Previous
// Main class
class GFG {
// Static block
static
{
// Print statement
System.out.print(
"Static block can be printed without main method");
}
}
Output:
Static block can be printed without main method
Example 1-B: Running on JDK version 1.6 and Later
// Java Program Running on JDK version 1.6 and Later
// Main class
class GFG {
// Static block
static
{
// Print statement
System.out.print(
"Static block can be printed without main method");
}
}
Output:
Execution of Static Block
Example 1:
// Java Program to Illustrate How Static block is Called
// Class 1
// Helper class
class Test {
// Case 1: Static variable
static int i;
// Case 2: non-static variables
int j;
// Case 3: Static block
// Start of static block
static
{
i = 10;
System.out.println("static block called ");
}
// End of static block
}
// Class 2
// Main class
class GFG {
// Main driver method
public static void main(String args[])
{
// Although we don't have an object of Test, static
// block is called because i is being accessed in
// following statement.
System.out.println(Test.i);
}
}
// Java Program to Illustrate How Static block is Called// Class 1// Helper classclass Test { // Case 1: Static variable static int i; // Case 2: non-static variables int j; // Case 3: Static block // Start of static block static { i = 10; System.out.println("static block called "); } // End of static block}// Class 2// Main classclass GFG { // Main driver method public static void main(String args[]) { // Although we don't have an object of Test, static // block is called because i is being accessed in // following statement. System.out.println(Test.i); }}Output
static block called 10
Remember: Static blocks can also be executed before constructors.
Example 2:
// Java Program to Illustrate Execution of Static Block
// Before Constructors
// Class 1
// Helper class
class Test {
// Case 1: Static variable
static int i;
// Case 2: Non-static variable
int j;
// Case 3: Static blocks
static
{
i = 10;
System.out.println("static block called ");
}
// Constructor calling
Test() { System.out.println("Constructor called"); }
}
// Class 2
// Main class
class GFG {
// Main driver method
public static void main(String args[])
{
// Although we have two objects, static block is
// executed only once.
Test t1 = new Test();
Test t2 = new Test();
}
}
// Java Program to Illustrate Execution of Static Block// Before Constructors// Class 1// Helper classclass Test { // Case 1: Static variable static int i; // Case 2: Non-static variable int j; // Case 3: Static blocks static { i = 10; System.out.println("static block called "); } // Constructor calling Test() { System.out.println("Constructor called"); }}// Class 2// Main classclass GFG { // Main driver method public static void main(String args[]) { // Although we have two objects, static block is // executed only once. Test t1 = new Test(); Test t2 = new Test(); }}Output
static block called Constructor called Constructor called
A class can have any number of static initialization blocks, and they can appear anywhere in the class body. The runtime system guarantees that static initialization blocks are called in the order that they appear in the source code.
Note: We use Initializer Block in Java if we want to execute a fragment of code for every object which is seen widely in enterprising industries in development.
