class A { static{ get(); } static void get(){ System.out.println("HELLO"); } } class B extends A { public static void main(String[] args) { } } Output:
HELLO
How is static method of class A getting called. I can understand static method of B getting called but not of A.
Because B extends from A, if the B class is loaded, then the A class has to get loaded, and if it is loaded, its static initializer block must be called.
Because, since B is inheriting A, B has to load A when main() executes. This activates the get() function since it is in the static block.
To execute the main method of class B, class B has to be loaded.
Class B extends class A, so to load B, class A must be loaded.
When class A is loaded, its static initializer is invoked, calling the get() method and printing "HELLO".
In inheritance class loading is happens from top to bottom i.e. Parent to child. It first loads the parent class in your case A and then loads the child class in your case B. And when class loads its static block get called first that why its printing HELLO
All Static blocks are executed at class loading time ... duplicate question When is the static block of a class executed?
During compilation the compiler knows that B is dependant on A. It passes the same data to the JVM. During loading phase of class B, the JVM reads metadata sent by the compiler and finds out that B is dependant on A and hence loads (and runs static initializers) of A then continues to load B and then initializes B.
If we had 2 classes Sample and Test extends Sample the Class Constant Pool (part of byte code ) for Test would have :
Constant pool: #1 = Class #2 // Test #2 = Utf8 Test #3 = Class #4 // Sample <---- reference #4 = Utf8 Sample #5 = Utf8 <init> #6 = Utf8 ()V #7 = Utf8 Code #8 = Methodref #3.#9 // Sample."<init>":()V <-- init Sample #9 = NameAndType #5:#6 // "<init>":()V #10 = Utf8 LineNumberTable #11 = Utf8 LocalVariableTable #12 = Utf8 this #13 = Utf8 LTest; #14 = Utf8 main #15 = Utf8 ([Ljava/lang/String;)V #16 = Utf8 args #17 = Utf8 [Ljava/lang/String; #18 = Utf8 SourceFile #19 = Utf8 Sample.java If you run java with verbose:class option then grep it, you will be able to see that the dependant class is being loaded.
java -verbose:class Test | grep 'Sample' [Loaded Sample from file:/Users/XXXX/Workspaces/SampleTest/Sample/bin/] <== Sample loaded first because test depends on Sample. [Loaded Test from file:/Users/XXXX/Workspaces/SampleTest/Sample/bin/]