Develop Reference

Unfamiliar Java Syntax [duplicate]

We can put code in a constructor or a method or an initialization block. What is the use of initialization block? Is it necessary that every java program must have it?

First of all, there are two types of initialization blocks:
instance initialization blocks, and
static initialization blocks.
This code should illustrate the use of them and in which order they are executed:
public class Test {
static int staticVariable;
int nonStaticVariable;
// Static initialization block:
// Runs once (when the class is initialized)
static {
System.out.println("Static initalization.");
staticVariable = 5;
}
// Instance initialization block:
// Runs each time you instantiate an object
{
System.out.println("Instance initialization.");
nonStaticVariable = 7;
}
public Test() {
System.out.println("Constructor.");
}
public static void main(String[] args) {
new Test();
new Test();
}
}
Prints:
Static initalization.
Instance initialization.
Constructor.
Instance initialization.
Constructor.
Instance initialization blocks are useful if you want to have some code run regardless of which constructor is used or if you want to do some instance initialization for anonymous classes.

would like to add to #aioobe's answer
Order of execution:
static initialization blocks of super classes
static initialization blocks of the class
instance initialization blocks of super classes
constructors of super classes
instance initialization blocks of the class
constructor of the class.
A couple of additional points to keep in mind (point 1 is reiteration of #aioobe's answer):
The code in static initialization block will be executed at class load time (and yes, that means only once per class load), before any instances of the class are constructed and before any static methods are called.
The instance initialization block is actually copied by the Java compiler into every constructor the class has. So every time the code in instance initialization block is executed exactly before the code in constructor.

nice answer by aioobe
adding few more points
public class StaticTest extends parent {
static {
System.out.println("inside satic block");
}
StaticTest() {
System.out.println("inside constructor of child");
}
{
System.out.println("inside initialization block");
}
public static void main(String[] args) {
new StaticTest();
new StaticTest();
System.out.println("inside main");
}
}
class parent {
static {
System.out.println("inside parent Static block");
}
{
System.out.println("inside parent initialisation block");
}
parent() {
System.out.println("inside parent constructor");
}
}
this gives
inside parent Static block
inside satic block
inside parent initialisation block
inside parent constructor
inside initialization block
inside constructor of child
inside parent initialisation block
inside parent constructor
inside initialization block
inside constructor of child
inside main
its like stating the obvious but seems a little more clear.

The sample code, which is approved as an answer here is correct, but I disagree with it. It does not shows what is happening and I'm going to show you a good example to understand how actually the JVM works:
package test;
class A {
A() {
print();
}
void print() {
System.out.println("A");
}
}
class B extends A {
static int staticVariable2 = 123456;
static int staticVariable;
static
{
System.out.println(staticVariable2);
System.out.println("Static Initialization block");
staticVariable = Math.round(3.5f);
}
int instanceVariable;
{
System.out.println("Initialization block");
instanceVariable = Math.round(3.5f);
staticVariable = Math.round(3.5f);
}
B() {
System.out.println("Constructor");
}
public static void main(String[] args) {
A a = new B();
a.print();
System.out.println("main");
}
void print() {
System.out.println(instanceVariable);
}
static void somethingElse() {
System.out.println("Static method");
}
}
Before to start commenting on the source code, I'll give you a short explanation of static variables of a class:
First thing is that they are called class variables, they belong to the class not to particular instance of the class. All instances of the class share this static(class) variable. Each and every variable has a default value, depending on primitive or reference type. Another thing is when you reassign the static variable in some of the members of the class (initialization blocks, constructors, methods, properties) and doing so you are changing the value of the static variable not for particular instance, you are changing it for all instances. To conclude static part I will say that the static variables of a class are created not when you instantiate for first time the class, they are created when you define your class, they exist in JVM without the need of any instances. Therefor the correct access of static members from external class (class in which they are not defined) is by using the class name following by dot and then the static member, which you want to access (template: <CLASS_NAME>.<STATIC_VARIABLE_NAME>).
Now let's look at the code above:
The entry point is the main method - there are just three lines of code. I want to refer to the example which is currently approved. According to it the first thing which must be printed after printing "Static Initialization block" is "Initialization block" and here is my disagreement, the non-static initialization block is not called before the constructor, it is called before any initializations of the constructors of the class in which the initialization block is defined. The constructor of the class is the first thing involved when you create an object (instance of the class) and then when you enter the constructor the first part called is either implicit (default) super constructor or explicit super constructor or explicit call to another overloaded constructor (but at some point if there is a chain of overloaded constructors, the last one calls a super constructor, implicitly or explicitly).
There is polymorphic creation of an object, but before to enter the class B and its main method, the JVM initializes all class(static) variables, then goes through the static initialization blocks if any exist and then enters the class B and starts with the execution of the main method. It goes to the constructor of class B then immediately (implicitly) calls constructor of class A, using polymorphism the method(overridden method) called in the body of the constructor of class A is the one which is defined in class B and in this case the variable named instanceVariable is used before reinitialization. After closing the constructor of class B the thread is returned to constructor of class B but it goes first to the non-static initialization block before printing "Constructor". For better understanding debug it with some IDE, I prefer Eclipse.

Initializer block contains the code that is always executed whenever
an instance is created. It is used to declare/initialise the common
part of various constructors of a class.
The order of initialization constructors and initializer block doesn’t matter, initializer block is always executed before constructor.
What if we want to execute some code once for all objects of a class?
We use Static Block in Java.

In addition to what was said in previous answers, blocks can be synchronized .. never felt I need to use it, however,it's there

Initialization blocks are executed whenever the class is initialized and before constructors are invoked. They are typically placed above the constructors within braces. It is not at all necessary to include them in your classes.
They are typically used to initialize reference variables. This page gives a good explanation

The question is not entirely clear, but here's a brief description of ways you can initialise data in an object. Let's suppose you have a class A that holds a list of objects.
1) Put initial values in the field declaration:
class A {
private List<Object> data = new ArrayList<Object>();
}
2) Assign initial values in the constructor:
class A {
private List<Object> data;
public A() {
data = new ArrayList<Object>();
}
}
These both assume that you do not want to pass "data" as a constructor argument.
Things get a little tricky if you mix overloaded constructors with internal data like above. Consider:
class B {
private List<Object> data;
private String name;
private String userFriendlyName;
public B() {
data = new ArrayList<Object>();
name = "Default name";
userFriendlyName = "Default user friendly name";
}
public B(String name) {
data = new ArrayList<Object>();
this.name = name;
userFriendlyName = name;
}
public B(String name, String userFriendlyName) {
data = new ArrayList<Object>();
this.name = name;
this.userFriendlyName = userFriendlyName;
}
}
Notice that there is a lot of repeated code. You can fix this by making constructors call each other, or you can have a private initialisation method that each constructor calls:
class B {
private List<Object> data;
private String name;
private String userFriendlyName;
public B() {
this("Default name", "Default user friendly name");
}
public B(String name) {
this(name, name);
}
public B(String name, String userFriendlyName) {
data = new ArrayList<Object>();
this.name = name;
this.userFriendlyName = userFriendlyName;
}
}
or
class B {
private List<Object> data;
private String name;
private String userFriendlyName;
public B() {
init("Default name", "Default user friendly name");
}
public B(String name) {
init(name, name);
}
public B(String name, String userFriendlyName) {
init(name, userFriendlyName);
}
private void init(String _name, String _userFriendlyName) {
data = new ArrayList<Object>();
this.name = name;
this.userFriendlyName = userFriendlyName;
}
}
The two are (more or less) equivalent.
I hope that gives you some hints on how to initialise data in your objects. I won't talk about static initialisation blocks as that's probably a bit advanced at the moment.
EDIT: I've interpreted your question as "how do I initialise my instance variables", not "how do initialiser blocks work" as initialiser blocks are a relatively advanced concept, and from the tone of the question it seems you're asking about the simpler concept. I could be wrong.

public class StaticInitializationBlock {
static int staticVariable;
int instanceVariable;
// Static Initialization Block
static {
System.out.println("Static block");
staticVariable = 5;
}
// Instance Initialization Block
{
instanceVariable = 7;
System.out.println("Instance Block");
System.out.println(staticVariable);
System.out.println(instanceVariable);
staticVariable = 10;
}
public StaticInitializationBlock() {
System.out.println("Constructor");
}
public static void main(String[] args) {
new StaticInitializationBlock();
new StaticInitializationBlock();
}
}
Output:
Static block
Instance Block
5
7
Constructor
Instance Block
10
7
Constructor

Just to add to the excellent answers from #aioobe and #Biman Tripathy.
A static initializer is the equivalent of a constructor in the static context. which is needed to setup the static environment.
A instance initializer is best for anonymous inner classes.
It is also possible to have multiple initializer blocks in class
When we have multiple initializer blocks they are executed (actually copied to constructors by JVM) in the order they appear
Order of initializer blocks matters, but order of initializer blocks mixed with Constructors doesn't
Abstract classes can also have both static and instance initializer blocks.
Code Demo -
abstract class Aircraft {
protected Integer seatCapacity;
{ // Initial block 1, Before Constructor
System.out.println("Executing: Initial Block 1");
}
Aircraft() {
System.out.println("Executing: Aircraft constructor");
}
{ // Initial block 2, After Constructor
System.out.println("Executing: Initial Block 2");
}
}
class SupersonicAircraft extends Aircraft {
{ // Initial block 3, Internalizing a instance variable
seatCapacity = 300;
System.out.println("Executing: Initial Block 3");
}
{ // Initial block 4
System.out.println("Executing: Initial Block 4");
}
SupersonicAircraft() {
System.out.println("Executing: SupersonicAircraft constructor");
}
}
An instance creation of SupersonicAircraft will produce logs in below order
Executing: Initial Block 1
Executing: Initial Block 2
Executing: Aircraft constructor
Executing: Initial Block 3
Executing: Initial Block 4
Executing: SupersonicAircraft constructor
Seat Capacity - 300

The execute order of the static method(JAVA) [duplicate]

This question already has answers here:
In what order do static blocks and initialization blocks execute when using inheritance?
(11 answers)
Closed 6 years ago.
I am searching for a long time on net. But no use. Please help or try to give some ideas how to achieve this?why the result is not to print the "int main"in the first? What I want to know is why the result of this program is as follow?Thanks in advance.
super static block
static block 4
in main
super constructor
constructor
class StaticSuper {
static {
System.out.println("super static block");
}
StaticSuper() {
System.out.println("super constructor");
}
}
public class StaticTests extends StaticSuper {
static int rand;
//static initialise
static {
rand = (int) (Math.random() * 6);
System.out.println("static block " + rand);
}
StaticTests() {
System.out.println("constructor");
}
public static void main(String[] args) {
System.out.println("in main");
StaticTests st = new StaticTests();
}
}

The firing order is:
Parent static blocks and fields in order of appearance
Child static blocks and fields in order of appearance
Parent non-static field initializers
Parent constructor
Child non-static field initializers
Child constructor
You can read more about here
Update:
Run this and you will understand why you should not call non-final method in super class constructor.
public class Derived extends Super
{
#Override
void initialise()
{
System.out.println("Now you can't initialise field \"a\" anymore");
}
Derived()
{
}
public static void main(String[] args)
{
Derived d = new Derived();
}
}
class Super
{
private int a;
void initialise()
{
a = 10;
}
Super()
{
initialise();
}
}
So you can't initialise your field a anymore it might break your super class code.
final methods cannot be overridden. So, you can initialise field a and you won't break anything in your super class.
I hope it clarifies your doubt.

The sequence is as follows:
You type java StaticTests
The JVM loads the StaticTests.
The JVM locates the static void main(String[]) method and attempts to call it.
The call to main(...) triggers the static initialization of StaticTests. (A class is initialized before the first call to a static method.)
The static initialization of StaticTests triggeres the static initialization of StaticSuper. (A classes superclasses must be initialized before the class is initialized.)
The "super static block" is printed by the super static init.
The "static block 4" is printed by the subclass static init.
The main(...) call starts.
The System.out.println("in main") statement is executed, printing "in main".
The new StaticTests calls the StaticTests constructor.
The StaticTests constructor calls super() implicitly. (Normal Java behavior.)
The superclass constructor prints "super constructor".
The subclass constructor prints "constructor"

Confusion in execution of class [duplicate]

We can put code in a constructor or a method or an initialization block. What is the use of initialization block? Is it necessary that every java program must have it?

First of all, there are two types of initialization blocks:
instance initialization blocks, and
static initialization blocks.
This code should illustrate the use of them and in which order they are executed:
public class Test {
static int staticVariable;
int nonStaticVariable;
// Static initialization block:
// Runs once (when the class is initialized)
static {
System.out.println("Static initalization.");
staticVariable = 5;
}
// Instance initialization block:
// Runs each time you instantiate an object
{
System.out.println("Instance initialization.");
nonStaticVariable = 7;
}
public Test() {
System.out.println("Constructor.");
}
public static void main(String[] args) {
new Test();
new Test();
}
}
Prints:
Static initalization.
Instance initialization.
Constructor.
Instance initialization.
Constructor.
Instance initialization blocks are useful if you want to have some code run regardless of which constructor is used or if you want to do some instance initialization for anonymous classes.

would like to add to #aioobe's answer
Order of execution:
static initialization blocks of super classes
static initialization blocks of the class
instance initialization blocks of super classes
constructors of super classes
instance initialization blocks of the class
constructor of the class.
A couple of additional points to keep in mind (point 1 is reiteration of #aioobe's answer):
The code in static initialization block will be executed at class load time (and yes, that means only once per class load), before any instances of the class are constructed and before any static methods are called.
The instance initialization block is actually copied by the Java compiler into every constructor the class has. So every time the code in instance initialization block is executed exactly before the code in constructor.

nice answer by aioobe
adding few more points
public class StaticTest extends parent {
static {
System.out.println("inside satic block");
}
StaticTest() {
System.out.println("inside constructor of child");
}
{
System.out.println("inside initialization block");
}
public static void main(String[] args) {
new StaticTest();
new StaticTest();
System.out.println("inside main");
}
}
class parent {
static {
System.out.println("inside parent Static block");
}
{
System.out.println("inside parent initialisation block");
}
parent() {
System.out.println("inside parent constructor");
}
}
this gives
inside parent Static block
inside satic block
inside parent initialisation block
inside parent constructor
inside initialization block
inside constructor of child
inside parent initialisation block
inside parent constructor
inside initialization block
inside constructor of child
inside main
its like stating the obvious but seems a little more clear.

The sample code, which is approved as an answer here is correct, but I disagree with it. It does not shows what is happening and I'm going to show you a good example to understand how actually the JVM works:
package test;
class A {
A() {
print();
}
void print() {
System.out.println("A");
}
}
class B extends A {
static int staticVariable2 = 123456;
static int staticVariable;
static
{
System.out.println(staticVariable2);
System.out.println("Static Initialization block");
staticVariable = Math.round(3.5f);
}
int instanceVariable;
{
System.out.println("Initialization block");
instanceVariable = Math.round(3.5f);
staticVariable = Math.round(3.5f);
}
B() {
System.out.println("Constructor");
}
public static void main(String[] args) {
A a = new B();
a.print();
System.out.println("main");
}
void print() {
System.out.println(instanceVariable);
}
static void somethingElse() {
System.out.println("Static method");
}
}
Before to start commenting on the source code, I'll give you a short explanation of static variables of a class:
First thing is that they are called class variables, they belong to the class not to particular instance of the class. All instances of the class share this static(class) variable. Each and every variable has a default value, depending on primitive or reference type. Another thing is when you reassign the static variable in some of the members of the class (initialization blocks, constructors, methods, properties) and doing so you are changing the value of the static variable not for particular instance, you are changing it for all instances. To conclude static part I will say that the static variables of a class are created not when you instantiate for first time the class, they are created when you define your class, they exist in JVM without the need of any instances. Therefor the correct access of static members from external class (class in which they are not defined) is by using the class name following by dot and then the static member, which you want to access (template: <CLASS_NAME>.<STATIC_VARIABLE_NAME>).
Now let's look at the code above:
The entry point is the main method - there are just three lines of code. I want to refer to the example which is currently approved. According to it the first thing which must be printed after printing "Static Initialization block" is "Initialization block" and here is my disagreement, the non-static initialization block is not called before the constructor, it is called before any initializations of the constructors of the class in which the initialization block is defined. The constructor of the class is the first thing involved when you create an object (instance of the class) and then when you enter the constructor the first part called is either implicit (default) super constructor or explicit super constructor or explicit call to another overloaded constructor (but at some point if there is a chain of overloaded constructors, the last one calls a super constructor, implicitly or explicitly).
There is polymorphic creation of an object, but before to enter the class B and its main method, the JVM initializes all class(static) variables, then goes through the static initialization blocks if any exist and then enters the class B and starts with the execution of the main method. It goes to the constructor of class B then immediately (implicitly) calls constructor of class A, using polymorphism the method(overridden method) called in the body of the constructor of class A is the one which is defined in class B and in this case the variable named instanceVariable is used before reinitialization. After closing the constructor of class B the thread is returned to constructor of class B but it goes first to the non-static initialization block before printing "Constructor". For better understanding debug it with some IDE, I prefer Eclipse.

Initializer block contains the code that is always executed whenever
an instance is created. It is used to declare/initialise the common
part of various constructors of a class.
The order of initialization constructors and initializer block doesn’t matter, initializer block is always executed before constructor.
What if we want to execute some code once for all objects of a class?
We use Static Block in Java.

In addition to what was said in previous answers, blocks can be synchronized .. never felt I need to use it, however,it's there

Initialization blocks are executed whenever the class is initialized and before constructors are invoked. They are typically placed above the constructors within braces. It is not at all necessary to include them in your classes.
They are typically used to initialize reference variables. This page gives a good explanation

The question is not entirely clear, but here's a brief description of ways you can initialise data in an object. Let's suppose you have a class A that holds a list of objects.
1) Put initial values in the field declaration:
class A {
private List<Object> data = new ArrayList<Object>();
}
2) Assign initial values in the constructor:
class A {
private List<Object> data;
public A() {
data = new ArrayList<Object>();
}
}
These both assume that you do not want to pass "data" as a constructor argument.
Things get a little tricky if you mix overloaded constructors with internal data like above. Consider:
class B {
private List<Object> data;
private String name;
private String userFriendlyName;
public B() {
data = new ArrayList<Object>();
name = "Default name";
userFriendlyName = "Default user friendly name";
}
public B(String name) {
data = new ArrayList<Object>();
this.name = name;
userFriendlyName = name;
}
public B(String name, String userFriendlyName) {
data = new ArrayList<Object>();
this.name = name;
this.userFriendlyName = userFriendlyName;
}
}
Notice that there is a lot of repeated code. You can fix this by making constructors call each other, or you can have a private initialisation method that each constructor calls:
class B {
private List<Object> data;
private String name;
private String userFriendlyName;
public B() {
this("Default name", "Default user friendly name");
}
public B(String name) {
this(name, name);
}
public B(String name, String userFriendlyName) {
data = new ArrayList<Object>();
this.name = name;
this.userFriendlyName = userFriendlyName;
}
}
or
class B {
private List<Object> data;
private String name;
private String userFriendlyName;
public B() {
init("Default name", "Default user friendly name");
}
public B(String name) {
init(name, name);
}
public B(String name, String userFriendlyName) {
init(name, userFriendlyName);
}
private void init(String _name, String _userFriendlyName) {
data = new ArrayList<Object>();
this.name = name;
this.userFriendlyName = userFriendlyName;
}
}
The two are (more or less) equivalent.
I hope that gives you some hints on how to initialise data in your objects. I won't talk about static initialisation blocks as that's probably a bit advanced at the moment.
EDIT: I've interpreted your question as "how do I initialise my instance variables", not "how do initialiser blocks work" as initialiser blocks are a relatively advanced concept, and from the tone of the question it seems you're asking about the simpler concept. I could be wrong.

public class StaticInitializationBlock {
static int staticVariable;
int instanceVariable;
// Static Initialization Block
static {
System.out.println("Static block");
staticVariable = 5;
}
// Instance Initialization Block
{
instanceVariable = 7;
System.out.println("Instance Block");
System.out.println(staticVariable);
System.out.println(instanceVariable);
staticVariable = 10;
}
public StaticInitializationBlock() {
System.out.println("Constructor");
}
public static void main(String[] args) {
new StaticInitializationBlock();
new StaticInitializationBlock();
}
}
Output:
Static block
Instance Block
5
7
Constructor
Instance Block
10
7
Constructor

Just to add to the excellent answers from #aioobe and #Biman Tripathy.
A static initializer is the equivalent of a constructor in the static context. which is needed to setup the static environment.
A instance initializer is best for anonymous inner classes.
It is also possible to have multiple initializer blocks in class
When we have multiple initializer blocks they are executed (actually copied to constructors by JVM) in the order they appear
Order of initializer blocks matters, but order of initializer blocks mixed with Constructors doesn't
Abstract classes can also have both static and instance initializer blocks.
Code Demo -
abstract class Aircraft {
protected Integer seatCapacity;
{ // Initial block 1, Before Constructor
System.out.println("Executing: Initial Block 1");
}
Aircraft() {
System.out.println("Executing: Aircraft constructor");
}
{ // Initial block 2, After Constructor
System.out.println("Executing: Initial Block 2");
}
}
class SupersonicAircraft extends Aircraft {
{ // Initial block 3, Internalizing a instance variable
seatCapacity = 300;
System.out.println("Executing: Initial Block 3");
}
{ // Initial block 4
System.out.println("Executing: Initial Block 4");
}
SupersonicAircraft() {
System.out.println("Executing: SupersonicAircraft constructor");
}
}
An instance creation of SupersonicAircraft will produce logs in below order
Executing: Initial Block 1
Executing: Initial Block 2
Executing: Aircraft constructor
Executing: Initial Block 3
Executing: Initial Block 4
Executing: SupersonicAircraft constructor
Seat Capacity - 300

overriding protected method of Superclass

In the below example why does the String b prints null and String c prints "gg".
Correct me if I am wrong, whenever a subclass (BClass) overrides a protected method (i.e initClass()) of the superclass (AClass).If you instantiate the subclass. The superclass must make use of overriden method specified by the subclass.
public class Example {
public class AClass {
private String a;
public AClass() {
initClass();
}
protected void initClass() {
a = "randomtext";
}
}
public class BClass extends AClass {
private String b = null;
private String c;
#Override
protected void initClass() {
b = "omg!";
c = "gg";
}
public void bValue() {
System.out.println(b); // prints null
System.out.println(c); // prints "gg"
}
}
public static void main(String[] args) {
Example.BClass b = new Example().new BClass();
b.bValue();
}
}

As of the JSF 12.5
In the example you can see the execution order. The first steps are the callings of the Constructor down to the Object constructor.
Afterwards this happens:
Next, all initializers for the instance variables of class [...] are executed.
Since your instance variable b is initialized to null it will be null again afterwards

This is happening because the superclass constructor is called before the fields of ClassB is initialized. Hence the initClass() method is called which sets b = "omg!" but then again when the super class constructor returns, b is initialized to the value declared in ClassB which is null.
To debug, put a break point and go step by step, you will find that b is first set to null and then changes to omg! and then comes back to null.

There have been already given several correct answers about what's happening. I just wanted to add that it is generally bad practice to call overridden methods from constructor (except of course if you know exactly what you are doing). As you can see, the subclass may not be completely initialised at the time its instance method is invoked (subclass constructor logic has not been executed yet, so effectively overridden method is invoked on an unconstructed object which is dangerous) which might lead to confusions like the one described in this question.
It is much better to write initialisation logic in the constructor and if it is too long then divide it between several private methods invoked from the constructor.

This is happening like this because, first constructor of AClass, which set value of b = omg! and c=gg. After that When BClass gets load in memory it set b=null and c remain as it is which is gg, this is happening because, because in BClass, for b you are doing declaration as well as initialization and for c you are doing only declaration, so as c is already in the memory it even won't get it's default value and as you are not doing any initialization for c, it remain with it's earlier state.

I believe that this example explains the issue:
public class Main {
private static class PrintOnCreate {
public PrintOnCreate(String message) {
System.out.println(message);
}
}
private static class BaseClass {
private PrintOnCreate member =
new PrintOnCreate("BaseClass: member initialization");
static {
System.out.println("BaseClass: static initialization");
}
public BaseClass() {
System.out.println("BaseClass: constructor");
memberCalledFromConstructor();
}
public void memberCalledFromConstructor() {
System.out.println("BaseClass: member called from constructor");
}
}
private static class DerivedClass extends BaseClass {
private PrintOnCreate member =
new PrintOnCreate("DerivedClass: member initialization");
static {
System.out.println("DerivedClass: static initialization");
}
public DerivedClass() {
System.out.println("DerivedClass: constructor");
}
#Override
public void memberCalledFromConstructor() {
System.out.println("DerivedClass: member called from constructor");
}
}
public static void main (String[] args) {
BaseClass obj = new DerivedClass();
}
}
The output from this program is:
BaseClass: static initialization
DerivedClass: static initialization
BaseClass: member initialization
BaseClass: constructor
DerivedClass: member called from constructor
DerivedClass: member initialization
DerivedClass: constructor
... which demonstrates that the derived class's members are initialized after the base class's constructor (and the invocation of the derived class's member function have completed). This also demonstrates a key danger of invoking an overridable function from a constructor, namely that the function can be invoked before the members of the class on which it depends have been initialized. For this reason, constructors should generally avoid invoking member functions (and, when they do, those functions should either be final or static, so that they either depend only on the current class which has been initialized or on none of the instance variables).

What is an initialization block?

We can put code in a constructor or a method or an initialization block. What is the use of initialization block? Is it necessary that every java program must have it?

First of all, there are two types of initialization blocks:
instance initialization blocks, and
static initialization blocks.
This code should illustrate the use of them and in which order they are executed:
public class Test {
static int staticVariable;
int nonStaticVariable;
// Static initialization block:
// Runs once (when the class is initialized)
static {
System.out.println("Static initalization.");
staticVariable = 5;
}
// Instance initialization block:
// Runs each time you instantiate an object
{
System.out.println("Instance initialization.");
nonStaticVariable = 7;
}
public Test() {
System.out.println("Constructor.");
}
public static void main(String[] args) {
new Test();
new Test();
}
}
Prints:
Static initalization.
Instance initialization.
Constructor.
Instance initialization.
Constructor.
Instance initialization blocks are useful if you want to have some code run regardless of which constructor is used or if you want to do some instance initialization for anonymous classes.

would like to add to #aioobe's answer
Order of execution:
static initialization blocks of super classes
static initialization blocks of the class
instance initialization blocks of super classes
constructors of super classes
instance initialization blocks of the class
constructor of the class.
A couple of additional points to keep in mind (point 1 is reiteration of #aioobe's answer):
The code in static initialization block will be executed at class load time (and yes, that means only once per class load), before any instances of the class are constructed and before any static methods are called.
The instance initialization block is actually copied by the Java compiler into every constructor the class has. So every time the code in instance initialization block is executed exactly before the code in constructor.

nice answer by aioobe
adding few more points
public class StaticTest extends parent {
static {
System.out.println("inside satic block");
}
StaticTest() {
System.out.println("inside constructor of child");
}
{
System.out.println("inside initialization block");
}
public static void main(String[] args) {
new StaticTest();
new StaticTest();
System.out.println("inside main");
}
}
class parent {
static {
System.out.println("inside parent Static block");
}
{
System.out.println("inside parent initialisation block");
}
parent() {
System.out.println("inside parent constructor");
}
}
this gives
inside parent Static block
inside satic block
inside parent initialisation block
inside parent constructor
inside initialization block
inside constructor of child
inside parent initialisation block
inside parent constructor
inside initialization block
inside constructor of child
inside main
its like stating the obvious but seems a little more clear.

The sample code, which is approved as an answer here is correct, but I disagree with it. It does not shows what is happening and I'm going to show you a good example to understand how actually the JVM works:
package test;
class A {
A() {
print();
}
void print() {
System.out.println("A");
}
}
class B extends A {
static int staticVariable2 = 123456;
static int staticVariable;
static
{
System.out.println(staticVariable2);
System.out.println("Static Initialization block");
staticVariable = Math.round(3.5f);
}
int instanceVariable;
{
System.out.println("Initialization block");
instanceVariable = Math.round(3.5f);
staticVariable = Math.round(3.5f);
}
B() {
System.out.println("Constructor");
}
public static void main(String[] args) {
A a = new B();
a.print();
System.out.println("main");
}
void print() {
System.out.println(instanceVariable);
}
static void somethingElse() {
System.out.println("Static method");
}
}
Before to start commenting on the source code, I'll give you a short explanation of static variables of a class:
First thing is that they are called class variables, they belong to the class not to particular instance of the class. All instances of the class share this static(class) variable. Each and every variable has a default value, depending on primitive or reference type. Another thing is when you reassign the static variable in some of the members of the class (initialization blocks, constructors, methods, properties) and doing so you are changing the value of the static variable not for particular instance, you are changing it for all instances. To conclude static part I will say that the static variables of a class are created not when you instantiate for first time the class, they are created when you define your class, they exist in JVM without the need of any instances. Therefor the correct access of static members from external class (class in which they are not defined) is by using the class name following by dot and then the static member, which you want to access (template: <CLASS_NAME>.<STATIC_VARIABLE_NAME>).
Now let's look at the code above:
The entry point is the main method - there are just three lines of code. I want to refer to the example which is currently approved. According to it the first thing which must be printed after printing "Static Initialization block" is "Initialization block" and here is my disagreement, the non-static initialization block is not called before the constructor, it is called before any initializations of the constructors of the class in which the initialization block is defined. The constructor of the class is the first thing involved when you create an object (instance of the class) and then when you enter the constructor the first part called is either implicit (default) super constructor or explicit super constructor or explicit call to another overloaded constructor (but at some point if there is a chain of overloaded constructors, the last one calls a super constructor, implicitly or explicitly).
There is polymorphic creation of an object, but before to enter the class B and its main method, the JVM initializes all class(static) variables, then goes through the static initialization blocks if any exist and then enters the class B and starts with the execution of the main method. It goes to the constructor of class B then immediately (implicitly) calls constructor of class A, using polymorphism the method(overridden method) called in the body of the constructor of class A is the one which is defined in class B and in this case the variable named instanceVariable is used before reinitialization. After closing the constructor of class B the thread is returned to constructor of class B but it goes first to the non-static initialization block before printing "Constructor". For better understanding debug it with some IDE, I prefer Eclipse.

Initializer block contains the code that is always executed whenever
an instance is created. It is used to declare/initialise the common
part of various constructors of a class.
The order of initialization constructors and initializer block doesn’t matter, initializer block is always executed before constructor.
What if we want to execute some code once for all objects of a class?
We use Static Block in Java.

In addition to what was said in previous answers, blocks can be synchronized .. never felt I need to use it, however,it's there

Initialization blocks are executed whenever the class is initialized and before constructors are invoked. They are typically placed above the constructors within braces. It is not at all necessary to include them in your classes.
They are typically used to initialize reference variables. This page gives a good explanation

The question is not entirely clear, but here's a brief description of ways you can initialise data in an object. Let's suppose you have a class A that holds a list of objects.
1) Put initial values in the field declaration:
class A {
private List<Object> data = new ArrayList<Object>();
}
2) Assign initial values in the constructor:
class A {
private List<Object> data;
public A() {
data = new ArrayList<Object>();
}
}
These both assume that you do not want to pass "data" as a constructor argument.
Things get a little tricky if you mix overloaded constructors with internal data like above. Consider:
class B {
private List<Object> data;
private String name;
private String userFriendlyName;
public B() {
data = new ArrayList<Object>();
name = "Default name";
userFriendlyName = "Default user friendly name";
}
public B(String name) {
data = new ArrayList<Object>();
this.name = name;
userFriendlyName = name;
}
public B(String name, String userFriendlyName) {
data = new ArrayList<Object>();
this.name = name;
this.userFriendlyName = userFriendlyName;
}
}
Notice that there is a lot of repeated code. You can fix this by making constructors call each other, or you can have a private initialisation method that each constructor calls:
class B {
private List<Object> data;
private String name;
private String userFriendlyName;
public B() {
this("Default name", "Default user friendly name");
}
public B(String name) {
this(name, name);
}
public B(String name, String userFriendlyName) {
data = new ArrayList<Object>();
this.name = name;
this.userFriendlyName = userFriendlyName;
}
}
or
class B {
private List<Object> data;
private String name;
private String userFriendlyName;
public B() {
init("Default name", "Default user friendly name");
}
public B(String name) {
init(name, name);
}
public B(String name, String userFriendlyName) {
init(name, userFriendlyName);
}
private void init(String _name, String _userFriendlyName) {
data = new ArrayList<Object>();
this.name = name;
this.userFriendlyName = userFriendlyName;
}
}
The two are (more or less) equivalent.
I hope that gives you some hints on how to initialise data in your objects. I won't talk about static initialisation blocks as that's probably a bit advanced at the moment.
EDIT: I've interpreted your question as "how do I initialise my instance variables", not "how do initialiser blocks work" as initialiser blocks are a relatively advanced concept, and from the tone of the question it seems you're asking about the simpler concept. I could be wrong.

public class StaticInitializationBlock {
static int staticVariable;
int instanceVariable;
// Static Initialization Block
static {
System.out.println("Static block");
staticVariable = 5;
}
// Instance Initialization Block
{
instanceVariable = 7;
System.out.println("Instance Block");
System.out.println(staticVariable);
System.out.println(instanceVariable);
staticVariable = 10;
}
public StaticInitializationBlock() {
System.out.println("Constructor");
}
public static void main(String[] args) {
new StaticInitializationBlock();
new StaticInitializationBlock();
}
}
Output:
Static block
Instance Block
5
7
Constructor
Instance Block
10
7
Constructor

Just to add to the excellent answers from #aioobe and #Biman Tripathy.
A static initializer is the equivalent of a constructor in the static context. which is needed to setup the static environment.
A instance initializer is best for anonymous inner classes.
It is also possible to have multiple initializer blocks in class
When we have multiple initializer blocks they are executed (actually copied to constructors by JVM) in the order they appear
Order of initializer blocks matters, but order of initializer blocks mixed with Constructors doesn't
Abstract classes can also have both static and instance initializer blocks.
Code Demo -
abstract class Aircraft {
protected Integer seatCapacity;
{ // Initial block 1, Before Constructor
System.out.println("Executing: Initial Block 1");
}
Aircraft() {
System.out.println("Executing: Aircraft constructor");
}
{ // Initial block 2, After Constructor
System.out.println("Executing: Initial Block 2");
}
}
class SupersonicAircraft extends Aircraft {
{ // Initial block 3, Internalizing a instance variable
seatCapacity = 300;
System.out.println("Executing: Initial Block 3");
}
{ // Initial block 4
System.out.println("Executing: Initial Block 4");
}
SupersonicAircraft() {
System.out.println("Executing: SupersonicAircraft constructor");
}
}
An instance creation of SupersonicAircraft will produce logs in below order
Executing: Initial Block 1
Executing: Initial Block 2
Executing: Aircraft constructor
Executing: Initial Block 3
Executing: Initial Block 4
Executing: SupersonicAircraft constructor
Seat Capacity - 300