Chapter 1. Declarations, Initialization and Scoping

Develop code that declares classes (including abstract and all forms of nested classes), interfaces, and enums, and includes the appropriate use of package and import statements (including static imports).

Class declarations define new reference types and describe how they are implemented.

A nested class is any class whose declaration occurs within the body of another class or interface. A top level class is a class that is not a nested class.

A named class may be declared abstract and must be declared abstract if it is incompletely implemented; such a class cannot be instantiated, but can be extended by subclasses. A class may be declared final, in which case it cannot have subclasses. If a class is declared public, then it can be referred to from other packages.

Each class except Object is an extension of (that is, a subclass of) a single existing class and may implement interfaces.

The body of a class declares members (fields and methods and nested classes and interfaces), instance and static initializers, and constructors. The scope of a member is the entire declaration of the class to which the member belongs. Field, method, member class, member interface, and constructor declarations may include the access modifiers public, protected, or private. The members of a class include both declared and inherited members. Newly declared fields can hide fields declared in a superclass or superinterface. Newly declared class members and interface members can hide class or interface members declared in a superclass or superinterface. Newly declared methods can hide, implement, or override methods declared in a superclass or superinterface.

Field declarations describe class variables, which are incarnated once, and instance variables, which are freshly incarnated for each instance of the class. A field may be declared final, in which case it can be assigned to only once. Any field declaration may include an initializer.

Member class declarations describe nested classes that are members of the surrounding class. Member classes may be static, in which case they have no access to the instance variables of the surrounding class; or they may be inner classes.

Member interface declarations describe nested interfaces that are members of the surrounding class.

Method declarations describe code that may be invoked by method invocation expressions. A class method is invoked relative to the class type; an instance method is invoked with respect to some particular object that is an instance of the class type. A method whose declaration does not indicate how it is implemented MUST be declared abstract. A method may be declared final, in which case it cannot be hidden or overridden. A method may be implemented by platform-dependent native code. A synchronized method automatically locks an object before executing its body and automatically unlocks the object on return, as if by use of a synchronized statement, thus allowing its activities to be synchronized with those of other threads.

Method names may be overloaded.

Instance initializers are blocks of executable code that may be used to help initialize an instance when it is created.

Static initializers are blocks of executable code that may be used to help initialize a class when it is first loaded.

Constructors are similar to methods, but cannot be invoked directly by a method call; they are used to initialize new class instances. Like methods, they may be overloaded.

Class declaration

A class declaration specifies a new named reference type:

	ClassModifiers(optional) class Identifier Super(optional) Interfaces(optional) ClassBody

The "Identifier" in a class declaration specifies the name of the class. A compile-time error occurs if a class has the same simple name as any of its enclosing classes or interfaces.

Class modifiers

A class declaration may include class modifiers (one of):

public protected private
abstract static final strictfp					

Not all modifiers are applicable to all kinds of class declarations. The access modifier public pertains only to top level classes and to member classes. The access modifiers protected and private pertain only to member classes within a directly enclosing class declaration. The access modifier static pertains only to member classes. A compile-time error occurs if the same modifier appears more than once in a class declaration. If two or more class modifiers appear in a class declaration, then it is customary, though not required, that they appear in the order consistent with that shown above.

Abstract classes

An abstract class is a class that is incomplete, or to be considered incomplete. Only abstract classes may have abstract methods, that is, methods that are declared but not yet implemented. If a class that is not abstract contains an abstract method, then a compile-time error occurs. A class C has abstract methods if any of the following is true:

  • C explicitly contains a declaration of an abstract method.

  • Any of C's superclasses declares an abstract method that has not been implemented in C or any of its superclasses.

  • A direct superinterface of C declares or inherits a method (which is therefore necessarily abstract) and C neither declares nor inherits a method that implements it.

In the example:

abstract class Point {
	int x = 1, y = 1;
	void move(int dx, int dy) {
		x += dx;
		y += dy;
	abstract void alert();

abstract class ColoredPoint extends Point {
	int color;

class SimplePoint extends Point {
	void alert() { }
a class Point is declared that must be declared abstract, because it contains a declaration of an abstract method named alert. The subclass of Point named ColoredPoint inherits the abstract method alert, so it must also be declared abstract. On the other hand, the subclass of Point named SimplePoint provides an implementation of alert(), so it need not be abstract.

A compile-time error occurs if an attempt is made to create an instance of an abstract class using a class instance creation expression.

Thus, continuing the example just shown, the statement:

Point p = new Point(); // wrong
would result in a compile-time error; the class Point cannot be instantiated because it is abstract. However, a Point variable could correctly be initialized with a reference to any subclass of Point, and the class SimplePoint is not abstract, so the statement:
Point p = new SimplePoint(); // correct
would be correct.

A subclass of an abstract class that is not itself abstract may be instantiated, resulting in the execution of a constructor for the abstract class and, therefore, the execution of the field initializers for instance variables of that class. Thus, in the example just given, instantiation of a SimplePoint causes the default constructor and field initializers for x and y of Point to be executed. It is a compile-time error to declare an abstract class type such that it is not possible to create a subclass that implements all of its abstract methods. This situation can occur if the class would have as members two abstract methods that have the same method signature but different return types.

As an example, the declarations:

interface Colorable { 
	void setColor(int color); 

abstract class Colored implements Colorable {
	abstract int setColor(int color);
result in a compile-time error: it would be impossible for any subclass of class Colored to provide an implementation of a method named setColor, taking one argument of type int, that can satisfy both abstract method specifications, because the one in interface Colorable requires the same method to return no value, while the one in class Colored requires the same method to return a value of type int.

A class type should be declared abstract only if the intent is that subclasses can be created to complete the implementation. If the intent is simply to prevent instantiation of a class, the proper way to express this is to declare a constructor of no arguments, make it private, never invoke it, and declare no other constructors. A class of this form usually contains class methods and variables. The class Math is an example of a class that cannot be instantiated; its declaration looks like this:

public final class Math {
	private Math() { }		// never instantiate this class
	// ... declarations of class variables and methods ...

Inner Classes and Enclosing Instances

An inner class is a nested class that is not explicitly or implicitly declared static. Inner classes may not declare static initializers or member interfaces. Inner classes may not declare static members, unless they are compile-time constant fields.

To illustrate these rules, consider the example below:

class HasStatic{
	static int j = 100;

class Outer{
	class Inner extends HasStatic{
		static final x = 3;		// ok - compile-time constant
		static int y = 4; 		// compile-time error, an inner class
	static class NestedButNotInner{
		static int z = 5; 		// ok, not an inner class
	interface NeverInner{}		// interfaces are never inner

Inner classes may inherit static members that are not compile-time constants even though they may not declare them. Nested classes that are not inner classes may declare static members freely, in accordance with the usual rules of the Java programming language. Member interfaces are always implicitly static so they are never considered to be inner classes.

A statement or expression occurs in a static context if and only if the innermost method, constructor, instance initializer, static initializer, field initializer, or explicit constructor statement enclosing the statement or expression is a static method, a static initializer, the variable initializer of a static variable, or an explicit constructor invocation statement.

Any local variable, formal method parameter or exception handler parameter used but not declared in an inner class must be declared final, and must be definitely assigned before the body of the inner class.

Inner classes include local, anonymous and non-static member classes. Here are some examples:

class Outer {
	int i = 100;
	static void classMethod() {
		final int l = 200;
		class LocalInStaticContext{
			int k = i; // compile-time error
			int m = l; // ok
	void foo() {
		class Local { // a local class
			int j = i;

The declaration of class LocalInStaticContext occurs in a static context-within the static method classMethod. Instance variables of class Outer are not available within the body of a static method. In particular, instance variables of Outer are not available inside the body of LocalInStaticContext. However, local variables from the surrounding method may be referred to without error (provided they are marked final).

Inner classes whose declarations do not occur in a static context may freely refer to the instance variables of their enclosing class. An instance variable is always defined with respect to an instance. In the case of instance variables of an enclosing class, the instance variable must be defined with respect to an enclosing instance of that class. So, for example, the class Local above has an enclosing instance of class Outer. As a further example:

class WithDeepNesting {
	boolean toBe;
	WithDeepNesting(boolean b) { toBe = b;}
	class Nested {
		boolean theQuestion;
		class DeeplyNested {
				theQuestion = toBe || !toBe;
Here, every instance of WithDeepNesting.Nested.DeeplyNested has an enclosing instance of class WithDeepNesting.Nested (its immediately enclosing instance) and an enclosing instance of class WithDeepNesting (its 2nd lexically enclosing instance).

Typesafe enumerations

The J2SE 5.0 enum declaration looks as follows:

public enum MainMenu {FILE, EDIT, FORMAT, VIEW};

enum MachineState { BUSY, IDLE, BLOCKED } // Canonical form

enum Day {

This approach has many advantages including:

  1. It provides strong compile-time type safety

  2. It provides a separate namespace for each enum type and thus eliminates the need to include a prefix in each constant name

  3. Constants are not compiled into clients so you can freely add, remove, or reorder them without recompiling the clients

  4. Printed values are informative instead of just numbers

  5. Enum constants can be used wherever objects can be used

There are a couple of important things to note about enum declarations. As an example, consider the following declaration:

public enum MainMenu {FILE, EDIT, FORMAT, VIEW};  

  1. Keyword enum is used to declare an enum type.

  2. The word enum is reserved and therefore if you have been using it as an identifier, you should adjust your code when compiling with a J2SE 5.0 compiler.

  3. The above enum declaration generates a class (MainMenu in the above example), which automatically implements the Comparable<MainMenu> and Serializable interfaces, and provides several members including:

    • Static variables FILE, EDIT, FORMAT, and VIEW.

    • Static method values(), which is an array containing the constants in the enum.

    • Static method valueOf(String) that returns the appropriate enum for the string passed in.

    • Appropriately overloaded equals(), hasCode(), toString(), and compareTo() methods.

Here is a complete example that declares an enumeration and then prints the values:

public class Example {
	public enum MainMenu {FILE, EDIT, FORMAT, VIEW}

	public static void main(String[] argv) {
		for (MainMenu menu : MainMenu.values()) {
And the following segment of code shows another example using the switch statement:
for(MainMenu menu : MainMenu.values()) {
	switch(menu) {
		case FILE:
			System.out.println("FILE Menu");
		case EDIT:
			System.out.println("EDIT Menu");
		case FORMAT:
			System.out.println("FORMAT Menu");
		case VIEW:
			System.out.println("VIEW Menu");
It is worth noting that two classes have been added to java.util in support of enums: EnumSet (a high-performance Set implementation for enums; all members of an enum set must be of the same enum type) and EnumMap (a high-performance Map implementation for use with enum keys).

Properties of the enum type

An enum declaration is a special kind of class declaration:

  • It can be declared at the top-level and as static enum declaration.

  • It is implicitly static, i.e. no outer object is associated with an enum constant.

  • It is implicitly final unless it contains constant-specific class bodies, but it can implement interfaces.

  • It cannot be declared abstract unless each abstract method is overridden in the constant-specific class body of every enum constant.

// (1) Top level enum declaration
public enum SimpleMeal { 
public class EnumTypeDeclarations {
	// (2) Static enum declaration is OK.
	public enum SimpleMeal { 

	public void foo() {
		// (3) Local (inner) enum declaration is NOT OK!
		enum SimpleMeal { 

Enum constructors

Each constant declaration can be followed by an argument list that is passed to the constructor of the enum type having the matching parameter signature.

An implicit standard constructor is created if no constructors are provided for the enum type.

As an enum cannot be instantiated using the new operator, the constructors cannot be called explicitly.

Example of enum constructors:

public enum Meal {
	BREAKFAST(7, 30), LUNCH(12, 15), DINNER(19, 45);
	private int hh;

	private int mm;

	Meal(int hh, int mm) {
		assert (hh >= 0 && hh <= 23) : "Illegal hour.";
		assert (mm >= 0 && mm <= 59) : "Illegal mins.";
		this.hh = hh; = mm;

	public int getHour() {
		return hh;

	public int getMins() {
		return mm;


Methods provided for the enum types

Names of members declared in an enum type cannot conflict with automatically generated member names:

  • The enum constant names cannot be redeclared.

  • The following methods cannot be redeclared:

    // Returns an array containing the constants of this enum class, in the order 
    // they are declared.
    static <this enum class>[] values()
    // Return the enum constant with the specified name
    static <this enum class> valueOf(String name)

Enum types are based on the java.lang.Enum class which provides the default behavior.

Enums cannot declare methods which override the final methods of the java.lang.Enum class:

  • clone(), compareTo(Object), equals(Object), getDeclaringClass(), hashCode(), name(), ordinal().

  • The final methods do what their names imply, but the clone() method throws an CloneNotSupportedException, as an enum constant cannot be cloned.

Note that the enum constants must be declared before any other declarations in an enum type.

public enum Meal {
	int q = 1; // WRONG ! Compilation error !
	BREAKFAST(7, 30), LUNCH(12, 15), DINNER(19, 45);
	private int hh;

Example of using enum type:

public class MealClient {
	public static void main(String[] args) {
		for (Meal meal : Meal.values())
			System.out.println(meal + " served at " + meal.getHour() + ":"
					+ meal.getMins() + ", has the ordinal value "
					+ meal.ordinal());
The output will be:
BREAKFAST served at 7:30, has the ordinal value 0
LUNCH served at 12:15, has the ordinal value 1
DINNER served at 19:45, has the ordinal value 2					

Extending enum types: constant-specific class bodies

Constant-specific class bodies define anonymous classes inside an enum type that extend the enclosing enum type.

Instance methods declared in these class bodies are accessible outside the enclosing enum type only if they override accessible methods in the enclosing enum type.

An enum type that contains constant-specific class bodies cannot be declared final:

public enum Meal {
	// Each enum constant defines a constant-specific class body
	BREAKFAST(7, 30) {
		public double mealPrice() {
			double breakfastPrice = 10.50;
			return breakfastPrice;
	// WRONG! Compilation error! Cannot override the final method from Meal
	final double mealPrice() { return 0; }

Correct example:

public enum Meal {
	// Each enum constant defines a constant-specific class body
	BREAKFAST(7, 30) {
		public double mealPrice() {
			double breakfastPrice = 10.50;
			return breakfastPrice;

	// Abstract method which the constant-specific class body
	abstract double mealPrice();


Enums can be used in a special purpose Set implementation (EnumSet) which provides better performance.

All of the elements in an enum set MUST come from a single enum type.

Enum sets are represented internally as bit vectors.

The EnumSet class defines new methods and inherits the ones in the Set/Collection interfaces.

NOTE, all methods are static except for the clone() method.

All methods return an EnumSet<E>.

The null reference CANNOT be stored in an enum set.

Method summary for the abstract class EnumSet<E extends Enum<E>>:

// Creates an enum set containing all of the elements in the specified element type.
allOf(Class<E> elementType)

// Returns a copy of this set.

// Creates an enum set with the same element type as the specified enum set, initially
// containing all the elements of this type that are not contained in the specified set.
complementOf(EnumSet<E> s)

// Creates an enum set initialized from the specified collection.
copyOf(Collection<E> c)

// Creates an enum set with the same element type as the specified enum set, initially
// containing the same elements (if any).
copyOf(EnumSet<E> s)

// Creates an empty enum set with the specified element type.
noneOf(Class<E> elementType)

// These factory methods creates an enum set initially containing the specified
// element(s). Note that enums cannot be used in varargs as it is not legal to use varargs
// with parameterized types.
of(E e)
of(E e1, E e2)
of(E e1, E e2, E e3)
of(E e1, E e2, E e3, E e4)
of(E e1, E e2, E e3, E e4, E e5)

// Creates an enum set initially containing all of the elements in the range defined by
// the two specified endpoints.
range(E from, E to)

Example of using java.util.EnumSet:

import java.util.EnumSet;
import static java.lang.System.out;

public class UsingEnumSet {
	enum Day {

	public static void main(String[] args) {

		EnumSet<Day> allDays = EnumSet.range(Day.MONDAY, Day.SUNDAY);
		out.println("All days: " + allDays);

		EnumSet<Day> weekend = EnumSet.range(Day.SATURDAY, Day.SUNDAY);
		out.println("Weekend: " + weekend);

		EnumSet<Day> oddDays = EnumSet.of(Day.MONDAY, Day.WEDNESDAY,
				Day.FRIDAY, Day.SUNDAY);
		out.println("Odd days: " + oddDays);

		EnumSet<Day> evenDays = EnumSet.complementOf(oddDays);
		out.println("Even days: " + evenDays);

		EnumSet<Day> weekDays = EnumSet.complementOf(weekend);
		out.println("Week days: " + weekDays);


Enums can be used in a special purpose Map implementation (EnumMap) which provides better performance.

Enum maps are represented internally as arrays.

All of the keys in an enum map MUST come from a single enum type.

Enum maps are maintained in the natural order of their keys (i.e. the order of the enum constant declarations).

The EnumMap class re-implements most of the methods in the Map interface.

The null reference as a key is NOT permitted.

Constructor summary of the class EnumMap<K extends Enum<K>,V>:

// Creates an empty enum map with the specified key type.
EnumMap(Class<K> keyType)

// Creates an enum map with the same key type as the specified enum map, initially
// containing the same mappings (if any).
EnumMap(EnumMap<K,? extends V> m)

// Creates an enum map initialized from the specified map.
EnumMap(Map<K,? extends V> m)


Method summary of the class EnumMap<K extends Enum<K>,V>:

// Removes all mappings from this map.
void clear()

// Returns a shallow copy of this enum map.
EnumMap<K,V> clone()

// Returns true if this map contains a mapping for the specified key.
boolean containsKey(Object key)

// Returns true if this map maps one or more keys to the specified
// value.
boolean containsValue(Object value)

// Returns a Set view of the mappings contained in this map.
Set<Map.Entry<K,V>> entrySet()

// Compares the specified object with this map for equality.
boolean equals(Object o)

// Returns the value to which this map maps the specified key, or null
// if this map contains no mapping for the specified key.
V get(Object key)

// Returns a Set view of the keys contained in this map.
Set<K> keySet()

// Associates the specified value with the specified key in this map.
V put(K key, V value)

// Copies all of the mappings from the specified map to this map.
void putAll(Map<? extends K,? extends V> m)

// Removes the mapping for this key from this map if present.
V remove(Object key)

// Returns the number of key-value mappings in this map.
int size()

// Returns a Collection view of the values contained in this map.
Collection<V> values()

Example of using EnumMap:

import java.util.Collection;
import java.util.EnumMap;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import static java.lang.System.out;

public class UsingEnumMap {

	enum Day {

	public static void main(String[] args) {

		int[] freqArray = { 12, 34, 56, 23, 5, 13, 78 };

		// Create a Map of frequencies
		Map<Day, Integer> ordinaryMap = new HashMap<Day, Integer>();
		for (Day day : Day.values()) {
			ordinaryMap.put(day, freqArray[day.ordinal()]);
		out.println("Frequency Map: " + ordinaryMap);

		// Create an EnumMap of frequencies
		EnumMap<Day, Integer> frequencyEnumMap = new EnumMap<Day, Integer>(

		// Change some frequencies
		frequencyEnumMap.put(Day.MONDAY, 100);
		frequencyEnumMap.put(Day.FRIDAY, 123);
		out.println("Frequency EnumMap: " + frequencyEnumMap);

		// Values
		Collection<Integer> frequencies = frequencyEnumMap.values();
		out.println("Frequencies: " + frequencies);

		// Keys
		Set<Day> days = frequencyEnumMap.keySet();
		out.println("Days: " + days);
Frequency EnumMap: {MONDAY=100, TUESDAY=34, WEDNESDAY=56, THURSDAY=23, FRIDAY=123, SATURDAY=13, SUNDAY=78}
Frequencies: [100, 34, 56, 23, 123, 13, 78]

Creating and using packages

To make classes easier to find and to use, to avoid naming conflicts, and to control access, programmers bundle groups of related classes and interfaces into packages. A package is a collection of related classes and interfaces providing access protection and namespace management.

The classes and interfaces that are part of the Java platform are members of various packages that bundle classes by function: fundamental classes are in java.lang, classes for reading and writing (input and output) are in, and so on. You can put your classes and interfaces in packages, too.

To create a package, you put a class or an interface in it. To do this, you put a package statement at the top of the source file in which the class or the interface is defined. For example, the following code appears in the source file and puts the Circle class in the graphics package:

package graphics;

public class Circle extends Graphic implements Draggable {
The Circle class is a public member of the graphics package. You must include a package statement at the top of every source file that defines a class or an interface that is to be a member of the graphics package. So you would also include the statement in and so on:
package graphics;

public class Rectangle extends Graphic implements Draggable {
The scope of the package statement is the entire source file, so all classes and interfaces defined in and are also members of the graphics package. If you put multiple classes in a single source file, only one may be public, and it must share the name of the source files base name. Only public package members are accessible from outside the package. If you do not use a package statement, your class or interface ends up in the default package, which is a package that has no name. Generally speaking, the default package is only for small or temporary applications or when you are just beginning development. Otherwise, classes and interfaces belong in named packages.

Only public package members are accessible outside the package in which they are defined. To use a public package member from outside its package, you must do one or more of the following:

  • Refer to the member by its long (qualified) name.

    You can use a package members simple name if the code you are writing is in the same package as that member or if the members package has been imported. However, if you are trying to use a member from a different package and that package has not been imported, you must use the members qualified name, which includes the package name. This is the qualified name for the Rectangle class declared in the graphics package:

    You could use this long name to create an instance of graphics.Rectangle:
    graphics.Rectangle myRect = new graphics.Rectangle();
    Using long names is okay for one-shot uses. You'd likely get annoyed if you had to write graphics.Rectangle again and again. Also, your code would get very messy and difficult to read. In such cases, you can just import the member instead.

  • Import the package member

    To import a specific member into the current file, put an import statement at the beginning of your file before any class or interface definitions but after the package statement, if there is one. Here's how you would import the Circle class from the graphics package:

    import graphics.Circle;
    Now you can refer to the Circle class by its simple name:
    Circle myCircle = new Circle();
    This approach works well if you use just a few members from the graphics package. But if you use many classes and interfaces from a package, you can import the entire package.

  • Import the members entire package

    To import all the classes and interfaces contained in a particular package, use the import statement with the asterisk (*) wildcard character:

    import graphics.*;
    Now you can refer to any class or interface in the graphics package by its short name:
    Circle myCircle = new Circle();
    Rectangle myRectangle = new Rectangle();
    The asterisk in the import statement can be used only to specify all the classes within a package, as shown here. It cannot be used to match a subset of the classes in a package. For example, the following does not match all the classes in the graphics package that begin with A:
    import graphics.A*;	// does not work
    Instead, it generates a compiler error. With the import statement, you can import only a single package member or an entire package. For your convenience, the Java runtime system automatically imports two entire packages: java.lang and the current package.

Static imports

The static import feature enables you to import static members from a class or an interface and thus use them without a qualifying name. As an example, consider the following interface that contains two constant values:


interface XYZ {
  public static final double Constant1 = someValue;
  public static final double Constant2 = anotherValue;

Now, the constants in the XYZ interface can be used as follows:

public class MyClass implements XYZ {
	double value = 2 * Constant1;

As you can see, a class must implement the interface in order to have access to the constants defined in the interface.

In J2SE 5.0, static import solves this problem as shown in the following example:

import static*;

public class MyClass {
	double value = 2 * Constant1;

As another example, consider the following static imports:

import static java.lang.Math.*;
import static java.lang.System.*;
With these two static imports, you now can use:
double r = cos(PI * theta);
instead of:
double r = Math.cos(PI * theta);
out.println("Hello there"); 
instead of:
System.out.println("Hello there");

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