Nested Classes
Nested Classes
The Java programming language allows you to define a class within another class. Such a class is called a nested class and is illustrated here:
class OuterClass {
...
class NestedClass {
...
}
}
Terminology: Nested classes are divided into two categories: non-static and static. Non-static nested classes are called inner classes. Nested classes that are declared static are called static nested classes.
class OuterClass {
...
class InnerClass {
...
}
static class StaticNestedClass {
...
}
}
A nested class is a member of its enclosing class. Non-static nested classes (inner classes) have access to other members of the enclosing class, even if they are declared private. Static nested classes do not have access to other members of the enclosing class. As a member of the OuterClass, a nested class can be declared private, public, protected, or package private. Recall that outer classes can only be declared public or package private.
Why Use Nested Classes?
Compelling reasons for using nested classes include the following:
- It is a way of logically grouping classes that are only used in one place: If a class is useful to only one other class, then it is logical to embed it in that class and keep the two together. Nesting such "helper classes" makes their package more streamlined.
- It increases encapsulation: Consider two top-level classes,
AandB, whereBneeds access to members ofAthat would otherwise be declared private. By hiding classBwithin classA,A's members can be declaredprivateandBcan access them. In addition,Bitself can be hidden from the outside world. - It can lead to more readable and maintainable code: Nesting small classes within top-level classes places the code closer to where it is used.
Inner Classes
As with instance methods and variables, an inner class is associated with an instance of its enclosing class and has direct access to that object's methods and fields. Also, because an inner class is associated with an instance, it cannot define any static members itself.
Objects that are instances of an inner class exist within an instance of the outer class. Consider the following classes:
class OuterClass {
...
class InnerClass {
...
}
}
An instance of InnerClass can exist only within an instance of OuterClass and has direct access to the methods and fields of its enclosing instance.
To instantiate an inner class, you must first instantiate the outer class. Then, create the inner object within the outer object with this syntax:
OuterClass outerObject = new OuterClass();
OuterClass.InnerClass innerObject = outerObject.new InnerClass();
There are two special kinds of inner classes: local classes and anonymous classes.
Static Nested Classes
As with class methods and variables, a static nested class is associated with its outer class. And like static class methods, a static nested class cannot refer directly to instance variables or methods defined in its enclosing class: it can use them only through an object reference. Inner Class and Nested Static Class Example demonstrates this.
Note: A static nested class interacts with the instance members of its outer class (and other classes) just like any other top-level class. In effect, a static nested class is behaviorally a top-level class that has been nested in another top-level class for packaging convenience. Inner Class and Nested Static Class Example also demonstrates this.
You instantiate a static nested class the same way as a top-level class:
StaticNestedClass staticNestedObject = new StaticNestedClass();
Inner Class and Nested Static Class Example
The following example, OuterClass, along with TopLevelClass, demonstrates which class members of OuterClass an inner class (InnerClass), a nested static class (StaticNestedClass), and a top-level class (TopLevelClass) can access:
OuterClass.java
public class OuterClass {
String outerField = "Outer field";
static String staticOuterField = "Static outer field";
class InnerClass {
void accessMembers() {
System.out.println(outerField);
System.out.println(staticOuterField);
}
}
static class StaticNestedClass {
void accessMembers(OuterClass outer) {
// Compiler error: Cannot make a static reference to the non-static
// field outerField
// System.out.println(outerField);
System.out.println(outer.outerField);
System.out.println(staticOuterField);
}
}
public static void main(String[] args) {
System.out.println("Inner class:");
System.out.println("------------");
OuterClass outerObject = new OuterClass();
OuterClass.InnerClass innerObject = outerObject.new InnerClass();
innerObject.accessMembers();
System.out.println("\nStatic nested class:");
System.out.println("--------------------");
StaticNestedClass staticNestedObject = new StaticNestedClass();
staticNestedObject.accessMembers(outerObject);
System.out.println("\nTop-level class:");
System.out.println("--------------------");
TopLevelClass topLevelObject = new TopLevelClass();
topLevelObject.accessMembers(outerObject);
}
}
TopLevelClass.java
public class TopLevelClass {
void accessMembers(OuterClass outer) {
// Compiler error: Cannot make a static reference to the non-static
// field OuterClass.outerField
// System.out.println(OuterClass.outerField);
System.out.println(outer.outerField);
System.out.println(OuterClass.staticOuterField);
}
}
This example prints the following output:
Inner class:
------------
Outer field
Static outer field
Static nested class:
--------------------
Outer field
Static outer field
Top-level class:
--------------------
Outer field
Static outer field
Note that a static nested class interacts with the instance members of its outer class just like any other top-level class. The static nested class StaticNestedClass cannot directly access outerField because it is an instance variable of the enclosing class, OuterClass. The Java compiler generates an error at the highlighted statement:
static class StaticNestedClass {
void accessMembers(OuterClass outer) {
// Compiler error: Cannot make a static reference to the non-static
// field outerField
System.out.println(outerField);
}
}
To fix this error, access outerField through an object reference:
System.out.println(outer.outerField);
Similarly, the top-level class TopLevelClass cannot directly access outerField either.
Shadowing
If a declaration of a type (such as a member variable or a parameter name) in a particular scope (such as an inner class or a method definition) has the same name as another declaration in the enclosing scope, then the declaration shadows the declaration of the enclosing scope. You cannot refer to a shadowed declaration by its name alone. The following example, ShadowTest, demonstrates this:
public class ShadowTest {
public int x = 0;
class FirstLevel {
public int x = 1;
void methodInFirstLevel(int x) {
System.out.println("x = " + x);
System.out.println("this.x = " + this.x);
System.out.println("ShadowTest.this.x = " + ShadowTest.this.x);
}
}
public static void main(String... args) {
ShadowTest st = new ShadowTest();
ShadowTest.FirstLevel fl = st.new FirstLevel();
fl.methodInFirstLevel(23);
}
}
The following is the output of this example:
x = 23
this.x = 1
ShadowTest.this.x = 0
This example defines three variables named x: the member variable of the class ShadowTest, the member variable of the inner class FirstLevel, and the parameter in the method methodInFirstLevel(). The variable x defined as a parameter of the method methodInFirstLevel() shadows the variable of the inner class FirstLevel. Consequently, when you use the variable x in the method methodInFirstLevel(), it refers to the method parameter. To refer to the member variable of the inner class FirstLevel, use the keyword this to represent the enclosing scope:
System.out.println("this.x = " + this.x);
Refer to member variables that enclose larger scopes by the class name to which they belong. For example, the following statement accesses the member variable of the class ShadowTest from the method methodInFirstLevel():
System.out.println("ShadowTest.this.x = " + ShadowTest.this.x);
Serialization
Serialization of inner classes, including local and anonymous classes, is strongly discouraged. When the Java compiler compiles certain constructs, such as inner classes, it creates synthetic constructs; these are classes, methods, fields, and other constructs that do not have a corresponding construct in the source code. Synthetic constructs enable Java compilers to implement new Java language features without changes to the JVM.
However, synthetic constructs can vary among different Java compiler implementations, which means that .class files can vary among different implementations as well. Consequently, you may have compatibility issues if you serialize an inner class and then deserialize it with a different JRE implementation.
Inner Class Example
To see an inner class in use, first consider an array. In the following example, you create an array, fill it with integer values, and then output only values of even indices of the array in ascending order.
The DataStructure.java example that follows consists of:
- The
DataStructureouter class, which includes a constructor to create an instance ofDataStructurecontaining an array filled with consecutive integer values (0, 1, 2, 3, and so on) and a method that prints elements of the array that have an even index value. - The
EvenIteratorinner class, which implements theDataStructureIteratorinterface, which extends theIterator< Integer>interface. Iterators are used to step through a data structure and typically have methods to test for the last element, retrieve the current element, and move to the next element. - A main method that instantiates a
DataStructureobject (ds), then invokes theprintEven()method to print elements of the arrayarrayOfIntsthat have an even index value.
public class DataStructure {
// Create an array
private final static int SIZE = 15;
private int[] arrayOfInts = new int[SIZE];
public DataStructure() {
// fill the array with ascending integer values
for (int i = 0; i < SIZE; i++) {
arrayOfInts[i] = i;
}
}
public void printEven() {
// Print out values of even indices of the array
DataStructureIterator iterator = this.new EvenIterator();
while (iterator.hasNext()) {
System.out.print(iterator.next() + " ");
}
System.out.println();
}
interface DataStructureIterator extends java.util.Iterator<Integer> { }
// Inner class implements the DataStructureIterator interface,
// which extends the Iterator<Integer> interface
private class EvenIterator implements DataStructureIterator {
// Start stepping through the array from the beginning
private int nextIndex = 0;
public boolean hasNext() {
// Check if the current element is the last in the array
return (nextIndex <= SIZE - 1);
}
public Integer next() {
// Record a value of an even index of the array
Integer retValue = Integer.valueOf(arrayOfInts[nextIndex]);
// Get the next even element
nextIndex += 2;
return retValue;
}
}
public static void main(String s[]) {
// Fill the array with integer values and print out only
// values of even indices
DataStructure ds = new DataStructure();
ds.printEven();
}
}
The output is:
0 2 4 6 8 10 12 14
Note that the EvenIterator class refers directly to the arrayOfInts instance variable of the DataStructure object.
You can use inner classes to implement helper classes such as the one shown in the this example. To handle user interface events, you must know how to use inner classes, because the event-handling mechanism makes extensive use of them.
Local and Anonymous Classes
There are two additional types of inner classes. You can declare an inner class within the body of a method. These classes are known as local classes. You can also declare an inner class within the body of a method without naming the class. These classes are known as anonymous classes.
Modifiers
You can use the same modifiers for inner classes that you use for other members of the outer class. For example, you can use the access specifiers private, public, and protected to restrict access to inner classes, just as you use them to restrict access do to other class members.
Local Classes
Local classes are classes that are defined in a block, which is a group of zero or more statements between balanced braces. You typically find local classes defined in the body of a method.
This section covers the following topics:
- Declaring Local Classes
- Accessing Members of an Enclosing Class
- Shadowing and Local Classes
- Local Classes Are Similar To Inner Classes
Declaring Local Classes
You can define a local class inside any block (see Expressions, Statements, and Blocks for more information). For example, you can define a local class in a method body, a for loop, or an if clause.
The following example, LocalClassExample, validates two phone numbers. It defines the local class PhoneNumber in the method validatePhoneNumber():
public class LocalClassExample {
static String regularExpression = "[^0-9]";
public static void validatePhoneNumber(
String phoneNumber1, String phoneNumber2) {
final int numberLength = 10;
// Valid in JDK 8 and later:
// int numberLength = 10;
class PhoneNumber {
String formattedPhoneNumber = null;
PhoneNumber(String phoneNumber){
// numberLength = 7;
String currentNumber = phoneNumber.replaceAll(
regularExpression, "");
if (currentNumber.length() == numberLength)
formattedPhoneNumber = currentNumber;
else
formattedPhoneNumber = null;
}
public String getNumber() {
return formattedPhoneNumber;
}
// Valid in JDK 8 and later:
// public void printOriginalNumbers() {
// System.out.println("Original numbers are " + phoneNumber1 +
// " and " + phoneNumber2);
// }
}
PhoneNumber myNumber1 = new PhoneNumber(phoneNumber1);
PhoneNumber myNumber2 = new PhoneNumber(phoneNumber2);
// Valid in JDK 8 and later:
// myNumber1.printOriginalNumbers();
if (myNumber1.getNumber() == null)
System.out.println("First number is invalid");
else
System.out.println("First number is " + myNumber1.getNumber());
if (myNumber2.getNumber() == null)
System.out.println("Second number is invalid");
else
System.out.println("Second number is " + myNumber2.getNumber());
}
public static void main(String... args) {
validatePhoneNumber("123-456-7890", "456-7890");
}
}
The example validates a phone number by first removing all characters from the phone number except the digits 0 through 9. After, it checks whether the phone number contains exactly ten digits (the length of a phone number in North America). This example prints the following:
First number is 1234567890
Second number is invalid
Accessing Members of an Enclosing Class
A local class has access to the members of its enclosing class. In the previous example, the PhoneNumber() constructor accesses the member LocalClassExample.regularExpression.
In addition, a local class has access to local variables. However, a local class can only access local variables that are declared final. When a local class accesses a local variable or parameter of the enclosing block, it captures that variable or parameter. For example, the PhoneNumber() constructor can access the local variable numberLength because it is declared final; numberLength is a captured variable.
However, starting in Java SE 8, a local class can access local variables and parameters of the enclosing block that are final or effectively final. A variable or parameter whose value is never changed after it is initialized is effectively final. For example, suppose that the variable numberLength is not declared final, and you add the highlighted assignment statement in the PhoneNumber() constructor to change the length of a valid phone number to 7 digits:
PhoneNumber(String phoneNumber) {
numberLength = 7;
String currentNumber = phoneNumber.replaceAll(
regularExpression, "");
if (currentNumber.length() == numberLength)
formattedPhoneNumber = currentNumber;
else
formattedPhoneNumber = null;
}
Because of this assignment statement, the variable numberLength is not effectively final anymore. As a result, the Java compiler generates an error message similar to "local variables referenced from an inner class must be final or effectively final" where the inner class PhoneNumber tries to access the numberLength variable:
if (currentNumber.length() == numberLength)
Starting in Java SE 8, if you declare the local class in a method, it can access the method's parameters. For example, you can define the following method in the PhoneNumber local class:
public void printOriginalNumbers() {
System.out.println("Original numbers are " + phoneNumber1 +
" and " + phoneNumber2);
}
The method printOriginalNumbers() accesses the parameters phoneNumber1 and phoneNumber2 of the method validatePhoneNumber().
Declarations of a type (such as a variable) in a local class shadow declarations in the enclosing scope that have the same name. See Shadowing for more information.
Local Classes Are Similar To Inner Classes
Local classes are similar to inner classes because they cannot define or declare any static members. Local classes in static methods, such as the class PhoneNumber, which is defined in the static method validatePhoneNumber(), can only refer to static members of the enclosing class. For example, if you do not define the member variable regularExpression as static, then the Java compiler generates an error similar to "non-static variable regularExpression cannot be referenced from a static context."
Local classes are non-static because they have access to instance members of the enclosing block. Consequently, they cannot contain most kinds of static declarations.
You cannot declare an interface inside a block; interfaces are inherently static. For example, the following code excerpt does not compile because the interface HelloThere is defined inside the body of the method greetInEnglish():
public void greetInEnglish() {
interface HelloThere {
public void greet();
}
class EnglishHelloThere implements HelloThere {
public void greet() {
System.out.println("Hello " + name);
}
}
HelloThere myGreeting = new EnglishHelloThere();
myGreeting.greet();
}
You cannot declare static initializers or member interfaces in a local class. The following code excerpt does not compile because the method EnglishGoodbye.sayGoodbye() is declared static. The compiler generates an error similar to "modifier static is only allowed in constant variable declaration" when it encounters this method definition:
public void sayGoodbyeInEnglish() {
class EnglishGoodbye {
public static void sayGoodbye() {
System.out.println("Bye bye");
}
}
EnglishGoodbye.sayGoodbye();
}
A local class can have static members provided that they are constant variables. (A constant variable is a variable of primitive type or type String that is declared final and initialized with a compile-time constant expression. A compile-time constant expression is typically a string or an arithmetic expression that can be evaluated at compile time. See Understanding Class Members for more information.) The following code excerpt compiles because the static member EnglishGoodbye.farewell is a constant variable:
public void sayGoodbyeInEnglish() {
class EnglishGoodbye {
public static final String farewell = "Bye bye";
public void sayGoodbye() {
System.out.println(farewell);
}
}
EnglishGoodbye myEnglishGoodbye = new EnglishGoodbye();
myEnglishGoodbye.sayGoodbye();
}
Anonymous Classes
Anonymous classes enable you to make your code more concise. They enable you to declare and instantiate a class at the same time. They are like local classes except that they do not have a name. Use them if you need to use a local class only once.
Declaring Anonymous Classes
While local classes are class declarations, anonymous classes are expressions, which means that you define the class in another expression. The following example, HelloWorldAnonymousClasses, uses anonymous classes in the initialization statements of the local variables frenchGreeting and spanishGreeting, but uses a local class for the initialization of the variable englishGreeting:
public class HelloWorldAnonymousClasses {
interface HelloWorld {
public void greet();
public void greetSomeone(String someone);
}
public void sayHello() {
class EnglishGreeting implements HelloWorld {
String name = "world";
public void greet() {
greetSomeone("world");
}
public void greetSomeone(String someone) {
name = someone;
System.out.println("Hello " + name);
}
}
HelloWorld englishGreeting = new EnglishGreeting();
HelloWorld frenchGreeting = new HelloWorld() {
String name = "tout le monde";
public void greet() {
greetSomeone("tout le monde");
}
public void greetSomeone(String someone) {
name = someone;
System.out.println("Salut " + name);
}
};
HelloWorld spanishGreeting = new HelloWorld() {
String name = "mundo";
public void greet() {
greetSomeone("mundo");
}
public void greetSomeone(String someone) {
name = someone;
System.out.println("Hola, " + name);
}
};
englishGreeting.greet();
frenchGreeting.greetSomeone("Fred");
spanishGreeting.greet();
}
public static void main(String... args) {
HelloWorldAnonymousClasses myApp =
new HelloWorldAnonymousClasses();
myApp.sayHello();
}
}
Syntax of Anonymous Classes
As mentioned previously, an anonymous class is an expression. The syntax of an anonymous class expression is like the invocation of a constructor, except that there is a class definition contained in a block of code.
Consider the instantiation of the frenchGreeting object:
HelloWorld frenchGreeting = new HelloWorld() {
String name = "tout le monde";
public void greet() {
greetSomeone("tout le monde");
}
public void greetSomeone(String someone) {
name = someone;
System.out.println("Salut " + name);
}
};
The anonymous class expression consists of the following:
- The
newoperator - The name of an interface to implement or a class to extend. In this example, the anonymous class is implementing the interface
HelloWorld. - Parentheses that contain the arguments to a constructor, just like a normal class instance creation expression. Note: When you implement an interface, there is no constructor, so you use an empty pair of parentheses, as in this example.
- A body, which is a class declaration body. More specifically, in the body, method declarations are allowed but statements are not.
- Because an anonymous class definition is an expression, it must be part of a statement. In this example, the anonymous class expression is part of the statement that instantiates the
frenchGreetingobject. (This explains why there is a semicolon after the closing brace.)
Accessing Local Variables of the Enclosing Scope, and Declaring and Accessing Members of the Anonymous Class
Like local classes, anonymous classes can capture variables; they have the same access to local variables of the enclosing scope:
- An anonymous class has access to the members of its enclosing class.
- An anonymous class cannot access local variables in its enclosing scope that are not declared as final or effectively final.
- Like a nested class, a declaration of a type (such as a variable) in an anonymous class shadows any other declarations in the enclosing scope that have the same name. See Shadowing for more information.
Anonymous classes also have the same restrictions as local classes with respect to their members:
- You cannot declare static initializers or member interfaces in an anonymous class.
- An anonymous class can have static members provided that they are constant variables.
Note that you can declare the following in anonymous classes:
- Fields
- Extra methods (even if they do not implement any methods of the supertype)
- Instance initializers
- Local classes
However, you cannot declare constructors in an anonymous class.
Last update: September 23, 2021