Interfaces, Lambda Expressions, and Inner Classes in Java
You have now seen all the basic tools for object-oriented programming in Java. This chapter shows you several advanced techniques that are commonly used. Despite their less obvious nature, you will need to master them to complete your Java tool chest.
The first technique, calledinterfaces, is a way of describingwhatclasses should do, without specifyinghowthey should do it. A class canimplementone or more interfaces. You can then use objects of these implementing classes whenever conformance to the interface is required. After we cover interfaces, we move on tolambda expressions, a concise way for expressing a block of code that can be executed at a later point in time. Using lambda expressions, you can express code that uses callbacks or variable behavior in an elegant and concise fashion.
We then discuss the mechanism ofinner classes. Inner classes are technically somewhat complex—they are defined inside other classes, and their methods can access the fields of the surrounding class. Inner classes are useful when you design collections of cooperating classes.
This chapter concludes with a discussion ofproxies, objects that implement arbitrary interfaces. A proxy is a very specialized construct that is useful for building system-level tools. You can safely skip that section on first reading.
6.1 Interfaces
In the following sections, you will learn what Java interfaces are and how to use them. You will also find out how interfaces have been made more powerful in Java SE 8.
6.1.1 The Interface Concept
In the Java programming language, an interface is not a class but a set ofrequirementsfor the classes that want to conform to the interface.
Typically, the supplier of some service states: “If your class conforms to a particular interface, then I’ll perform the service.” Let’s look at a concrete example. Thesortmethod of theArraysclass promises to sort an array of objects, but under one condition: The objects must belong to classes that implement theComparableinterface.
Here is what theComparableinterface looks like:
public interface Comparable { int compareTo(Object other); }
This means that any class that implements theComparableinterface is required to have acompareTomethod, and the method must take anObjectparameter and return an integer.
All methods of an interface are automaticallypublic. For that reason, it is not necessary to supply the keywordpublicwhen declaring a method in an interface.
Of course, there is an additional requirement that the interface cannot spell out: When callingx.compareTo(y), thecompareTomethod must actually be able tocomparethe two objects and return an indication whetherxoryis larger. The method is supposed to return a negative number ifxis smaller thany, zero if they are equal, and a positive number otherwise.
This particular interface has a single method. Some interfaces have multiple methods. As you will see later, interfaces can also define constants. What is more important, however, is what interfacescannotsupply. Interfaces never have instance fields. Before Java SE 8, methods were never implemented in interfaces. (As you will see in Section 6.1.4, “Static Methods,” on p. 298 and Section 6.1.5, “Default Methods,” on p. 298, it is now possible to supply simple methods in interfaces. Of course, those methods cannot refer to instance fields—interfaces don’t have any.)
Supplying instance fields and methods that operate on them is the job of the classes that implement the interface. You can think of an interface as being similar to an abstract class with no instance fields. However, there are some differences between these two concepts—we look at them later in some detail.
Now suppose we want to use thesortmethod of theArraysclass to sort an array ofEmployeeobjects. Then theEmployeeclass mustimplementtheComparableinterface.
To make a class implement an interface, you carry out two steps:
- You declare that your class intends to implement the given interface.
- You supply definitions for all methods in the interface.
To declare that a class implements an interface, use theimplementskeyword:
class EmployeeimplementsComparable
Of course, now theEmployeeclass needs to supply thecompareTomethod. Let’s suppose that we want to compare employees by their salary. Here is an implementation of thecompareTomethod:
public int compareTo(Object otherObject) { Employee other = (Employee) otherObject; return Double.compare(salary, other.salary); }
Here, we use the staticDouble.comparemethod that returns a negative if the first argument is less than the second argument,0if they are equal, and a positive value otherwise.
We can do a little better by supplying a type parameter for the genericComparableinterface:
class Employee implements Comparable{ public int compareTo(Employeeother) { return Double.compare(salary, other.salary); } ... }
Note that the unsightly cast of theObjectparameter has gone away.
Now you saw what a class must do to avail itself of the sorting service—it must implement acompareTomethod. That’s eminently reasonable. There needs to be some way for thesortmethod to compare objects. But why can’t theEmployeeclass simply provide acompareTomethod without implementing theComparableinterface?
The reason for interfaces is that the Java programming language isstrongly typed. When making a method call, the compiler needs to be able to check that the method actually exists. Somewhere in thesortmethod will be statements like this:
if (a[i].compareTo(a[j]) > 0) { // rearrange a[i] and a[j] ... }
The compiler must know thata[i]actually has acompareTomethod. Ifais an array ofComparableobjects, then the existence of the method is assured because every class that implements theComparableinterface must supply the method.
Listing 6.1 presents the full code for sorting an array of instances of the classEmployee(Listing 6.2) for sorting an employee array.
Listing 6.1interfaces/EmployeeSortTest.java
1包接口;2 3导入java.util。*;4个5/** 6 * This program demonstrates the use of the Comparable interface. 7 * @version 1.30 2004-02-27 8 * @author Cay Horstmann 9 */ 10 public class EmployeeSortTest 11 { 12 public static void main(String[] args) 13 { 14 Employee[] staff = new Employee[3]; 15 16 staff[0] = new Employee("Harry Hacker", 35000); 17 staff[1] = new Employee("Carl Cracker", 75000); 18 staff[2] = new Employee("Tony Tester", 38000); 19 20 Arrays.sort(staff); 21 22 // print out information about all Employee objects 23 for (Employee e : staff) 24 System.out.println("name=" + e.getName() + ",salary=" + e.getSalary()); 25 } 26 }
Listing 6.2interfaces/Employee.java
1包接口;2 3 public class Employee implements Comparable4 { 5 private String name; 6 private double salary; 7 8 public Employee(String name, double salary) 9 { 10 this.name = name; 11 this.salary = salary; 12 } 13 14 public String getName() 15 { 16 return name; 17 } 18 19 public double getSalary() 20 { 21 return salary; 22 } 23 24 public void raiseSalary(double byPercent) 25 { 26 double raise = salary * byPercent / 100; 27 salary += raise; 28 } 29 30 /** 31 * Compares employees by salary 32 * @param other another Employee object 33 * @return a negative value if this employee has a lower salary than 34 * otherObject, 0 if the salaries are the same, a positive value otherwise 35 */ 36 public int compareTo(Employee other) 37 { 38 return Double.compare(salary, other.salary); 39 } 40 }
6.1.2 Properties of Interfaces
Interfaces are not classes. In particular, you can never use thenewoperator to instantiate an interface:
x = new Comparable(. . .); // ERROR
However, even though you can’t construct interface objects, you can still declare interface variables.
Comparable x; // OK
An interface variable must refer to an object of a class that implements the interface:
x = new Employee(. . .); // OK provided Employee implements Comparable
Next, just as you useinstanceofto check whether an object is of a specific class, you can useinstanceofto check whether an object implements an interface:
if (anObject instanceof Comparable) { . . . }
Just as you can build hierarchies of classes, you can extend interfaces. This allows for multiple chains of interfaces that go from a greater degree of generality to a greater degree of specialization. For example, suppose you had an interface calledMoveable.
public interface Moveable { void move(double x, double y); }
Then, you could imagine an interface calledPoweredthat extends it:
public interface Powered extends Moveable { double milesPerGallon(); }
Although you cannot put instance fields or static methods in an interface, you can supply constants in them. For example:
public interface Powered extends Moveable { double milesPerGallon(); double SPEED_LIMIT = 95; // a public static final constant }
Just as methods in an interface are automaticallypublic, fields are alwayspublic static final.
Some interfaces define just constants and no methods. For example, the standard library contains an interfaceSwingConstantsthat defines constantsNORTH,SOUTH,HORIZONTAL, and so on. Any class that chooses to implement theSwingConstantsinterface automatically inherits these constants. Its methods can simply refer toNORTHrather than the more cumbersomeSwingConstants.NORTH. However, this use of interfaces seems rather degenerate, and we do not recommend it.
While each class can have only one superclass, classes can implementmultipleinterfaces. This gives you the maximum amount of flexibility in defining a class’s behavior. For example, the Java programming language has an important interface built into it, calledCloneable. (We will discuss this interface in detail in Section 6.2.3, “Object Cloning,” on p. 306.) If your class implementsCloneable, theclonemethod in theObjectclass will make an exact copy of your class’s objects. If you want both cloneability and comparability, simply implement both interfaces. Use commas to separate the interfaces that you want to implement:
class Employee implements Cloneable, Comparable
6.1.3 Interfaces and Abstract Classes
If you read the section about abstract classes in Chapter 5, you may wonder why the designers of the Java programming language bothered with introducing the concept of interfaces. Why can’tComparablesimply be an abstract class:
abstract class Comparable // why not? { public abstract int compareTo(Object other); }
TheEmployeeclass would then simply extend this abstract class and supply thecompareTomethod:
class Employee extends Comparable // why not? { public int compareTo(Object other) { . . . } }
There is, unfortunately, a major problem with using an abstract base class to express a generic property. A class can only extend a single class. Suppose theEmployeeclass already extends a different class, say,Person. Then it can’t extend a second class.
class Employee extends Person, Comparable // Error
But each class can implement as many interfaces as it likes:
class Employee extends Person implements Comparable // OK
Other programming languages, in particular C++, allow a class to have more than one superclass. This feature is calledmultiple inheritance. The designers of Java chose not to support multiple inheritance, because it makes the language either very complex (as in C++) or less efficient (as in Eiffel).
Instead, interfaces afford most of the benefits of multiple inheritance while avoiding the complexities and inefficiencies.
6.1.4 Static Methods
As of Java SE 8, you are allowed to add static methods to interfaces. There was never a technical reason why this should be outlawed. It simply seemed to be against the spirit of interfaces as abstract specifications.
Up to now, it has been common to place static methods in companion classes. In the standard library, you find pairs of interfaces and utility classes such asCollection/CollectionsorPath/Paths.
Have a look at thePathsclass. It only has a couple of factory methods. You can construct a path to a file or directory from a sequence of strings, such asPaths.get("jdk1.8.0", "jre", "bin"). In Java SE 8, one could have added this method to thePathinterface:
public interface Path { public static Path get(String first, String... more) { return FileSystems.getDefault().getPath(first, more); } ... }
Then thePathsclass is no longer necessary.
It is unlikely that the Java library will be refactored in this way, but when you implement your own interfaces, there is no longer a reason to provide a separate companion class for utility methods.
6.1.5 Default Methods
You can supply adefaultimplementation for any interface method. You must tag such a method with thedefaultmodifier.
public interface Comparable{defaultint compareTo(T other) { return 0; } // By default, all elements are the same }
Of course, that is not very useful since every realistic implementation ofComparablewould override this method. But there are other situations where default methods can be useful. For example, as you will see in Chapter 11, if you want to be notified when a mouse click happens, you are supposed to implement an interface that has five methods:
public interface MouseListener { void mouseClicked(MouseEvent event); void mousePressed(MouseEvent event); void mouseReleased(MouseEvent event); void mouseEntered(MouseEvent event); void mouseExited(MouseEvent event); }
大多数时候,你只关心一个或两个of these event types. As of Java SE 8, you can declare all of the methods as default methods that do nothing.
public interface MouseListener { default void mouseClicked(MouseEvent event) {} default void mousePressed(MouseEvent event) {} default void mouseReleased(MouseEvent event) {} default void mouseEntered(MouseEvent event) {} default void mouseExited(MouseEvent event) {} }
Then programmers who implement this interface only need to override the listeners for the events they actually care about.
A default method can call other methods. For example, aCollectioninterface can define a convenience method
public interface Collection { int size(); // An abstract method default boolean isEmpty() { return size() == 0; } ... }
Then a programmer implementingCollectiondoesn’t have to worry about implementing anisEmptymethod.
An important use for default methods isinterface evolution. Consider for example theCollectioninterface that has been a part of Java for many years. Suppose that a long time ago, you provided a class
public class Bag implements Collection
Later, in Java SE 8, astreammethod was added to the interface.
Suppose thestreammethod was not a default method. Then theBagclass no longer compiles since it doesn’t implement the new method. Adding a nondefault method to an interface is notsource compatible.
But suppose you don’t recompile the class and simply use an old JAR file containing it. The class will still load, even with the missing method. Programs can still constructBaginstances, and nothing bad will happen. (Adding a method to an interface isbinary compatible.) However, if a program calls thestreammethod on aBaginstance, anAbstractMethodErroroccurs.
Making the method adefaultmethod solves both problems. TheBagclass will again compile. And if the class is loaded without being recompiled and thestreammethod is invoked on aBaginstance, theCollection.streammethod is called.
6.1.6 Resolving Default Method Conflicts
What happens if the exact same method is defined as a default method in one interface and then again as a method of a superclass or another interface? Languages such as Scala and C++ have complex rules for resolving such ambiguities. Fortunately, the rules in Java are much simpler. Here they are:
- Superclasses win. If a superclass provides a concrete method, default methods with the same name and parameter types are simply ignored.
- Interfaces clash. If a superinterface provides a default method, and another interface supplies a method with the same name and parameter types (default or not), then you must resolve the conflict by overriding that method.
Let’s look at the second rule. Consider another interface with agetNamemethod:
interface Named { default String getName() { return getClass().getName() + "_" + hashCode(); } }
What happens if you form a class that implements both of them?
class Student implements Person, Named {...}
The class inherits two inconsistentgetNamemethods provided by thePersonandNamedinterfaces. Instead of choosing one over the other, the Java compiler reports an error and leaves it up to the programmer to resolve the ambiguity. Simply provide agetNamemethod in theStudentclass. In that method, you can choose one of the two conflicting methods, like this:
class Student implements Person, Named { public String getName() { returnPerson.super.getName();} ... }
Now assume that theNamedinterface does not provide a default implementation forgetName:
interface Named { String getName(); }
Can theStudentclass inherit the default method from thePersoninterface? This might be reasonable, but the Java designers decided in favor of uniformity. It doesn’t matter how two interfaces conflict. If at least one interface provides an implementation, the compiler reports an error, and the programmer must resolve the ambiguity.
我们只讨论两个数据之间的名称冲突ces. Now consider a class that extends a superclass and implements an interface, inheriting the same method from both. For example, suppose thatPersonis a class andStudentis defined as
class Student extends Person implements Named { ... }
In that case, only the superclass method matters, and any default method from the interface is simply ignored. In our example,Studentinherits thegetNamemethod fromPerson, and it doesn’t make any difference whether theNamedinterface provides a default forgetNameor not. This is the “class wins” rule.
The “class wins” rule ensures compatibility with Java SE 7. If you add default methods to an interface, it has no effect on code that worked before there were default methods.