Chapter 6 ■ Generics and Collections

A.It prints the following: StrLastError: setError.

B.It prints the following: LastError: setError.

C.It results in a compilation error.

D.It results in a runtime exception.

Answer: C.  It results in a compilation error.

(It looks like the setError() method in StrLastError is overriding setError() in the LastError class. However, it is not the case. At the time of compilation, the knowledge of type S is not available. Therefore, the compiler records the signatures of these two methods as setError(String) in superclass and setError(S_extends_CharSequence) in subclass—treating them as overloaded methods (not overridden).

In this case, when the call to setError() is found, the compiler finds both the overloaded methods matching, resulting in the ambiguous method call error. Here is the error message

Test.java:22: error: reference to setError is ambiguous, both method setError(T) in LastError and method setError(S) in StrLastError match

err.setError("Last error");

^

where T and S are type-variables:

T extends Object declared in class LastError.

S extends CharSequence declared in class StrLastError).

Summary

Generics

•Generics will ensure that any attempts to add elements of types other than the specified type(s) will be caught at compile time itself. Hence, generics offer generic implementation with type safety.

•Java 7 introduced diamond syntax where the type parameters (after new operator and class name) can be omitted. The compiler will infer the types from the type declaration.

•Generics are not covariant. That is, subtyping doesn’t work with generics; you cannot assign a derived generic type parameter to a base type parameter.

•The <?> specifies an unknown type in generics and is known as a wildcard. For example, List<?> refers to list of unknowns.

•Wildcards can be bounded. For example, <? extends Runnable> specifies that ? can match any type as long as it is Runnable or any of its derived types. Note that extends is inclusive, so you can replace X in ? extends X. However, in <? super Runnable> , ? would match only the super types of Runnable, and Runnable itself will not match (i.e., it is an exclusive clause).

•You use the extends keyword for both class type as well as an interface when specifying bounded types in generics. For specifying multiple base types, you use the & symbol. For example, in List<? extends X & Y>, ? will match types, extending both the types X and Y.

203

Chapter 6 ■ Generics and Collections

Collections Framework

•Avoid mixing raw types with generic types. In other cases, make sure of the type safety manually.

•The terms Collection, Collections, and collection are different.

Collection— java.util.Collection<E>—is the root interface in the collection hierarchy. Collections—java.util.Collections—is a utility class that contains only static methods. The general term collection(s) refers to containers like map, stack, queue, etc.

•The container classes store references to objects, so you cannot use primitive types with any of the collection classes.

•The methods hashCode() and equals() need to be consistent for a class. For practical purposes, ensure that you follow this one rule: the hashCode() method should return the same hash value for two objects if the equals() method returns true for them.

•If you’re using an object in containers like HashSet or HashMap, make sure you override the hashCode() and equals() methods correctly.

•The Map interface does not extend the Collection interface.

•It is not recommended that you store null as an argument, since there are methods in the Deque interface that return null, and it would be difficult for you to distinguish between the success or failure of the method call.

•Implement the Comparable interface for your classes where a natural order is possible. If you want to compare the objects other than the natural order or if there is no natural ordering present for your class type, then create separate classes implementing the Comparator interface. Also, if you have multiple alternative ways to decide the order, then go for the

Comparator interface.

204