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CSC 243 – Java Programming,
Spring 2014
March, 2014
Week 7ish, Generics
Generic Interfaces / Classes
• We have been using generic interfaces and
classes from the library.
• java.util.Set<E>, java.util.HashSet<E>
• java.util.Map<K,V>, java.util.HashMap<K,V>
• They allow containers to contain different
types (classes) of objects.
• Generic parameters such as E (element type),
K (key type) and V (value type) above are
types bound by client code at compile time.
Using Generic Classes
• Client code supplies a binding for each generic
parameter that must be an interface or class
• It cannot be a primitive type.
• A generic parameter may constrain the type of
class or interface that client code can bind
using a wildcard.
• ?, ? extends T, ? super T – details follow.
Building Generics
• A class or interface specifies its generic type.
HashSet<E>, HashMap<K, V>, etc.
? extends T is a bounded wildcard, T or a subclass
? super T is a lower-bounded wildcard, T or a superclass
? is an unbounded wildcard, same as ? Extends Object
The bound constrains client code type binding.
• The writer of a generic class must ensure type
consistency within the class.
• The user can safely assume type compatibility if the
compiler does not flag an error.
Erasure and Restrictions
• Unlike C++ template classes, the compiler
generates only 1 copy of the class code for a
generic class, erasing the generic type T
information at run time.
• Every generic type T is treated as a java.lang.Object at
run time.
• Writers of generic classes may need to cast to generic
type T, E, K, V etc. in places.
• Writers of generic classes must ensure that those casts
are valid. The compiler cannot ensure this.
• 1. No constructor – no “new T()” for generic T.
• 2. No “new T[]” – you must cast an “Object []” to a “T
[]” within the generic class code.
• This cast often fails at run time with a wildcard T. Make
sure to test if you use arrays of wildcard generic types.
• 3. No generic T in a static field, static method or
static initialization block of code.
• 4. Exception classes cannot be generic.
Non-template class topics
• java.lang.Cloneable
• Marker interface that says (to humans and tools), “Objects of
this interface provide a public clone() method.
• See java.lang.Object.clone()
• clone() returns a new copy of its object.
• Its implementation often uses a private constructor to build
the new object.
• An immutable object can return a reference to itself.
– It is like a C++ copy constructor.
– Do not clone 2D arrays. An array clone() clones only the
first dimension.
Non-template class topics
• java.util.Comparable<T>
– Specifies method int compareTo(T o).
– Returns -1, 0 or 1 if object is logically <, == or > o.
– May require redefining equals() and hashCode() in class
• Just exclusive-OR (^) the hashCodes of the fields.
• java.util.Serializable
– Useful for storing objects in binary data files or sending
them across a network.
– All fields in a Serializable object should be Serializable,
primitive types or transient.