1. CSC 220 – Processing Spring, 2017
March / April, 2017
Java Generics in Processing

2. 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.

3. Using Generic Classes
Application code supplies a binding for each generic parameter that must be an interface or class name.
It cannot be a primitive type such as int or float.
Java Generic classes are comparable to Template classes in C++. Their main use is to allow a container class such as java.util.HashSet<E> to work with any Java class substituted by your code for class variable E.

4. 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.
Struck-through parts not in CSC220 final.
The writer of a generic class must ensure type consistency within the class.
The application programmer can safely assume type compatibility if the compiler does not flag an error.

5. 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.

6. Restrictions 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.

7. 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.

8. 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 java.lang.Object.
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.