Slides on Java Generics - Organization of Programming Languages | CMSC 330, Study notes of Programming Languages

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CMSC 330: Organization of

Programming Languages

Java Generics

An Integer Stack Implementation

class Stack { class Entry { Integer elt; Entry next; Entry(Integer i, Entry n) { elt = i; next = n; } } Entry theStack; void push(Integer i) { theStack = new Entry(i, theStack); } Integer pop() throws EmptyStackException { if (theStack == null) throw new EmptyStackException(); else { Integer i = theStack.elt; theStack = theStack.next; return i; }}}

Polymorphism Using Object

class Stack { class Entry { Object elt; Entry next; Entry(Object i, Entry n) { elt = i; next = n; } } Entry theStack; void push(Object i) { theStack = new Entry(i, theStack); } Object pop() throws EmptyStackException { if (theStack == null) throw new EmptyStackException(); else { Object i = theStack.elt; theStack = theStack.next; return i; }}}

New Stack Client

Stack is = new Stack(); Integer i; is.push(new Integer(3)); is.push(new Integer(4)); i = (Integer) is.pop();

• Now Stacks are reusable

• (^) push() works the same
• (^) But now pop() returns an Object
  • (^) Have to downcast back to Integer
  • (^) Not checked until run-time

Parametric Polymorphism (for Classes)

• After Java 1.5 we can parameterize the Stack

class by its element type

• Syntax:

• (^) Class declaration: class A { ... }
  • (^) A is the class name, as before
  • (^) T is a type variable , can be used in body of class (...)
• (^) Client usage declaration:A x;
  • (^) We instantiate A with the Integer type

class Stack { class Entry { ElementType elt; Entry next; Entry(ElementType i, Entry n) { elt = i; next = n; } } Entry theStack; void push(ElementType i) { theStack = new Entry(i, theStack); } ElementType pop() throws EmptyStackException { if (theStack == null) throw new EmptyStackException(); else { ElementType i = theStack.elt; theStack = theStack.next; return i; }}}

Parametric Polymorphism for Stack

Parametric Polymorphism for Methods

• (^) String is a subtype of Object
  • (^) static Object id(Object x) { return x; }
  • (^) static Object id(String x) { return x; }
  • (^) static String id(Object x) { return x; }
  • (^) static String id(String x) { return x; }
• (^) Can’t pass an Object to 2 or 4
• (^) 3 doesn’t type check
• (^) Can pass a String to 1 but you get an Object back

Parametric Polymorphism, Again

• But id() doesn’t care about the type of x

• (^) It works for any type

• So parameterize the static method :

static T id(T x) { return x; } Integer i = id(new Integer(3));

• (^) Notice no need to instantiate id; compiler figures out the correct type at usage
• (^) The formal parameter has type T, the actual parameter has type Integer

Translation via Erasure

• Replace uses of type variables with Object

• (^) class A { ...T x;... } becomes
• (^) class A { ...Object x;... }

• Add downcasts wherever necessary

• (^) Integer x = A.get(); becomes
• (^) Integer x = (Integer) (A.get());

• So why did we bother with generics if they’re

just going to be removed?

• (^) Because the compiler still did type checking for us
• (^) We know that those casts will not fail at run time

Limitations of Translation

• Some type information not available at compile-

time

• (^) Recall type variables T are rewritten to Object

• Disallowed, assuming T is type variable:

• (^) new T() would translate to new Object() (error)
• (^) new T[n] would translate to new Object[n] (warning)
• (^) Some casts/instanceofs that use T
  • (^) (Only ones the compiler can figure out are allowed)

Polymorphism Quiz

• What kind of polymorphism?

• Possible answers: subtype, parametric (classes or methods), or ad-hoc

• foo(x) takes floats and ints because the

programmer provided two different versions of

the function, one for each type

int foo(int x) { return x+1; } float foo(float x) { return x+1.0; }

Polymorphism Quiz

• What kind of polymorphism?

• Possible answers: subtype, parametric (classes or methods), or ad-hoc

• foo(x) takes Floats and Integers because both

are subclasses of Numbers

class Number { } class Float extends Number { } class Integer extends Number { } String foo(Number x) { return x.toString(); }

Polymorphism Quiz

• What kind of polymorphism?

• Possible answers: subtype, parametric (classes or methods), or ad-hoc

• foo(x) takes Floats and Integers because the

programmer used generics

T foo(T x) { return x; }

Polymorphism Quiz

• What kind of polymorphism?

• Possible answers: subtype, parametric (classes or methods), or ad-hoc

• the constructor for class Foo takes Floats and

Integers because the programmer used generics

class Foo { Foo(T x) { } }