Proposal: Indexing access syntax for Lists and Maps

[Shams Mahmood]

Indexing access syntax for Lists and Maps

VERSION
This is version 1.0.

AUTHOR(S):
Shams Mahmood Imam

OVERVIEW

FEATURE SUMMARY:
Collection classes are among the most frequently used in the Java SDK.
Currently Lists and Maps do not provide any additional language 
feature to access individual elements unlike Arrays. This proposal 
aims to provide these Collection citizens of java additional 
language support to access elements like Arrays have currently. 

MAJOR ADVANTAGE:
Will provide a consistent syntax for accessing elements of Arrays, 
Lists and Maps. In addition, the language grammar will not change 
much since the subscript operator is already supported for Arrays.

MAJOR BENEFIT:
Apart from the consistency mentioned above, implementation fo this 
feature will result in fewer characters needed to be typed to achieve 
simple access to elements in Maps/Lists. 

MAJOR DISADVANTAGE:
Like the for-each loop construct, it will expose the client to 
NullPointerException(NPE)s when used with a null List/Map. However, 
this shouldn't be such a major issue as NPEs are also generated 
by arrays when the operator is used in a null array.

ALTERNATIVES:
The comparatively more verbose get/set methods for Lists and get/put 
methods for Maps.

EXAMPLES

SIMPLE EXAMPLE:

public class Main {
  public static void main(String[] arguments) {
    List<String> l1 = Arrays.asList(new String[] {"a", "b", "c"});
    String firstElement = l1[0];
    Map<Integer, String> m1 = new HashMap<Integer, String>(4);
    m1[Integer.valueOf(1)] = "One";
  }
}

ADVANCED EXAMPLE:

public class Main {
  public static void main(String[] arguments) {
    List<String> l1 = Arrays.asList(new String[] {"a", "b", "c"});
    Map<Integer, String> m1 = new HashMap<Integer, String>(4);
    Map<String, Integer> m2 = new HashMap<String, Integer>(4);
    
    m2[l1[2]] = m2[m1[1]] = 4; // same as m2.put(l1.get(2), m2.put(m1.get(1), 4));
  }
}

DETAILS

SPECIFICATION:
Java Language Specification changes:

15.29 (NEW CHAPTER): Collection Access Expressions
A collection access expression contains two subexpressions, the List/Map reference expression (before the left bracket) and the index expression (within the brackets). Note that the List/Map reference expression may be a name or any expression that evaluates to a List/Map. The index experssion is expected to evaluate to an int for Lists and a valid key type for Maps.

CollectionAccess:
  Expression [ Expression ]

15.8 Primary Expressions
original:
---------
PrimaryNoNewArray:
  Literal
  Type . class
  void . class
  this
  ClassName.this
  ( Expression )
  ClassInstanceCreationExpression
  FieldAccess
  MethodInvocation
  ArrayAccess
         
replaced with:
--------------
PrimaryNoNewArray:
  Literal
  Type . class
  void . class
  this
  ClassName.this
  ( Expression )
  ClassInstanceCreationExpression
  FieldAccess
  MethodInvocation
  ArrayAccess
  CollectionAccess
         
15.26 Assignment Operators
original:
---------
LeftHandSide:
  ExpressionName
  FieldAccess
  ArrayAccess

replaced with:
--------------
LeftHandSide:
  ExpressionName
  FieldAccess
  ArrayAccess
  CollectionAccess


COMPILATION:

After successful creation of the AST handling the additional grammar for Collection Access expressions, the syntactic sugar will be replaced by JDK1.4 compatible code during the Code Generation phase. This is consistent with how JDK5.0 constructs like the for-each loop is handled by the compiler.

e.g.
public class TestConcise {
  public static void main(String[] args) {
    java.util.Map<Integer, String> m1 = new java.util.HashMap<Integer, String>();
    m1[2] = "two";
    
    java.util.LinkedList<String> l1 = java.util.Arrays.asList( new String[] {"a", "b", "c" });
    m1[3] = l1[2];
    l1[0] = m1[0];
    l1[1] = "one";
  }
}

is converted to 
public class TestConcise {
  public TestConcise() {
    super();
  }

  public static void main(String[] args) {
    java.util.Map<Integer, String> m1 = new java.util.HashMap<Integer, String>();
    m1.put(Integer.valueOf(2), "two");
    
    java.util.LinkedList<String> l1 = java.util.Arrays.asList( new String[] {"a", "b", "c" });
    m1.put(Integer.valueOf(3), l1.get(2));
    l1.set(0, m1.get(Integer.valueOf(0)));
    l1.set(1, "one");
  }
}


TESTING:

LIBRARY SUPPORT:
No additional library support is needed. 

REFLECTIVE APIS:
This proposal does not require any reflective API changes. 

OTHER CHANGES:
No changes required.

MIGRATION:
No migration is needed. 

COMPATIBILITY

BREAKING CHANGES:
No breaking changes.

EXISTING PROGRAMS:
Existing programs are not affected by this change.

REFERENCES
My Java7 Wishlist regarding Collections, http://shamsmi.blogspot.com/2008/04/my-java7-wishlist-regarding-collections.html
Implementation of My Java7 Wishlist, http://shamsmi.blogspot.com/2008/05/implementation-of-my-java7-wishlist.html

EXISTING BUGS:
None.

URL FOR PROTOTYPE (optional):
Projects kijaro's concisecollections branch: https://kijaro.dev.java.net/source/browse/kijaro/branches/concisecollections/