Whats new in Java 7

SE 7 : What's New!

Ayushman Jain
JDT/Core committer
IBM

© IBM 2011. Licensed under EPL v1.0

New features at a glance

● Project Coin – small enhancement and new language
features.
● Support for dynamic languages.
● Unicode 6.0 – new rupee symbol, brahmi script,
emoticons.
● New I/O and concurrent APIs.


Project coin features
● Binary literals and underscores in literals.
• Strings in switch.
• @SafeVarargs - Varargs warnings.
• Diamond - improved type inference for generic instance
creation.
• Multi-catch and more precise rethrow.
• try-with-resources (formerly known as Automatic
Resource Management or ARM).


Strings in switch
● When do you use a switch statement?
● Valid case labels upto javaSE 6 can be
– Int constants
– Enum constants
● String can also be constants!


Safe Varargs

● Variable arity method - public static <T> List<T>
java.util.Arrays.asList(T... a)
● If T is the parameter type with variable arity and is also
non-reifiable, a new warning on declaration of variable
arity methods with parameter of type T.
● A new annotation @SafeVarargs to suppress the
warning at both declaration site and call site.


Reifiable types
● A type whose type information is fully available at
runtime, that is, a type that does not lose information
in the course of type erasure.
● Any type with type parameters is available to the JVM as
the raw type because of type erasure.
● So List<Number> and List<String> are both seen as List
by the JVM.
● Hence parameterized types are non-reifiable.
● On the other hand, types such as “String” are reifiable.


Safe Varargs
● A variable arity parameter of a generic type can cause
heap pollution.
● public static <T> List<T> java.util.Arrays.asList(T... a)
● Heap pollution - A situation where a variable of a
parameterized type refers to an object that is not of
that parameterized type.
● Usually, the method body is well behaved and only
iterates over the elements.
● Hence, unchecked warning is mostly a distraction.


@SafeVarargs

● Annotation legal on static or final variable arity methods
or constructors.
● Not legal on
– Fixed arity methods or constructors.
– Variable arity methods or constructors that are
neither final nor static.
● Some java APIs already retrofitted with the annotation in
JDK 7.


Binary Integer Literals and
Underscores in numeric literals
● Binary integer literals structured just like hex interger
literals. Differences:
– Binary digits used instead of hex digits
– 0b now used instead of 0x
● In numeric literals, underscores now permitted as separators
between digits.
● Applies to literals in any base: binary, octal, hexadecimal, or
decimal
● Applies to both integer literals and floating point literals


Multi-catch and more precise
rethrow

● Different catch blocks performing the same action on the
caught exception.
● They can now be combined into one single catch block
using Disjunctive types.
● Disjunctive type = ExceptionA | ExceptionB | ....
● Disjunctive types implicitly final.
● Also, now only the actually thrown exception is now
rethrown and not its parent type.


Try With Resources
● Language & Library changes to
– Ease management of objects that require explicit
freeing/disposal/destruction.
– Prevent resource leaks (handles, streams ...)
– A la Destructors in C++ (and others)
● Library changes:
– A new interface java.lang.AutoCloseable.
– Libraries retrofitted to implement the new interface
– Facilities to manage suppressed exceptions on
java.lang.Throwable

What is a Resource
● Basically a final variable local to the try block.
● Must be of type AutoCloseable.
● Must be initialized in the resource section.
● Language guarantees that every successfully
initialized non-null resource will be “closed”.
● Will be closed regardless of normal or abrupt
completion.
● In LIFC order: Last initialized first closed.

Suppressed exceptions

● “Primary” exception will be the exception seen by a user
supplied catch block.
● Exception from close methods are added to the
suppressed list of primary exception (unless it is the
primary exception.)
● An interested party can query and process suppressed
exceptions. New APIs in class Throwable:
– public final void addSuppressed(Throwable);
– public final Throwable [] getSuppressed();


Diamond
● Improved type inference for generic instance
creation.
● Before,
● List<String> list = new ArrayList<String>();
● Now,
● List<String> list = new ArrayList<>();
● Why not List<String> list = new ArrayList() ?


Support for dynamic languages
● A growing interest in running programs written in
dynamic languages on JVM.
● Particularly scripting languages-JRuby,Jpython,Groovy.
● Motivation – make implementation in such languages
efficient and fast.
● Current Problem - JVM instruction to invoke a method takes
method descriptor as the argument
– Method descriptor is the method name, argument
types and the return type.
● Solution – new invokeDynamic instruction and dynamic
linking through method handles.

JVM bytecode instructions
● invokevirtual - Invokes a method on a class. This is the
typical type of method invocation.
● invokeinterface - Invokes a method on an interface.
● invokestatic - Invokes a static method on a class. This is
the only kind of invocation that lacks a receiver
argument.
● invokespecial - Invokes a method without reference to
the type of the receiver.
● invokedynamic - enables an implementer of a dynamic
language to translate a method invocation into
bytecode without having to specify a target type that
contains the method. © IBM 2011. Licensed under EPL v1.0

Method handles
● Consider hypothetical dynamic language code
function max (x,y) {
if x.lessThan(y) then y else x }
● To compile this for JVM
MyObject function max (MyObject x,MyObject y) {
if x.lessThan(y) then y else x }
● Or use reflection.
● With Java7, use method handles!


Use Eclipse for java 7

● No builds available as of today.
● Setup Eclipse to work for java 7 as described
in http://wiki.eclipse.org/JDT_Core/Java7


Thank You!


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