14: GENERICS in Java

Encoder QR Code 2d barcode in Java 14: GENERICS
CHAPTER 14: GENERICS
Making QR-Code In Java
Using Barcode generator for Java Control to generate, create QR Code JIS X 0510 image in Java applications.
Earlier in this chapter we saw the constraint T extends Comparable<T>, which is also a recursive bound Another example of recursive bounds is the declaration of the Enum<E extends Enum<E>> class in the Java Standard Library
Bar Code Encoder In Java
Using Barcode generator for Java Control to generate, create barcode image in Java applications.
1411 Type Erasure
Barcode Scanner In Java
Using Barcode scanner for Java Control to read, scan read, scan image in Java applications.
Understanding translation by type erasure helps to understand the restrictions and limitations that arise when using generics in Java Although the compiler generates generic-free byte code, we can view the process as a source-to-source translation that generates non-generic code from generic code The translated code has no information about type parameters, ie, the type parameters have been erased hence the term, type erasure This involves replacing the usage of the type parameters with concrete types and inserting suitable type conversions to ensure type correctness In certain situations, bridge methods are also inserted for backward compatibility The process of determining the erasure of a type, ie, what a type in the source code should be replaced with, uses the following rules: 1 2 Drop all type parameter specifications from parameterized types Replace any type parameter as follows: a Replace it with the erasure of its bound, if it has one b Replace it with Object, if it has none c Replace it with the erasure of the first bound, if it has multiple bounds Table 144 shows examples of translation by erasure for some representative types, and the rules that are applied
QR-Code Creation In Visual C#.NET
Using Barcode generator for VS .NET Control to generate, create QR Code image in VS .NET applications.
Table 144
Paint QR Code ISO/IEC18004 In .NET
Using Barcode encoder for ASP.NET Control to generate, create Denso QR Bar Code image in ASP.NET applications.
Examples of Type Erasure Type
Making QR Code ISO/IEC18004 In Visual Studio .NET
Using Barcode generator for .NET framework Control to generate, create Denso QR Bar Code image in VS .NET applications.
List<E> List<Integer> List<String> List<List<String>> List< super Integer> List< extends Number> List<Integer>[] List int Integer List
QR Generator In Visual Basic .NET
Using Barcode generator for .NET Control to generate, create QR image in VS .NET applications.
Erasure 1
Make Bar Code In Java
Using Barcode maker for Java Control to generate, create barcode image in Java applications.
Rule no
UPCA Creation In Java
Using Barcode generation for Java Control to generate, create Universal Product Code version A image in Java applications.
List[] List int Integer
Making Data Matrix 2d Barcode In Java
Using Barcode creation for Java Control to generate, create Data Matrix 2d barcode image in Java applications.
1 1 For any primitive type For any non-generic type (continues)
Paint Code 128C In Java
Using Barcode encoder for Java Control to generate, create Code 128 Code Set C image in Java applications.
1411: TYPE ERASURE Table 144
Printing Barcode In Java
Using Barcode creator for Java Control to generate, create barcode image in Java applications.
Examples of Type Erasure (continued) Type
Painting ISSN In Java
Using Barcode generator for Java Control to generate, create ISSN - 13 image in Java applications.
class Subclass extends Superclass implements Comparable<Subclass> {} public static <T extends Comparable< super T>> T max(T obj1, T obj2) { } public static <T> T doIt(T t) { T lv = t; } T extends MyClass & Comparable<T> & Serializable
Draw ECC200 In Visual Basic .NET
Using Barcode generation for Visual Studio .NET Control to generate, create Data Matrix image in Visual Studio .NET applications.
Erasure
Print Bar Code In Visual C#
Using Barcode printer for .NET framework Control to generate, create barcode image in .NET applications.
class Subclass extends Superclass implements Comparable {} public static Comparable max(Comparable obj1, Comparable obj2) { } public static Object doIt(Object t) { Object lv = t; } MyClass
UPC-A Supplement 2 Printer In .NET Framework
Using Barcode creation for .NET framework Control to generate, create UPCA image in .NET applications.
Rule no 1
Data Matrix 2d Barcode Creator In Visual Studio .NET
Using Barcode creation for VS .NET Control to generate, create Data Matrix ECC200 image in .NET applications.
2a The first bound is Comparable
Data Matrix 2d Barcode Maker In Visual C#.NET
Using Barcode creator for .NET framework Control to generate, create Data Matrix ECC200 image in .NET framework applications.
2c The first bound is MyClass
Draw Bar Code In Visual Basic .NET
Using Barcode generator for Visual Studio .NET Control to generate, create bar code image in .NET framework applications.
The following code mixes legacy and generic code Note that a ClassCastException is expected at (5) because the type-safety of the stack of String has been compromised
Creating Code 3/9 In .NET Framework
Using Barcode creation for ASP.NET Control to generate, create Code-39 image in ASP.NET applications.
// Pre-erasure code List<String> strList = new ArrayList<String>(); // (0) List list = strList; // (1) Assignment to non-generic reference is ok strList = list; // (2) warning: unchecked conversion strListadd("aha"); // (3) Method call type-safe listadd(23); // (4) warning: [unchecked] unchecked call to add(E) // as a member of the raw type javautilList Systemoutprintln(strListget(1)length()); // (5) ClassCastException
It is instructive to compare the corresponding lines of code in the pre-erasure code above and the post-erasure results shown below A cast is inserted to convert from Object type to String type in (5') This is necessary because post-erasure code can only get an Object from the list, and in order to call the length() method, the reference value of this object must be converted to String It is this cast that is the cause of the exception at runtime
// Post-erasure code List strList = new ArrayList(); List list = strList; strList = list; strListadd("aha"); listadd(IntegervalueOf(23)); Systemoutprintln(((String)strListget(1))length()); // // // // // // (0') (1') (2') (3') (4') (5')
CHAPTER 14: GENERICS
Bridge Methods
Bridge methods are inserted in subclasses by the compiler to ensure that overriding of method works correctly The canonical example is the implementation of the Comparable interface The post-erasure code of the class CmpNode<E> from Section 146 on page 684 is shown below A second compareTo() method has been inserted by the compiler at (2), whose method signature is compareTo(Object) This is necessary because, without this method, the class would not implement the Comparable interface, as the compareTo() method of the interface would not be overridden correctly
class CmpNode extends Node implements Comparable { CmpNode(Object data, CmpNode next) { super(data, next); } public int compareTo(CmpNode node2) { // (1) return thisgetData()compareTo(node2getData()); } public int compareTo(Object node2) { // (2) return thiscompareTo((CmpNode)node2); // Calls the method at (1) } }
Such a bridge method cannot be invoked in the source code, and is provided for backward compatibility with legacy code There are Java decompilers readily available that can be used to examine the code generated by the compiler