149: WILDCARD CAPTURE in Java

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149: WILDCARD CAPTURE
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(17), the type parameter N specifies that the element type of the set of vertices to be returned, the type of the keys in the map, the element type of the collections that are values of the map, and the type of the start vertex, are the same
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149 Wildcard Capture
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As we have seen, a wildcard can represent a family of types However, the compiler needs to have a more concrete notion of a type than a wildcard in order to do the necessary type checking Internally, the compiler represents the wildcard by some anonymous, but specific, type Although this type is unknown, it belongs to the family of types represented by the wildcard This specific, but unknown, type is called the capture of the wildcard Compiler messages about erroneous usage of wildcards often refer to the capture of a wildcard Here are some examples of such error messages, based on compiling the following code:
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// Filename: WildcardCapturejava Node< > anyNode; Node< super Number> supNumNode;
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// (1) // (2)
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Node<Integer> intNode = anyNode; // (3) Compile-time error! Node< extends Number> extNumNode = supNumNode; // (4) Compile-time error! anyNodesetData("Trash"); // (5) Compile-time error!
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The assignment at (3) results in the following error message:
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WildcardCapturejava:9: incompatible types found : Node<capture#10 of > required: Node<javalangInteger> Node<Integer> intNode = anyNode; ^
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// (3) Compile-time error!
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The type of the reference anyNode is Node<capture#10 of > The string "capture#10 of " is the designation used by the compiler for the type capture of the wildcard ( ) at (3) The number 10 in "capture#10 of " distinguishes it from the type capture of any other occurrences of wildcards in the statement The type of the reference intNode is Node<Integer> The reference value of a Node<capture#10 of > cannot be assigned to a Node<Integer> reference Whatever the type capture of the wildcard is, it cannot be guaranteed to be Integer, and the assignment is rejected To put it another way, the assignment involves a narrowing reference conversion, requiring an explicit cast which is not provided: Node< > is the supertype of all invocations of the generic class Node<E> The error message below for the assignment at (4) shows the type capture of the lower bounded wildcard at (4) to be "capture#311 of super javalangNumber" Figure 145, p 678, also shows that the Node<capture#311 of super javalangNumber> and Node< extends javalangNumber> types are unrelated
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CHAPTER 14: GENERICS WildcardCapturejava:10: incompatible types found : Node<capture#311 of super javalangNumber> required: Node< extends javalangNumber> Node< extends Number> extNumNode = supNumNode; // (4) Compile-time error! ^
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The method call at (5) results in the following error message:
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WildcardCapturejava:11: setData(capture#351 of ) in Node<capture#351 of > cannot be applied to (javalangString) anyNodesetData("Trash"); // (5) Compile-time error! ^
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The type of the reference anyNode is Node<capture#351 of > and the type of the formal parameter in the method declaration is "capture#351 of " The type of the actual parameter in the method call is String, which is not compatible with "capture#351 of " The call is not allowed As we have seen earlier, with a < > reference we cannot put anything into a data structure If we have the following method in the class MyStack:
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public static <T> void move(MyStack< super T> dstStack, MyStack< extends T> srcStack) { while (!srcStackisEmpty()) dstStackpush(srcStackpop()); }
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and we try to compile the following client code:
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MyStack< > anyStack; MyStackmove(anyStack, anyStack); // Compile-time error!
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the compiler issues the following error message:
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MyStackUserjava:67: <T>move(MyStack< super T>,MyStack< extends T>) in MyStack cannot be applied to (MyStack<capture#774 of >,MyStack<capture#371 of >) MyStackmove(anyStack, anyStack); // Compile-time error! ^
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The error message shows that each occurrence of a wildcard in a statement is represented by a distinct type capture We see that the signature of the move() method is move(MyStack< super T>, MyStack< extends T>) The type of the reference anyStack is MyStack< > The static types of the two arguments in the method call are MyStack<capture#774 of > and MyStack<capture#371 of > The numbers in the type capture names differentiate between the type captures of different wildcard occurrences The signature of the argument list is (MyStack<capture#774 of >, MyStack<capture#371 of >) The type parameter T cannot be inferred from the types of the arguments, as the stacks are considered to be of two different types and, therefore, the call is rejected