710: METHOD INVOCATION CONVERSIONS INVOLVING REFERENCES in Java

Printing QR-Code in Java 710: METHOD INVOCATION CONVERSIONS INVOLVING REFERENCES
710: METHOD INVOCATION CONVERSIONS INVOLVING REFERENCES
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If the SourceType is an interface type, the reference value in srcRef may be assigned to the destRef reference, provided the DestinationType is one of the following:
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DestinationType is Object DestinationType is a superinterface of subinterface SourceType
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objRef = iStackRef; // (5) Always possible iStackRef = iSafeStackRef; // (6) Subinterface to superinterface assignment
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If the SourceType is an array type, the reference value in srcRef may be assigned to the destRef reference, provided the DestinationType is one of the following:
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DestinationType is Object DestinationType is an array type, where the element type of the SourceType is assignable to the element type of the DestinationType
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objRef = objArray; // objRef = stackArray; // objArray = stackArray; // objArray = iSafeStackArray; // objRef = intArray; // // objArray = intArray; // stackArray = safeStackArray; // iSafeStackArray = safeStackArray;// (7) Always possible (8) Always possible (9) Always possible (10) Always possible (11) Always possible (12) Compile-time error (13) Subclass array to superclass array (14) SafeStackImpl implements ISafeStack
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The rules for assignment are enforced at compile time, guaranteeing that no type conversion error will occur during assignment at runtime Such conversions are type safe The reason the rules can be enforced at compile time is that they concern the declared type of the reference (which is always known at compile time) rather than the actual type of the object being referenced (which is known at runtime)
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710 Method Invocation Conversions Involving References
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The conversions for reference value assignment are also applicable for method invocation conversions, except for the narrowing conversion for constant expressions of non-long integer type (Table 51, p 163) This is reasonable, as parameters in Java are passed by value (see Section 37, p 81), requiring that values of actual parameters must be assignable to formal parameters of compatible types In Example 79, the method sendParams() at (17) has the following signature, showing the types of the formal parameters:
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sendParams(Object, StackImpl, IStack, StackImpl[], IStack[])
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The method call at (15) has the following signature, showing the types of the actual parameters:
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sendParams(StackImpl, SafeStackImpl, IStack, SafeStackImpl[], ISafeStack[]);
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Note that the assignment of the values of the actual parameters to the corresponding formal parameters is legal, according to the rules for assignment discussed earlier
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CHAPTER 7: OBJECT-ORIENTED PROGRAMMING
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The method call at (16) provides another example of the parameter passing conversion It has the following signature:
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sendParams(ISafeStack[], StackImpl, ISafeStack, StackImpl[], SafeStackImpl[]);
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Analogous to assignment, the rules for parameter passing conversions are based on the reference type of the parameters and are enforced at compile time The output in Example 79 shows the class of the actual objects referenced by the formal parameters at runtime, which in this case turns out to be either SafeStackImpl or SafeStackImpl[] The characters [L in the output indicate a onedimensional array of a class or interface type (see the ClassgetName() method in the Java API documentation)
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Overloaded Method Resolution
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In this subsection, we take a look at some aspects regarding overloaded method resolution, ie, how the compiler determines which overloaded method will be invoked by a given method call at runtime Resolution of overloaded methods selects the most specific method for execution One method is more specific than another method if all actual parameters that can be accepted by the one can be accepted by the other If there is more than one such method, the call is ambiguous The following overloaded methods illustrate this situation
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private static void outprintln(str } private static void outprintln(str } flipFlop(String str, int i, Integer iRef) { // (1) + " ==> (String, int, Integer)"); flipFlop(String str, int i, int j) { + " ==> (String, int, int)"); // (2)
Their method signatures are, as follows:
flipFlop(String, int, Integer) flipFlop(String, int, int) // See (1) above // See (2) above
The following method call is ambiguous:
flipFlop("(String, Integer, int)", new Integer(4), 2004); // (3) Ambiguous call
It has the call signature: