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} // Show type of ob void showType() { Systemoutprintln("Type of ob is " + obgetClass()getName()); } } // Demonstrate the non-generic class class NonGenDemo { public static void main(String args[]) { NonGen iOb; // Create NonGen Object and store // an Integer in it Autoboxing still occurs iOb = new NonGen(88); // Show the type of data used by iOb iObshowType(); // Get the value of iOb // This time, a cast is necessary int v = (Integer) iObgetob(); Systemoutprintln("value: " + v); Systemoutprintln(); // Create another NonGen object and // store a String in it NonGen strOb = new NonGen("Non-Generics Test"); // Show the type of data used by strOb strObshowType(); // Get the value of strOb // Again, notice that a cast is necessary String str = (String) strObgetob(); Systemoutprintln("value: " + str); // This compiles, but is conceptually wrong! iOb = strOb; v = (Integer) iObgetob(); // run-time error! } }
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There are several things of interest in this version First, notice that NonGen replaces all uses of T with Object This makes NonGen able to store any type of object, as can the generic version However, it also prevents the Java compiler from having any real knowledge about the type of data actually stored in NonGen, which is bad for two reasons First, explicit casts must be employed to retrieve the stored data Second, many kinds of type mismatch errors cannot be found until run time Let s look closely at each problem
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int v = (Integer) iObgetob();
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Because the return type of getob( ) is Object, the cast to Integer is necessary to enable that value to be auto-unboxed and stored in v If you remove the cast, the program will not compile With the generic version, this cast was implicit In the non-generic version, the cast must be explicit This is not only an inconvenience, but also a potential source of error Now, consider the following sequence from near the end of the program:
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// This compiles, but is conceptually wrong! iOb = strOb; v = (Integer) iObgetob(); // run-time error!
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Here, strOb is assigned to iOb However, strOb refers to an object that contains a string, not an integer This assignment is syntactically valid because all NonGen references are the same, and any NonGen reference can refer to any other NonGen object However, the statement is semantically wrong, as the next line shows Here, the return type of getob( ) is cast to Integer, and then an attempt is made to assign this value to v The trouble is that iOb now refers to an object that stores a String, not an Integer Unfortunately, without the use of generics, the Java compiler has no way to know this Instead, a run-time exception occurs when the cast to Integer is attempted As you know, it is extremely bad to have run-time exceptions occur in your code! The preceding sequence can t occur when generics are used If this sequence were attempted in the generic version of the program, the compiler would catch it and report an error, thus preventing a serious bug that results in a run-time exception The ability to create type-safe code in which type-mismatch errors are caught at compile time is a key advantage of generics Although using Object references to create generic code has always been possible, that code was not type safe, and its misuse could result in run-time exceptions Generics prevent this from occurring In essence, through generics, what were once run-time errors have become compile-time errors This is a major advantage
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