Java generic understanding (efficient java generic)

Java generic understanding (efficient java generic)

1 * generic --> when a class or interface declaration contains one or more type parameters, it is called a generic class or interface, generic 2 * generics are more secure and expressive than the original ecological type. 3 * generics are immutable, sub extends Super (Sub extends Super, Sub [] extends Super []) 5 * (Sub extends Super, list <Sub> extends List <Super>) 6 * Check the element type information of a wildcard during compilation, discard element type information during runtime. 7 * each class is its own child type and key form parameters in the 10 * K Map of the Form Type parameter in the 9 * E set. 11 * V Map value format parameter 12 *? Any matching parameter <? Extends E>/<? Super E> 13 * limited wildcard form parameters (PECS) 14*15 * Class <T> generic 16 * Class <String> parameterized type 17 * original Class type (all generic information is deleted) 18 * Class <?> Unrestricted wildcard type 19 * Class <? Extends E> restricted wildcard type 20 * <T> Format type parameter 21 * <String> actual type parameter 22*23 * static <E> E get (E e) generic Method 24 * (<E> This is called the list of type parameters, that is, to tell the compiler before calling the method, what type is used in this method) 25 * the compiler uses the generic method to deduce the type of the type parameter list through type derivation.

 

 

Test code

1 package com. undergrowth. lang; 2 3 import java. io. serializable; 4 import java. util. arrays; 5 import java. util. collection; 6 import java. util. collections; 7 import java. util. list; 8 import java. util. concurrent. copyOnWriteArrayList; 9 10 import org. junit. test; 11 12/** 13 * generic learning Test code 14*15 * generic --> when one or more type parameters are included in the class or interface declaration, it is called a generic class/generic interface. for short, generic 16 * generic is more advantageous than the original ecological type in terms of security and presentation. 17 * generics are immutable, cannot form a covariant relationship like an array 18 * (Su B extends Super, supports Sub [] extends Super []) 19 * (Sub extends Super, does not support List <Sub> extends List <Super>) 20 * when compiling a wildcard, check its element type information, in the runtime, discard the element type information 21 * each class is its own child type and the key form parameter in the 24 * K Map of the Form Type parameter in the 22 * T form type parameter 23 * E set 25 * V Map value format parameter 26 *? Any matching parameter <? Extends E>/<? Super E> 27 * limited wildcard form parameters (PECS) 28*29 * Class <T> generic 30 * Class <String> parameterized type 31 * Class Original Ecological Type (all generic information is deleted) 32 * Class <?> Unrestricted wildcard type 33 * Class <? Extends E> restricted wildcard type 34 * <T> formal type parameter 35 * <String> actual type parameter 36*37 * static <E> E get (E e) generic Method 38 * (<E> it is called the list of type parameters, that is, to tell the compiler before calling the method, what type is used in this method) 39 * the compiler can use the generic method to derive the type 40*41 * of the type parameter list through type derivation. This content mainly involves generic and generic methods, test Case --> testGenericeClass 42 * generic set --> testGeneCollection 43 * use of limited wildcards --> testBoundWildType 44 * generic compiler maintains type information, discard during running --> testEraseGenericsInfo 45*46 * @ author Administrator 47*48 */49 public class GenericsLearn {50 51/** 52 * Test generic classes and Methods 53 */54 @ Test 55 public void testGenericeClass () {56 GenericeClass <String> gClass1 = new GenericeClass <String> ("testing generic class Parameters"); 57 System. out. println (gClass1); 58 System. out. println (gClass1.getAddSome ("generic method"); 59 System. out. println (gClass1.getAddSome (123 )); 60} 61 62/** 63 * generic test 64*65 * @ author Administrator 66*67 * @ param <T> 68 */69 private static c Lass GenericeClass <T> {70 private final T; 71 72 public GenericeClass (t T) {73 super (); 74 this. t = t; 75} 76 77/** 78 * add additional information 79*80 * @ param t 81 * @ return 82 */83 public <E> E getAddSome (E t) after obtaining the object) {84 if (t instanceof String) {85 // because the type judgment has been made when entering this method, the annotation is used to eliminate the non-Checked Warning 86 @ SuppressWarnings ("unchecked ") 87 E t1 = (E) (String. valueOf (t) + ", Additional generic protection for new messages"); 88 return t1; 89} 90 return t; 91} 9 2 93 public void iterator (Iterable <? Extends T> src) {94 for (T t: src) {95 System. out. println (t); 96} 97} 98 99 @ Override100 public String toString () {101 return "GenericeClass [t =" + t + "]"; 102} 103 104} 105 106/** 107 * testing generic set 108 */109 @ Test110 public void testGeneCollection () {111 Collection <String> collection = createCollection ("123"); 112/* 113 * for (Iterator <String> iterator = collection. iterator (); 114 * iterator. hasNext ();) {String string = (String) iterator. next (); 115 * System. out. println (string);} 116 */117 iterator (collection ); 118} 119 120/** 121 * Create A Test Set 122*123 * @ return124 */125 private <E> Collection <E> createCollection (E t) {126 // TODO Auto-generated method stub127 Collection <E> collection = Collections. emptyList (); 128 if (t instanceof String) {129 collection = new CopyOnWriteArrayList <E> (); 130 // already entered The row class type is checked, so there is no problem during the conversion. Each class is its own sub-type and super-type 131 @ SuppressWarnings ("unchecked") 132 Collection <? Extends E> initData = (Collection <? Extends E>) Arrays133. asList (new String [] {"Test Set 1", "Test Set 2", "Test Set 3"}); 134 collection. addAll (initData); 135} 136 return collection; 137} 138 139/** 140 * Create Number set 141 * this method is not recommended here only test the use of 142 * @ return143 */144 @ Deprecated145 private <E> Collection <E> createNumber () {146 Collection <E> collection = new CopyOnWriteArrayList <E> (); 147 @ SuppressWarnings ("unchecked") 148 Collection <? Extends E> initData = (Collection <? Extends E>) Arrays149. asList (new Number [] {123.2, Integer. MAX_VALUE, 789}); 150 // System. out. println (initData. getClass (). getName (); 151 collection. addAll (initData); 152 return collection; 153} 154 155/** 156 * iterative implementation of the Iterable interface 157*158 * @ param src159 */160 public <E> void iterator (Iterable <E> src) {161 for (E: src) {// The iterator is used internally to traverse element 162 System. out. println (e); 163} 164} 165 166/** 167 * tested Limited wildcard type each class is its own child type and super type 168 * PECS principle --> producer-extends, consumer-super169 */170 @ Test171 public void testBoundWildType () {172 GenericeClass <Number> gClass1 = new GenericeClass <Number> (123456); 173 System. out. println (gClass1); 174 System. out. println (gClass1.getAddSome ("123456"); 175 // create Number array 176 Collection <Number> collection = createNumber (); 177 iterator (collection ); 178 // use the GenericeClass Iteration Method for Iteration Each class is its own sub-type and super-type 179 System. out180. println ("use a limited wildcard to iterate in the form of: Iterable <? Extends T> actually: Iterable <Number extends Number> each class is its own subtype and super type "); 181 gClass1.iterator (collection); 182 Collection <Integer> collection2 = createNumber (); 183 // use foreach to iterate 184 System. out. println ("use foreach for iteration"); 185 iterator (collection2); 186 // use a limited wildcard for iteration 187 System. out188. println ("use a limited wildcard to iterate in the form of: Iterable <? Extends T> actually: Iterable <Integer extends Number> "); 189 gClass1.iterator (collection2); 190 System. out. println ("foreach iteration of String sets"); 191 Collection <String> collection3 = createNumber (); 192 iterator (collection3); 193 System. out. println ("if the collection3.add (123) method is used, the generic security check will be reflected, and the compiler will report an error"); 194 // collection3.add (123); 195 System. out196. println ("if you call the gClass1.iterator (collection3) method, there is a problem because the format is: Iterable <? Extends T> actually: Iterable <String extends Number> "); 197 // gClass1.iterator (collection3); 198 System. out. println ("Get Serializable set"); 199 Collection <Serializable> collection4 = createNumber (); 200 iterator (collection4); 201 System. out202. println ("I have been wondering, why can a Collection be returned when Collection <Number>, Collection <Integer>, Collection <String>, Collection <Serializable>, and Collection <List> are called, "203 +" because it is obvious that the createNumber method creates Is the List <Number> set of numbers. We know why generics cannot be changed. "); 204 System. out205. println ("because the generic type checks its type information in the compiler (therefore, createNumber is called when the Collection <Serializable> collection4 = createNumber (); is defined, and this method only performs type check, createNumber is a generic method, so there is no problem during compilation), and its element type information is discarded at runtime, and forced type conversion is performed at runtime, so in the (Collection <? Extends E>) Forced type conversion is actually a (Collection) List, so there is no problem during the runtime. After debugging, you will know, and I thought about it for more than an hour "); 206} 207 208/** 209 * testing generics only maintain the type information of the compiler at runtime, and do not maintain the type information of its elements. 210 * the following code reports an error in java. lang. classCastException: 211 * java. lang. double cannot be cast to java. util. list212 */213 @ Test214 public void testEraseGenericsInfo () {215 System. out. println ("Get List set"); 216 Collection <List> collection5 = createNumber (); 217 for (List list: collection5) {218 System. out. println (list. get (0); 219; 220} 221} 222 223 224 @ Test225 public void testOther () {226 Integer integer = (int) 13.2; 227 String aString = String. valueOf (13.2); 228} 229 230}