chap14.code

These files contain all of the code listings in Java: The Complete Reference, J2SE 5 Edition

 
// Now, implement MinMax 
class MyClass<T extends Comparable<T>> implements MinMax<T> { 
  T[] vals; 
 
  MyClass(T[] o) { vals = o; } 
 
  // Return the minimum value in vals. 
  public T min() { 
    T v = vals[0]; 
 
    for(int i=1; i < vals.length; i++) 
      if(vals[i].compareTo(v) < 0) v = vals[i]; 
 
    return v; 
  } 
 
  // Return the maximum value in vals. 
  public T max() { 
    T v = vals[0]; 
 
    for(int i=1; i < vals.length; i++) 
      if(vals[i].compareTo(v) > 0) v = vals[i]; 
 
    return v; 
  } 
} 
 
class GenIFDemo { 
  public static void main(String args[]) { 
    Integer inums[] = {3, 6, 2, 8, 6 }; 
    Character chs[] = {'b', 'r', 'p', 'w' }; 
 
    MyClass<Integer> iob = new MyClass<Integer>(inums); 
    MyClass<Character> cob = new MyClass<Character>(chs); 
 
    System.out.println("Max value in inums: " + iob.max()); 
    System.out.println("Min value in inums: " + iob.min()); 
 
    System.out.println("Max value in chs: " + cob.max()); 
    System.out.println("Min value in chs: " + cob.min()); 
  } 
}

listing 11
// Demonstrate a raw type. 
class Gen<T> {  
  T ob; // declare an object of type T  
    
  // Pass the constructor a reference to   
  // an object of type T.  
  Gen(T o) {  
    ob = o;  
  }  
  
  // Return ob.  
  T getob() {  
    return ob;  
  }  
}  
  
// Demonstrate raw type. 
class RawDemo {  
  public static void main(String args[]) {  
 
    // Create a Gen object for Integers. 
    Gen<Integer> iOb = new Gen<Integer>(88);  
   
    // Create a Gen object for Strings. 
    Gen<String> strOb = new Gen<String>("Generics Test");  
  
    // Create a raw-type Gen object and give it 
    // a Double value. 
    Gen raw = new Gen(new Double(98.6)); 
 
    // Cast here is necessary because type is unknown. 
    double d = (Double) raw.getob(); 
    System.out.println("value: " + d); 
 
    // The use of a raw type can lead to run-time. 
    // exceptions.  Here are some examples. 
 
    // The following cast causes a run-time error! 
//    int i = (Integer) raw.getob(); // run-time error 
 
    // This assigment overrides type safety. 
    strOb = raw; // OK, but potentially wrong 
//    String str = strOb.getob(); // run-time error  
     
    // This assingment also overrides type safety. 
    raw = iOb; // OK, but potentially wrong 
//    d = (Double) raw.getob(); // run-time error 
  }  
}

listing 12
// A simple generic class hierarchy. 
class Gen<T> {  
  T ob; 
    
  Gen(T o) {  
    ob = o;  
  }  
  
  // Return ob.  
  T getob() {  
    return ob;  
  }  
}  
 
// A subclass of Gen. 
class Gen2<T> extends Gen<T> { 
  Gen2(T o) { 
    super(o); 
  } 
}

listing 13
// A subclass can add its own type parameters. 
class Gen<T> {  
  T ob; // declare an object of type T  
    
  // Pass the constructor a reference to   
  // an object of type T.  
  Gen(T o) {  
    ob = o;  
  }  
  
  // Return ob.  
  T getob() {  
    return ob;  
  }  
}  
 
// A subclass of Gen that defines a second 
// type parameter, called V. 
class Gen2<T, V> extends Gen<T> { 
  V ob2; 
 
  Gen2(T o, V o2) { 
    super(o); 
    ob2 = o2; 
  } 
 
  V getob2() { 
    return ob2; 
  } 
} 
  
// Create an object of type Gen2. 
class HierDemo {  
  public static void main(String args[]) {  
    
    // Create a Gen2 object for String and Integer. 
    Gen2<String, Integer> x = 
      new Gen2<String, Integer>("Value is: ", 99);  
 
    System.out.print(x.getob()); 
    System.out.println(x.getob2()); 
  }  
}

listing 14
// A nongeneric class can be the superclass 
// of a generic subclass. 
 
// A nongeneric class. 
class NonGen { 
  int num; 
 
  NonGen(int i) { 
    num = i; 
  } 
 
  int getnum() { 
    return num; 
  } 
} 
 
// A generic subclass. 
class Gen<T> extends NonGen {  
  T ob; // declare an object of type T  
    
  // Pass the constructor a reference to   
  // an object of type T.  
  Gen(T o, int i) {  
    super(i); 
    ob = o;  
  }  
  
  // Return ob.  
  T getob() {  
    return ob;  
  }  
}  
  
// Create a Gen object. 
class HierDemo2 {  
  public static void main(String args[]) {  
    
    // Create a Gen object for String. 
    Gen<String> w = new Gen<String>("Hello", 47); 
    
    System.out.print(w.getob() + " "); 
    System.out.println(w.getnum()); 
  }  
}

listing 15
// Use the instanceof operator with a generic class hierarchy.  
class Gen<T> {   
  T ob;  
     
  Gen(T o) {   
    ob = o;   
  }   
   
  // Return ob.   
  T getob() {   
    return ob;   
  }   
}   
  
// A subclass of Gen.  
class Gen2<T> extends Gen<T> {  
  Gen2(T o) {  
    super(o);  
  }  
}  
  
// Demonstrate run-time type ID implications of generic 
// class hierarchy. 
class HierDemo3 {   
  public static void main(String args[]) {   
     
    // Create a Gen object for Integers.  
    Gen<Integer> iOb = new Gen<Integer>(88);  
  
    // Create a Gen2 object for Integers.  
    Gen2<Integer> iOb2 = new Gen2<Integer>(99);   
    
    // Create a Gen2 object for Strings.  
    Gen2<String> strOb2 = new Gen2<String>("Generics Test");   
  
    // See if iOb2 is some form of Gen2. 
    if(iOb2 instanceof Gen2<?>)   
      System.out.println("iOb2 is instance of Gen2");  
 
    // See if iOb2 is some form of Gen. 
    if(iOb2 instanceof Gen<?>)   
      System.out.println("iOb2 is instance of Gen");  
  
    System.out.println();  
  
    // See if strOb2 is a Gen2. 
    if(strOb2 instanceof Gen2<?>)   
      System.out.println("strOb is instance of Gen2");  
  
    // See if strOb2 is a Gen. 
    if(strOb2 instanceof Gen<?>)   
      System.out.println("strOb is instance of Gen");  
 
    System.out.println();  
  
    // See if iOb is an instance of Gen2, which it is not. 
    if(iOb instanceof Gen2<?>)   
      System.out.println("iOb is instance of Gen2");  
  
    // See if iOb is an instance of Gen, which it is. 
    if(iOb instanceof Gen<?>)   
      System.out.println("iOb is instance of Gen");  
  
    // The following can't be compiled because  
    // generic type info does not exist at run-time. 
//    if(iOb2 instanceof Gen2<Integer>)   
//      System.out.println("iOb2 is instance of Gen2<Integer>");  
  }   
}

listing 16
// Overriding a generic method in a generic class. 
class Gen<T> {  
  T ob; // declare an object of type T  
    
  // Pass the constructor a reference to   
  // an object of type T.  
  Gen(T o) {  
    ob = o;  
  }  
  
  // Return ob.  
  T getob() {  
    System.out.print("Gen's getob(): " ); 
    return ob;  
  }  
}  
 
// A subclass of Gen that overrides getob(). 
class Gen2<T> extends Gen<T> { 
 
  Gen2(T o) { 
    super(o); 
  } 
   
  // Override getob(). 
  T getob() {  
    System.out.print("Gen2's getob(): "); 
    return ob;  
  }  
} 
  
// Demonstrate generic method override. 
class OverrideDemo {  
  public static void main(String args[]) {  
    
    // Create a Gen object for Integers. 
    Gen<Integer> iOb = new Gen<Integer>(88); 
 
    // Create a Gen2 object for Integers. 
    Gen2<Integer> iOb2 = new Gen2<Integer>(99);  
   
    // Create a Gen2 object for Strings. 
    Gen2<String> strOb2 = new Gen2<String>("Generics Test");  
 
    System.out.println(iOb.getob()); 
    System.out.println(iOb2.getob()); 
    System.out.println(strOb2.getob()); 
  }  
}

listing 17
// Here, T is bound by Object by default. 
class Gen<T> {  
  T ob; // here, T will be replaced by Object 
    
  Gen(T o) {  
    ob = o;  
  }  
  
  // Return ob.  
  T getob() {  
    return ob;  
  }  
}  
 
// Here, T is bound by String. 
class GenStr<T extends String> { 
  T str; // here, T will be replaced by String 
 
  GenStr(T o) {  
    str = o;  
  }  
 
  T getstr() { return str; } 
}

listing 18
class GenTypeDemo {  
  public static void main(String args[]) {  
    Gen<Integer> iOb = new Gen<Integer>(99);  
    Gen<Float> fOb = new Gen<Float>(102.2F); 
 
    System.out.println(iOb.getClass().getName()); 
    System.out.println(fOb.getClass().getName()); 
  } 
}

listing 19
// A situation that creates a bridge method. 
class Gen<T> {  
  T ob; // declare an object of type T  
    
  // Pass the constructor a reference to   
  // an object of type T.  
  Gen(T o) {  
    ob = o;  
  }  
  
  // Return ob.  
  T getob() {  
    return ob;  
  }  
}  
 
// A subclass of Gen. 
class Gen2 extends Gen<String> { 
 
  Gen2(String o) { 
    super(o); 
  } 
 
  // A String-specific override of getob(). 
  String getob() { 
    System.out.print("You called String getob(): "); 
    return ob; 
  } 
} 
  
// Demonstrate a situation that requires a bridge method. 
class BridgeDemo {  
  public static void main(String args[]) {  
 
    // Create a Gen2 object for Strings. 
    Gen2 strOb2 = new Gen2("Generics Test");  
 
    System.out.println(strOb2.getob()); 
  }  
}

listing 20
// Ambiguity caused by erasure on  
// overloaded methods. 
class MyGenClass<T, V> {  
  T ob1;  
  V ob2;  
 
  // ... 
 
  // These two overloaded methods are ambiguous 
  // and will not compile. 
  void set(T o) { 
    ob1 = o; 
  } 
 
  void set(V o) { 
    ob2 = o; 
  } 
}

listing 21
// Can't create an instance of T. 
class Gen<T> {  
  T ob;  
  Gen() {  
    ob = new T(); // Illegal!!! 
  }  
} 

listing 22
class Wrong<T> {  
  // Wrong, no static variables of type T. 
  static T ob; 
    
  // Wrong, no static method can use T. 
  static T getob() { 
    return ob; 
  } 
 
  // Wrong, no static method can access object 
  // of type T. 
  static void showob() { 
    System.out.println(ob); 
 } 
}

listing 23
// Generics and arrays. 
class Gen<T extends Number> {  
  T ob;  
 
  T vals[]; // OK 
 
  Gen(T o, T[] nums) {  
    ob = o; 
 
    // This statement is illegal. 
    // vals = new T[10]; // can't create an array of T 
 
    // But, this statement is OK. 
    vals = nums; // OK to assign reference to existent array 
  }  
}  
  
class GenArrays {  
  public static void main(String args[]) {  
    Integer n[] = { 1, 2, 3, 4, 5 };   
 
    Gen<Integer> iOb = new Gen<Integer>(50, n); 
 
    // Can't create an array of type-specific generic references. 
    // Gen<Integer> gens[] = new Gen<Integer>[10]; // Wrong! 
 
    // This is OK. 
    Gen<?> gens[] = new Gen<?>[10]; // OK
  } 
}