Run-Time Type Comparisons Within a Generic Hierarchy : Generic Class Hierarchies « Generics « Java Tutorial

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2. Generics
3. Generic Class Hierarchies

The run-time type information operator instanceof: determines if an object is an instance of a class. The instanceof operator can be applied to objects of generic classes.

class Gen<T> {  
  T ob; 
    
  Gen(T o) {  
    ob = o;  
  }  
  
  T getObject() {  
    return ob;  
  }  
}  
 
class Gen2<T> extends Gen<T> { 
  Gen2(T o) { 
    super(o); 
  } 
} 
 
public class MainClass {  
  public static void main(String args[]) {  
    Gen<Integer> iOb = new Gen<Integer>(88); 
    Gen2<Integer> iOb2 = new Gen2<Integer>(99);  
    Gen2<String> strOb2 = new Gen2<String>("Generics Test");  
 
    if(iOb2 instanceof Gen2<?>){  
      System.out.println("iOb2 is instance of Gen2"); 
    }
    if(iOb2 instanceof Gen<?>){  
      System.out.println("iOb2 is instance of Gen"); 
    }
    if(strOb2 instanceof Gen2<?>){  
      System.out.println("strOb is instance of Gen2"); 
    }
    if(strOb2 instanceof Gen<?>){  
      System.out.println("strOb is instance of Gen"); 
    }
    if(iOb instanceof Gen2<?>){  
      System.out.println("iOb is instance of Gen2"); 
    }
    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 runtime.
//    if(iOb2 instanceof Gen2<Integer>)  
//      System.out.println("iOb2 is instance of Gen2<Integer>"); 
  }  
}

iOb2 is instance of Gen2
iOb2 is instance of Gen
strOb is instance of Gen2
strOb is instance of Gen
iOb is instance of Gen