Wildcard Parameterized Subtyping Example

This section provides a tutorial example on wildcard parameterized subtyping using the putAll() method in the java.util.Hashtable class.

Let's see how wildcard parameterized types are used on JDK generic classes. Here is good example in the generic class, java.util.HashMap, as defined in JDK 1.7:

package java.util;
class Hashtable<K,V> {
   ...
   void putAll(Map<? extends K,? extends V> t) {}
   ...
}

My understanding on putAll() definition is:

The tutorial example below has no compilation errors, because objects provided in the putAll() meet the type requirements:

/* HashtablePutAll.java
 - Copyright (c) 2014, HerongYang.com, All Rights Reserved.
 */
import java.util.*;
import java.awt.*; 
import javax.swing.*; 
class HashtablePutAll {
   public static void main(String[] a) {
      // Hashtable API: 
      //    Class Hashtable<K,V> {
      //       void putAll(Map<? extends K,? extends V> t) {}
      //    }

      Hashtable<Number,Container> hashNumberContainer 
         = new Hashtable<Number,Container>();
      
      // allowed - Integer extends Number & Box extends Container
      HashMap<Integer,Box> mapIntegerBox = new HashMap<Integer,Box>();
      mapIntegerBox.put(new Integer(777), new Box(BoxLayout.X_AXIS));
      hashNumberContainer.putAll(mapIntegerBox); 
      System.out.println("# of entries: "+mapIntegerBox.size());
      
      // not allowed - Component does not extend from Container
      // HashMap<Number,Component> mapNumberComponent 
      //   = new HashMap<Number,Component>();
      // hashNumberContainer.putAll(mapNumberComponent); 
   }
}

But if you uncomment the not allowed lines in the example, you will get compilation errors:

HashtablePutAll.java:26: error: no suitable method found for 
   putAll(HashMap<Number,Component>)
       hashNumberContainer.putAll(mapNumberComponent);
                          ^
    method Map.putAll(Map<? extends Number,? extends Container>) is 
    not applicable
      (argument mismatch; HashMap<Number,Component> cannot be 
      converted to Map<? extends Number,? extends Container>)
    method Hashtable.putAll(Map<? extends Number,? extends Container>)
    is not applicable
      (argument mismatch; HashMap<Number,Component> cannot be 
      converted to Map<? extends Number,? extends Container>)
1 error

Last update: 2014.

Table of Contents

 About This Book

 Installing JDK 1.8 on Windows

 Execution Process, Entry Point, Input and Output

 Primitive Data Types and Literals

 Bits, Bytes, Bitwise and Shift Operations

 Managing Bit Strings in Byte Arrays

 Reference Data Types and Variables

 StringBuffer - The String Buffer Class

 System Properties and Runtime Object Methods

Generic Classes and Parameterized Types

 What Is a Generic Class?

 Using a Generic Class

 Using a Generic Class - Example

 Creating a Generic Class

 Creating a Generic Class - Example

 Bounded Type Parameters

 Raw Type, Generic Type and Parameterized Type

 Parameterized Type and Subtyping

 Wildcard Parameterized Types

 Wildcard Parameterized Type Test

 Wildcard Parameterized Subtyping

Wildcard Parameterized Subtyping Example

 Generic Methods and Type Inference

 Lambda Expressions and Method References

 Execution Threads and Multi-Threading Java Programs

 ThreadGroup Class and "system" ThreadGroup Tree

 Synchronization Technique and Synchronized Code Blocks

 Deadlock Condition Example Programs

 Garbage Collection and the gc() Method

 Outdated Tutorials

 References

 PDF Printing Version