Using a Generic Class - Example

This section provides a tutorial example on using a generic class in 4 areas: calling the constructor, declaring a reference variable, passing an argument to methods and handling the returning reference from methods.

Let's see a tutorial example program that uses generic classes provided as part of JDK:

/* UsingGenericClass.java
 - Copyright (c) 2014, HerongYang.com, All Rights Reserved.
 */
import java.util.*;
class UsingGenericClass {
   public static void main(String[] a) {
      java.io.PrintStream o = System.out;

      // calling constructors
      new java.util.Stack<double[]>();
      new java.util.ArrayList<String>();
      new java.util.Vector<Number>(200);

      // declaring reference variables
      java.util.Stack<double[]> stackForDoubleArray; 
      java.util.ArrayList<String> arrayListForString;
      java.util.Vector<Number> vectorForNumber;

      vectorForNumber = new java.util.Vector<Number>(200);

      // passing arguments
      vectorForNumber.add(new Double(9.99)); 
         // allowed - Double is compatible with Number 
      
      // vectorForNumber.add(new String("9.99"));
         // not allowed - String is not compatible with Number
   
      // receiving returns
      Object firstTry = vectorForNumber.firstElement(); 
         // allowed - Return type Number can be up casted to Object
      
      // Double secondTry = vectorForNumber.firstElement(); 
         // not allowed - Double is not compatible with Number

      o.println("First element: "+(Number) firstTry);
   }
}

Compile and run the example, you will get:

First element: 9.99

The output proves that I used generic classes correctly in the example program. Now if you uncomment those 2 not allowed statements, you should see 2 compilation errors:

UsingGenericClass.java:25: error: no suitable method found for
   add(String)
      vectorForNumber.add(new String("9.99"));
                     ^
    method Collection.add(Number) is not applicable
      (argument mismatch; String cannot be converted to Number)
    method List.add(Number) is not applicable
      (argument mismatch; String cannot be converted to Number)
    method AbstractCollection.add(Number) is not applicable
      (argument mismatch; String cannot be converted to Number)
    method AbstractList.add(Number) is not applicable
      (argument mismatch; String cannot be converted to Number)
    method Vector.add(Number) is not applicable
      (argument mismatch; String cannot be converted to Number)

UsingGenericClass.java:32: error: incompatible types: Number cannot 
   be converted to Double
      Double secondTry = vectorForNumber.firstElement();
                                                     ^
Note: Some messages have been simplified; recompile with 
   -Xdiags:verbose to get full output
2 errors

The first error says that the parameterized class represented by vectorForNumber supports "add(Number)" method. But error statement tries to invoke it with "add(String)".

The second error says that the parameterized class represented by vectorForNumber supports "Number firstElement()" method. But error statement tries to cast the returning type "Number" to "Double".

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