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About Prasad Saya

Prasad Saya
Prasad Saya is a software engineer with over ten years’ experience in application development, maintenance, testing and consulting on various platforms. He is a certified Java and Java EE developer. At present his interest is in developing Java applications. He also has experience working with databases and ERP applications.

Generic Classes in Java Example

This article shows creating a generic class. Java generics were introduced with Java SE version 5.

Some commonly used generic classes are defined as collections in Java API; for example the ArrayList. The API javadoc documentation shows the ArrayList definition as public class ArrayList<E> (where E stands for an element type). Generics added the compile-time type safety and eliminated the need to cast when reading elements from collections.

Sample code of the array list usage:

ArrayList<String> list = new ArrayList<>();
list.add("one");
String s = list.get(0);

The code creates an array list of type String and adds an element to it. The code list.get(0) returns the element of type String.

1. An Example

This example shows the defining a generic class. This class defines an instance variable and its getset methods. The class is of generic type. The property, the getset method’s parameter and return type are of the class’s generic parameter type.

The class is Gen1.Java. The class’s type is specified as T (this can be specified as X, Y or any other alphabet; T stands for Type and it is a convention). The code and a class to test the Gen1‘s usage follow.

The example requires Java SE 7.

1.1. The code

Gen1.Java

public class Gen1<T> {

    private T t;

    public  Gen1() {
    }

    public Gen1(T t) {

        this.t = t;
    }

    public void set(T t) {

        this.t = t;
    }

    public T get() {

        return t;
    }
}

Gen1Tester.java

public class Gen1Tester {

    public static void main(String [] args) {

        Gen1<String> g1 = new Gen1<>();

        g1.set("Apple");
        System.out.println(g1.get());

        Gen1<Integer> g2 = new Gen1<>();
        g2.set(1234);
        System.out.println(g2.get());

        // g2.set("Banana");	
    }
}

1.2. The output

Apple
1234

1.3. From the class and the output:

The Gen1 object g1 is created with String type, and g2 is of Integer type. The program was able to set the value and print it for both the instances. The Gen1 class is of generic type so the object is able to accept any type that it is created with.

Consider the commented code on line 14 of Gen1Tester.java. When the code comments are removed and the class is compiled, the compiler will generate an error like this:

Gen1Tester.java:14: error: method set in class Gen1<T> cannot be applied to given types;
                g2.set("Banana");
                  ^
    required: Integer
    found: String
    reason: actual argument String cannot be converted to Integer by method invocation conversion
        where T is a type-variable: T extends Object declared in class Gen1
1 error

This is because the Gen1‘s g2 variable is defined as of type Integer and not a String. The compiler ensured the type safety of the object. Type safety is one of the main features of generics and this makes the software more reliable.

2. Notes

A class that can be typed to whatever the programmer chooses, and the compiler will enforce the type. Within a generic class, the type may be used with:

  • a variable
  • an array
  • a method argument
  • a method’s return type

2.1. Generic class with multiple parameter types

A generic class can be defined with more than one parametrized type:

public class MultiGenParams<X, Y> {

    ...
}

2.2. Generic class with wildcard notation

A generic class can also use a wildcard notation in a class definition. This notation specifies a range (called “bounds”) for the type that can be used for the type parameter.

public class Automobile<T extends Automobile> {

    ...
}

The type T can be any of a Automobile, Car, RaceCar or a Bus(where Car, RaceCar and Bus classes are sublasses of Automobile).

3. Download Java Source Code

This was an example of Generic Classes in Java

Download
You can download the full source code of this example here: GenericsExample.zip
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