Generics: a stronger typization in Java5

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I'd like to introduce you a very important forward leap towards a stronger typization in the Java 5 language: generics.

The Apple and Pear class are full working examples.

The TestApplePear class is a NOT working example, to show you the single most advantage from a stronger typization: a run-time error from the compiler.

the article

Comparable is a typical example of generic interface because it has a parameter of type T.

<T> is a declaration of formal parameter of type. After that declaration, T could be used in the code as a known type.

A generic class could be instantiated as in the following examples, and it will be clear that we can't compare pears and apples. Eheheh.

Comparable.java

public interface Comparable<T> {
 
    int compareTo(T o);
 
}

Apple.java

public class Apple implements Comparable<Apple>{
    private int weight;
    private String type;
 
    public Apple(String type, int weight) {
        this.type = type;
        this.weight = weight;
    }
 
    public int compareTo(Apple m){
        // we don't have to use an explicit cast
        return weight - p.weight;
    }
}

Pear.java

public class Pear implements Comparable<Pear>{
    private int weight;
    private String type;
 
    public Pear(String type, int weight){
        this.type = type;
        this.weight = weight;
    }
 
    public int compareTo(Pear p){
        // we don't have to use an explicit cast
        return weight - p.weight;
    }
}

TestApplePear.java

public class TestApplePear{
 
    public static void main (String [] args){
        Apple apple = new Apple("golden",135);
        Pear pear = new Pear("williams",120);
        int comp = pear.compareTo(apple); // compilation error
    }
}

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