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Generic Types

Generic Types

This lesson explains generic types in Java.

We'll cover the following...
1.

Explain generic types?

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Java
class Demonstration {
    public static void main( String args[] ) {
      (new Printer<Integer>()).print(Integer.valueOf(555));
      (new Printer<String>()).print("print away");
      (new Printer<Object>()).print(new Demonstration());
        
    }
}
class Printer<T> {
    void print(T t) {
        System.out.println("printing : " + t.toString());
    }
}
1.

What are generic methods?

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Java
class Demonstration {
    public static void main( String args[] ) {
        // Attempt to create a generic array
        // results in a ClassCastException
        String[] array = Demonstration.<String>createArray(5);
    }
    static <T> T[] createArray(int size) {
        T[] array = (T[]) new Object[size];
        return array;
    }
}

The above code fails because on line 5 there is an implicit cast from Object[] to String[]. Let's look at the erasure of the method to better understand the outcome. The rules of erasure tell us to replace the unbounded parameter T with Object.

Erasure of createArray method

    Object[] createArray(int size) {
        Object[] array = (Object[]) new Object[size];
        return array;
    }
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