GenericsExamples.java

package concept.examples.generics;

import java.util.ArrayList;

public class GenericsExamples {

	static ArrayList multiplyNumbersBy2(ArrayList numbers) {
		ArrayList result = new ArrayList();

		for (int i = 0; i < numbers.size(); i++) {
			// TYPE CAST is required
			int value = (Integer) numbers.get(i);
			result.add(value * 2);
		}

		return result;
	}

	static void addElement(ArrayList something) {
		something.add(new String("String"));
	}

	public static void main(String[] args) {

		// Older code - Before Java 5
		ArrayList<Integer> numbers = new ArrayList<>();
		numbers.add(5);
		numbers.add(6);

		// javac gives warning because multiplyNumbersBy2(ArrayList) is invoked
		// with a Specific ArrayList<Integer>
		// and in the method multiplyNumbersBy2 an element is added to ArrayList
		// com/rithus/generics/GenericsExamples.java uses unchecked or unsafe
		// operations.
		// Recompile with -Xlint:unchecked for details.

		System.out.println(multiplyNumbersBy2(numbers));// [10, 12]

		// javac gives warning because addElement(ArrayList) is invoked with a
		// Specific ArrayList<Integer>
		// and in the method addElement, an element is added to the list.
		// com/rithus/generics/GenericsExamples.java uses unchecked or unsafe
		// operations.
		addElement(numbers);

		// Throws runtime exception - java.lang.ClassCastException
		System.out.println(multiplyNumbersBy2(numbers));

		// New code - After

	}

	// javac -Xlint:unchecked com/rithus/generics/GenericsExamples.java
	// com/rithus/generics/GenericsExamples.java:14: warning: [unchecked]
	// unchecked call to add(E) as a member of the raw type java.util.ArrayList
	// result.add(value * 2);
	// ^
	// com/rithus/generics/GenericsExamples.java:21: warning: [unchecked]
	// unchecked call to add(E) as a member of the raw type java.util.ArrayList
	// something.add(new String("String"));
	// ^
	// 2 warnings

}