Java Reintroduction-Java Collection

background

Java Collection memo writing. Also, a generics-like writing memo.

References

• Java Tutorial - Collections --Java API --Collection --I don't feel like reading, but let's make a link for the time being ~ ^^; -[[Revised New Edition] Java Pocket Reference --Technical Review](https://www.amazon.co.jp/Java-%E3%83%9D%E3%82%B1%E3%83%83%E3% 83% 88% E3% 83% AA% E3% 83% 95% E3% 82% A1% E3% 83% AC% E3% 83% B3% E3% 82% B9-% E9% AB% 98% E6% B1 % 9F-% E8% B3% A2 / dp / 4774145920)

Verification environment

• JDK 8
• Eclipse Neon

Basic form of Collection

1. List --Managed by Index. So-called general array --Elements can be duplicated
2. Set --Elements are in no particular order --Elements cannot be duplicated
3. Queue / Deque

• Queue --Only the first or last element can be added / deleted

4. Map --Managed by key / value pairs --Keys cannot be duplicated

In the Java Tutorial, it is explained in this figure because it is a Core Collection Interface.

Generics The one who writes like this ... I can't explain it ...

Collection<String> col1 = new ArrayList<>();

See here for generics terms and more. Points of Java Generics

Type Parameter naming convention

ArrayList ArrayList sample code. It works, but it gets angry from Eclipse in the following places ... 「List myList = new ArrayList();」

		// Old usage of List - Raw type List
		List myList = new ArrayList();
		String s1 = "Apple";
		String s2 = "Orange";
		String s3 = "Banana";

		myList.add(s1);
		myList.add(s2);
		myList.add(s3);

		// Size, Print List
		System.out.println("List size : " + myList.size());
		for (Object obj : myList) {
			System.out.println(obj);
		}

		// Sort List
		Collections.sort(myList);
		for (Object obj : myList) {
			System.out.println(obj);
		}
		
		// Using Iterator
		Iterator itr = myList.iterator();
		while (itr.hasNext()) {
			System.out.println((String)itr.next());
		}

Click here for a modern way (Java 7 and above) using Generics (parameterized type). The correct answer is "List <String> myList = new ArrayList <> ();". The point is to thoroughly type-safe at the coding level.

		// New usage of List - Generics
		List<String> myList = new ArrayList<>();
		String s1 = "Apple";
		String s2 = "Orange";
		String s3 = "Banana";

		myList.add(s1);
		myList.add(s2);
		myList.add(s3);
		
		// Using Iterator
		Iterator<String> itr = myList.iterator();
		while (itr.hasNext()) {
			System.out.println((String)itr.next());
		}

HashSet HashSet sample code.

		// Use HashSet (Old Style)
		Set hs = new HashSet();
		hs.add("Hello");
		hs.add("Dolly");
		
		// Using Iterator
		Iterator itr = hs.iterator();
		while (itr.hasNext()) {
			System.out.println((String)itr.next());
		}
		
		// New Style
		Set<String> hs2 = new HashSet<>();

		// Using Iterator
		Iterator<String> itr2 = hs2.iterator();
		while (itr2.hasNext()) {
			System.out.println(itr2.next());
		}

HashMap HashMap sample code.

		Map<Integer, String> map1 = new HashMap<>();
		
		map1.put(1, "Cloud");
		map1.put(2, "Big Data");
		map1.put(3, "Machine Learning");
		
		// key objects
		Set<Integer> keys = map1.keySet();
		for (Object key : keys) {
			System.out.println(key + ": " + map1.get(key));
		}

? Wildcard (unbounded wildcard type) example

	// argument with ? wildcard
	public static void printList(List<?> list) {
		for (Object obj : list ) {
			System.out.println(obj);
		}
	}
	
	public static void main(String[] args) {
		List<String> list1 = new ArrayList<>();
		list1.add("Apple");
		list1.add("Grape");
		printList(list1);
		
		List<Integer> list2 = new ArrayList<>();
		list2.add(100);
		list2.add(300);
		printList(list2);
	}

Class / Method example using Generic Type

public class GenericTypeClassSample<T> {
	T height;
	T width;
	
	public GenericTypeClassSample(T width, T height) {
		super();
		this.width = width;
		this.height = height;
	}

	public T getWidth() {
		return width;
	}

	public void setWidth(T width) {
		this.width = width;
	}

	public T getHeight() {
		return height;
	}

	public void setHeight(T height) {
		this.height = height;
	}

	public static void main(String[] args) {
		GenericTypeClassSample<Integer> rect1 = new GenericTypeClassSample<>(10, 5);
		System.out.println("Area : " + rect1.getHeight() * rect1.getWidth());

		GenericTypeClassSample<Double> rect2 = new GenericTypeClassSample<>(4.3, 5.7);
		System.out.println("Area : " + rect2.getHeight() * rect2.getWidth());

	}
}