Container collection

Collection hierarchy root interface Collection represents a group of objects, which are also referred to as the collection element. Some collection allows duplicate elements, while others do not. Some collection is ordered, while others are unordered. JDK does not provide any direct implementations of this interface: it provides more specific sub-interfaces (such as Set and List) implementation. This interface is typically used to transfer collection, and operate the collection where needed largest universal.

Storing a plurality of data containers

Generics:
parameterized types
generics is only valid at compile time

set subinterface: disordered not duplicate
list subinterface: ordered repeat

method:

add (E e)
an additive element

addAll (Collection <? the extends E> c)
all of the elements in the specified collection to this collection (optional operation).

clear()

移除此 collection 中的所有元素（可选操作）。
Collection collection = new ArrayList();
		collection.add('a');
		collection.add("b");
		collection.add("c");
		System.out.println(collection);//[a, b, c]
		collection.clear();
		System.out.println(collection);//[]

the contains (Object O)
if this collection contains the specified element, it returns true.

Collection c = new ArrayList();
		c.add('a');
		c.add('b');
		c.add('c');
		System.out.println(c.contains('b'));//true

containsAll (Collection <?> c)
if this collection contains all of the elements in the specified collection, it returns true.

// An highlighted block
var foo = 'bar';
Collection c = new ArrayList();
		c.add('a');
		c.add('b');
		c.add('c');		
		Collection c2 = new ArrayList();
		c2.add('a');
		c2.add('b');
		System.out.println(c.containsAll(c2));//true

the equals (Object O)
comparing this collection is equal to the specified object.

Collection c = new ArrayList();
   	c.add('a');
   	c.add('b');
   	c.add('c');		
   	Collection c2 = new ArrayList();
   	c2.add('a');
   	c2.add('b');
   	System.out.println(c.equals(c2));//false

hashCode()
返回此 collection 的哈希码值。

Collection c = new ArrayList();
   	c.add('a');
   	c.add('b');
   	c.add('c');		
Collection c2 = new ArrayList();
   	c2.add('a');
   	c2.add('b');
   	System.out.println(c.hashCode());//126145
   	System.out.println(c2.hashCode());//4066

isEmpty()
如果此 collection 不包含元素，则返回 true。

Collection c = new ArrayList();
	c.add('a');
	c.add('b');
	c.add('c');	
System.out.println(c.isEmpty());//false

iterator()
返回在此 collection 的元素上进行迭代的迭代器
iterator中的方法
boolean iterator .hasNext()
如果仍有元素可以迭代，则返回 true。
E iterator .next()
返回迭代的下一个元素。
void iterator .remove()
从迭代器指向的 collection 中移除迭代器返回的最后一个元素（可选操作）。

Collection c = new ArrayList();
   	c.add("a");
   	c.add("b");
   	c.add("c");
   	System.out.println(c);//[a, b, c]
   	//迭代器
   	Iterator<String> it=c.iterator();
   	while(it.hasNext()){
   		System.out.print(it.next());//abc
   	}
   	it.remove();
   	System.out.println(c);//[a, b]

remove(Object o)
从此 collection 中移除指定元素的单个实例，如果存在的话（可选操作）。
removeAll(Collection<?> c)
移除此 collection 中那些也包含在指定 collection 中的所有元素（可选操作）。

Collection c = new ArrayList();
		c.add("a");
		c.add("b");
		c.add("c");
		c.add("d");
		Collection c2 = new ArrayList();//[b, c]
		c2.add("b");
		c2.add("c");
		System.out.println(c);// [a, b, c, d]
		c.remove("a");
		System.out.println(c);// [b, c, d]
		c.removeAll(c2);
		System.out.println(c);// [d]

retainAll(Collection<?> c)
仅保留此 collection 中那些也包含在指定 collection 的元素（可选操作）。

Collection c = new ArrayList();
   	c.add("a");
   	c.add("b");
   	c.add("c");
   	c.add("d");
   	Collection c2 = new ArrayList();//[b, c]
   	c2.add("b");
   	c2.add("c");
   	System.out.println(c);// [a, b, c, d]		
   	c.retainAll(c2);//true	
   	System.out.println(c);// [b,c]

size ()
Returns the number of elements in this collection.

Collection c = new ArrayList();
		c.add("a");
		c.add("b");
		c.add("c");
		c.add("d");
		System.out.println(c.size());//4

toArray ()
Returns an array of all the elements of the collection.
toArray (T [] A)
Returns an array of all the elements of the collection; the same runtime returned array of the type specified array type

Collection c = new ArrayList();
		c.add("a");
		c.add("b");
		c.add("c");
		c.add("d");
		System.out.println(c);//[a, b, c, d]//系统默认重写了toString方法
		c.toArray();//[a, b, c, d]