Use of collection

Common methods

Modifier and Type	Method and Description
boolean	add(E e) Make sure that this collection contains the specified elements （ Optional operation ）.
boolean	addAll(Collection<? extends E> c) Add all elements of the specified collection to this collection （ Optional operation ）.
void	clear() Remove all elements from this collection （ Optional operation ）.
boolean	contains(Object o) If this collection contains the specified element , Then return to true .
boolean	containsAll(Collection<?> c) If this collection contains the specified aggregate All elements in , Then return to true.
boolean	equals(Object o) Compares the specified object with this collection for equality .
int	hashCode() Returns the hash code value of this collection .
boolean	isEmpty() If this collection does not contain elements , Then return to true .
Iterator<E>	iterator() Returns the iterator of the elements in this collection .
default Stream<E>	parallelStream() Returns possible parallel Stream With this collection as its source .
boolean	remove(Object o) Remove a single instance of the specified element from the collection （ If there is ）（ Optional operation ）.
boolean	removeAll(Collection<?> c) Deletes all elements of the specified collection contained in the specified collection （ Optional operation ）.
default boolean	removeIf(Predicate<? super E> filter) Delete all elements of this collection that satisfy the given predicate .
boolean	retainAll(Collection<?> c) Keep only the elements contained in this collection in the specified collection （ Optional operation ）.
int	size() Returns the number of elements in this collection .
default Spliterator<E>	spliterator() Create a Spliterator The elements in this collection .
default Stream<E>	stream() Returns the order in which this collection is the source Stream .
Object[]	toArray() Returns an array containing all the elements in this collection .
<T> T[]	toArray(T[] a) Returns an array containing all the elements in this collection ; The runtime type of the specified array .

Demonstration of general methods

package  aggregate ;

import java.util.ArrayList;
import java.util.Collection;

public class CollectionDemo {
    
	public static void main(String[] args) {
    
		Collection coll = new ArrayList();
		Collection c = new ArrayList();
		// Additive elements 
		coll.add(" watermelon ");
		coll.add(" strawberry ");
		coll.add(" Apple ");
		c.addAll(coll);
		// Output all elements 
		System.out.println("====== Output all elements ========");
		System.out.println(coll);
		System.out.println(c);
		System.out.println(c);
		// Element number 
		System.out.println("======== Element number ===========");
		System.out.println(coll.size());
		System.out.println(c.size());
		// Contains elements 
		System.out.println("======= Determine whether it contains elements =======");
		System.out.println(coll.contains(" watermelon "));
		System.out.println(coll.isEmpty());
		System.out.println(coll.containsAll(c));
		c.remove(" Apple ");
		System.out.println(coll.equals(c));
		System.out.println("====== Remove elements ===========");
		coll.removeAll(c);
		System.out.println(coll);
		c.clear();
		System.out.println(c.size());
	}
}

A demonstration of the iteration of elements within a collection

The two methods

• enhance for loop

• Using the iterator method iterator() To operate

  iterator() Returns all the elements in the collection as iterators , After that, the operation on the iterator is the operation on the collection ;

There are three important functions in iterators

boolean	hasNext() If the iteration has more elements , Then return to true .
E	next() Returns the next element in the iteration .
default void	remove() Remove the last element returned by this iterator from the underlying collection （ Optional operation ）.

package  aggregate ;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;

public class CollectionDemo {
    
	public static void main(String[] args) {
    
		Collection coll = new ArrayList();
		// Additive elements 
		coll.add(" watermelon ");
		coll.add(" strawberry ");
		coll.add(" Apple ");
		System.out.println("======= enhance for loop =======");
		for (Object object : coll) {
    
			String o = (String)object;
			System.out.println(o);
		}
		System.out.println("========= Using Iterators =======");
		Iterator its = coll.iterator();
		while(its.hasNext()) {
    
			String o = (String) its.next();
			System.out.println(o);
// coll.remove(" Apple "); // When using iterators , You cannot modify the collection 
			its.remove();
		}
		System.out.println("======= After using the iterator to delete the elements in the collection =======");
		System.out.println(coll.size());
	}
}

As a collection element of an object

package  aggregate ;

import java.util.ArrayList;
import java.util.Collection;

public class Student {
    
	private String name;
	private int age;
	public Student(String name, int age) {
    
		super();
		this.name = name;
		this.age = age;
	}
	public String getName() {
    
		return name;
	}
	public void setName(String name) {
    
		this.name = name;
	}
	public int getAge() {
    
		return age;
	}
	public void setAge(int age) {
    
		this.age = age;
	}
	@Override
	public String toString() {
    
		return "Student [name=" + name + ", age=" + age + "]";
	}
	public static void main(String[] args) {
    
		Collection coll = new ArrayList();
		Student s1 = new Student(" Zhang San ", 22);
		Student s2 = new Student(" Li Si ", 21);
		Student s3 = new Student(" Wang Wu ", 19);
		// Additive elements 
		coll.add(s1);
		coll.add(s2);
		coll.add(s3);
		System.out.println(coll);
		// Element iteration 
		System.out.println(" enhance for loop ");
		for (Object object : coll) {
    
			Student s = (Student) object;
			System.out.println(s);
		}
		// Remove elements 
		System.out.println(coll.size());
		coll.remove(s2);
		System.out.println(coll.size());
		coll.remove(new Student(" Zhang San ", 22));
		System.out.println("new After the same element is deleted :" + coll.size()); // In this way, the elements in the collection cannot be deleted 
	}
}