Collection and method of traversing collection elements (1)

Catalog

One 、 An overview of the set framework

Two 、 Collections framework

3、 ... and 、Collection Testing of methods declared in the interface

Four 、JDK 5.0 Added foreach loop , Used to traverse a collection , Array

5、 ... and 、 Using Iterators Interator Interface traverses collection elements

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One 、 An overview of the set framework

1. aggregate 、 Arrays are structures that store multiple data , abbreviation java Containers 
  explain ： The storage at this time , Mainly refers to memory level storage , It doesn't involve persistent storage （.txt .jpg .avi  In the database ）

2.1  The characteristics of arrays in storing multiple data ：
      >— Once initialized , The length is determined .
      > Once the array is defined , The type of its elements is determined . We can only manipulate the data of the specified type . such as : String[] arr;int[] arr1;
2.2  The disadvantage of arrays in storing multiple data ：
      >— Once initialized , Its length cannot be modified 
      > The methods provided in the array are very limited , For adding 、 Delete 、 Inserting data is very inconvenient , The efficiency is not high 
      > There is no ready-made attribute or method in the array to represent the number of stored elements 
      > The characteristics of storing data in arrays ： Orderly 、 repeatable . For unordered and unrepeatable requirements , Can't meet 

Two 、 Collections framework

 /----Collection Interface ： Single column set , Objects used to store one by one 
       /----List Interface ： Stored in order , Repeatable data （ The dynamic array ）
              /----ArrayList、LinkedList、Vector
       /----Set Interface ： Disordered storage , Non repeatable data （ Similar to the collection of high school ）
              /----HashSet、LinkedHashSet、TreeSet
/----Map Interface ： Double column set , Used to store one-to-one data .（key、value）（ function ）
              /----HashMap、LinkedHashMap、TreeMap、Hashtable、Properties

3、 ... and 、Collection Testing of methods declared in the interface

 requirement ：Collection The implementation class of the interface needs to be rewritten equals(), Determine whether an element is included , Remove elements , lookup , All need to use 

  contains(Object obj): Determine whether the set contains this obj
  containsAll(Collection coll1): Judging formal parameters coll1 Whether all elements in are in the current collection 
  remove(Object obj): Delete... From the current collection obj Elements 
  removeAll(Collection coll1): Difference set ： Delete... From the current collection coll1 All elements in 
  retainAll(Collection coll1): intersection ： Keep the current set and coll1 Common elements 
  equals(Collection coll1): Determine whether the elements of two sets are equal , We need to consider whether the set is ordered or unordered 
  hashCode(): Returns the hash value of the current object 
  toArray()： aggregate ---> Array 
   Array ---> aggregate ： call Arrays Class asList Static methods 

public class CollectionTest1 {
    @Test
    public void test1() {
        Collection c = new ArrayList();
        c.add(123);
        c.add(456);
        c.add(new String("haiyu"));
        c.add(new Person(12, "haiuyu"));
        c.add(false);

        boolean contains = c.contains(123);
        System.out.println(contains);//true

        System.out.println(c.contains(new String("haiyu")));//true
        System.out.println(c.contains(new Person(12, "haiuyu")));//false
//         summary ：contains amount to equals,String Rewritten in equals, Compared to the content . The custom class has not been overridden .

//        containsAll(Collection coll1): Judging formal parameters coll1 Whether all elements in are in the current collection 
        Collection coll1 = Arrays.asList(123, 456);
        boolean b = c.containsAll(coll1);
        System.out.println(b);//true

        System.out.println("*************");
        c.remove(123);
        System.out.println(c);

        c.removeAll(coll1);
        System.out.println(c);
    }


    @Test
    public void test2() {
        Collection c = new ArrayList();
        c.add(123);
        c.add(456);
        c.add(new String("haiyu"));
        c.add(new Person(12, "haiuyu"));
        c.add(false);

        Collection coll1 = Arrays.asList(123, 456, 38383, 3);

        c.retainAll(coll1);
        System.out.println(c);

        System.out.println(c.hashCode());//5230

        Object[] a = c.toArray();
        for (int i = 0; i < a.length; i++) {
            System.out.println(a[i]);
        }
//         Array ---> aggregate ： call Arrays Class asList Static methods 
        List<String> list = Arrays.asList(new String[]{"aa", "bb", "cc"});//list Namely collection
        System.out.println(list);

        List<int[]> ints = Arrays.asList(new int[]{123, 456});
        System.out.println(ints.size());//1

        List<Integer> integers = Arrays.asList(new Integer[]{123, 456});
        System.out.println(integers.size());//2
    }
}

Four 、JDK 5.0 Added foreach loop , Used to traverse a collection , Array

public class ForTest {
    @Test
    public void test1() {
        Collection c = new ArrayList();
        c.add(123);
        c.add(456);
        c.add(new String("haiyu"));
        c.add(new Person(12, "haiuyu"));
        c.add(false);
        //for( The type of collection element   local variable ： A collection of objects )           for( The type of array element   local variable  :  The array itself )
        // Iterators that are still called internally 
        for (Object obj:c){
            System.out.println(obj);
        }
    }

5、 ... and 、 Using Iterators Interator Interface traverses collection elements

1、 There are two internal approaches ：hasNext()   next()
2、 There's another one inside remove() Method , It can be traversed , Delete elements from the collection , This method is different from a collection calling directly remove()

public class IteratorTest {
    @Test
    public void test1(){
        Collection c = new ArrayList();
        c.add(123);
        c.add(456);
        c.add(new String("haiyu"));
        c.add(new Person(12,"haiuyu"));
        c.add(false);

        Iterator iterator = c.iterator();// It can be understood as having a pointer , Point to an empty place before the first element of the collection 
//         Mode one ：
//        System.out.println(iterator.next());
//        System.out.println(iterator.next());
//        System.out.println(iterator.next());
//        System.out.println(iterator.next());
//        System.out.println(iterator.next());
//        System.out.println(iterator.next());         NoSuchElementException
//         Mode two ： Not recommended 
//        for (int i = 0; i < c.size(); i++) {
//            System.out.println(iterator.next());
//        }
//         Mode three ： recommend 
        while (iterator.hasNext()){// It can't be c.iterator.hasNext(), Because every time a new pointer structure appears 
            System.out.println(iterator.next());
        }
    }