Java programming ideology (9) -- Holding Objects (2), java programming ideology

Java programming ideology (9) -- Holding Objects (2), java programming ideology

11) Map

The author said that the ability to map objects to other objects is the best solution to programming problems.

Indeed, for example, to view the distribution of random numbers, if it is really a random number, then 10000 times to generate a random number of less than 20 times, each number should appear in a similar number.

Public class TestMap {public static void main (String [] args) {Map <Integer, Integer> map = new HashMap <Integer, Integer> (); random r = new Random (47); // The System is empty in map. out. println (map. get (1); for (int I = 0; I <10000; I ++) {int j = r. nextInt (10); Integer temp = map. get (j); map. put (j, temp = null? 1: temp + 1);} System. out. println (map) ;}// result: null // {0 = 994, 1 = 1033, 2 = 1010, 3 = 1014, 4 = 958, 5 = 1000, 6 = 1052, 7 = 980, 8 = 946, 9 = 1013}

Numbers are indeed randomly distributed, and Map is also useful.

Map also uses multiple dimensions.

Map<Person,List<TV>>;

12) Queue

First-in-first-out, when buying a fund, because of the additional purchase in the later stage, but the previous fund has been full for one month, so no redemption is required. Then, after asking the customer service, I found that the first buy is redeemed first, then I shot my thigh. Isn't this the so-called queue design?

The Queue List implements the Queue interface.

By the way, Random is often used. I forgot to say why the author used 47 parameter passing. In fact, the parameter passing through Random is the input of a computing seed. The default value is the system time, 47. In his opinion, it has always been a "magic number ".

Public class TestQueue {public static void main (String [] args) {Queue <Integer> q = new Queue list <Integer> (); Random r = new Random (47 ); for (int I = 0; I <10; I ++) {// insert an element to the end of the q. offer (r. nextInt (12);} // returns the System header. out. println (q. peek (); System. out. println (q );}}

1. PriorityQueue

Let's look at the example:

public class TestPriorityQueue {    public static void main(String[] args) {        String s = "What 's your favorite number ,1 or 2?";          List<String> l = Arrays.asList(s.split(""));          PriorityQueue<String> pq = new PriorityQueue<String>(l);          while(pq.peek()!=null){              System.out.print(pq.remove()+" ");          }    }}result：[,  ,  ,  ,  , b,  , 1,  , i, e, n, h, ',  , a, o, 2, ?, y, t, t, o, u, u, m, r, e, r, f, ,, s, a, v, r, o, W, r]                                       ' , 1 2 ? W a a b e e f h i m n o o o r r r r s t t u u v y 

First-level, highest-level, and first-level pop-up. The smallest value in the priority queue has the highest priority. If the value is String, the highest priority is given by spaces.

Direct output is not sorted, which is different from what I expected.

To output the data in sequence, you must use the peek method to return the header of the queue. If it is null, null is returned. If it is removed, the data with a higher priority will be displayed.

 public static void main(String[] args) {        String s = "werwerwer";        List<String> l = Arrays.asList(s.split(""));        PriorityQueue<String> pq = new PriorityQueue<String>(l);        while (pq.peek() != null) {            System.out.print(pq.remove() + " ");        }        pq = new PriorityQueue<String>(l);        System.out.println(pq.peek());        pq.remove();        System.out.println(pq.peek());        pq.remove();        System.out.println(pq.peek());    }result: e e e r r r w w w ee

The result surprised me that the String contains spaces. After reading the string, return to solve the problem.

13) Foreach and iterator

Foreach traversal:

public class Box {    public static void main(String[] args) {        Collection<String> c = new LinkedList<String>();        String s = "you are so great";        Collections.addAll(c,s.split(" "));        for(String string : c){            System.out.println(string);        }    }}

Previously, foreach was introduced by Java SE5 because the Iterable interface was also introduced. The Iterator method of Iterator is generated by the interface, and Iterable is used by Foreach to move in the sequence. Therefore, the classes that implement the Iterable interface can be used for Foreach statements.

All Known Subinterfaces: BeanContext, BeanContextServices, BlockingDeque<E>, BlockingQueue<E>, Collection<E>, Deque<E>, DirectoryStream<T>, List<E>, NavigableSet<E>, Queue<E>, Set<E>, SortedSet<E> 

In fact, there are so many Iterable sub-interfaces, the next implementation of the Collection interface will naturally implement the Iterable interface, so it is also applicable.

Summary:

First look at the container classification diagram:

The dot line is an interface, the solid line is a specific class, the hollow arrow refers to the implementation interface, and the solid arrow refers to the object of the class that a class can generate the arrow points.

1. Arrays can store objects and basic types of data. They can be multi-dimensional, that is, the capacity cannot be changed.

2. Collection stores a single element, and Map stores key-value pairs.

3. The List and array are similar, but the List can automatically expand the capacity. ArrayList is used for a large number of random access requests, and insertion and deletion operations are often performed.

public class Box {    public static void main(String[] args) {        long start = System.currentTimeMillis();       Random r = new Random(47);       LinkedList<Integer> l = new LinkedList<Integer>();       for(int i = 0; i< 1000000; i++){           l.add(r.nextInt(1000));       }       long end = System.currentTimeMillis();       System.out.println(end - start);             ArrayList<Integer> a = new ArrayList<Integer>();      for(int i = 0; i< 1000000; i++){          a.add(r.nextInt(1000));      }      long end2 = System.currentTimeMillis();      System.out.println(end2 -end);    }}

4. Map and HashMap are used for fast access. TreeMap keeps the key sorting, and the speed is not as fast as HashMap. LinkedHashMap keeps the element sorting, and also supports fast access through hash.

5. Set elements are not repeated. TreeSet, HashSet, and LinkedHashSet are similar to Map.

In the end, you will find that these containers can store objects in an infinite size and help you count them. How are these magical classes designed? This is the importance of learning the data structure well.

Which version of Thinking in java is better ?? Is it second, third, or fourth?

I have read the second edition. If you are a newbie, I suggest you read Lin Xinliang's <java Study Notes>. Tsinghua University Press

Is Java programming thought suitable for me?

It seems to be the same person. I started to learn java in my sophomore year. I suggest you do not read this book. I have read it. It is not difficult to write or translate it, we recommend that you read Sun weiqin's new version of "java object-oriented programming", which is simple and clear, with clear examples. I believe it will certainly help you.
Remember to learn the data structure well !! But do not learn the java data structure. I personally think the data structure of c ++ is better than that of c, object-oriented, better than java, and Operation pointer.