Map container-TreeMap and common APIs, Comparator and Comparable interfaces, and treemapcomparator

Map container-TreeMap and common APIs, Comparator and Comparable interfaces, and treemapcomparator

TreeMap and common APIs

① The TreeMap class implements the Map interface by using the red/black tree;

② TreeMap provides an effective means to store key/value pairs in order, and allows quick retrieval;

③ Unlike HashMap, tree ing ensures that its elements are sorted in ascending order by keywords;

④ TreeMap constructor:

A) TreeMap ()

B) TreeMap (Comparator comp)

C) TreeMap (Map m)

D) TreeMap (SortedMap sm)

⑤ TreeMap implements SortedMap and extends AbstractMap. It does not define other methods;

1 TreeMap <String, String> tmap = new TreeMap <String, String> (); 2 tmap. put ("zhang", "zhang San"); 3 tmap. put ("jack", "James"); 4 tmap. put ("mary", "Xiaohong"); 5 tmap. put ("free", "lobular"); 6 tmap. put ("mary", "Grass"); 7 // tmap. put (null, "Grass"); // The key cannot be null, and an exception 8 System is thrown. out. println (tmap); 9

By default, the output is in ascending order of keys.

Output result:

{Free = Xiaoye, jack = James, mary = xiaocao, zhang = James}

 

Output all key-value pairs

1     Set<Entry<String,String>> entrys=tmap.entrySet();2         for(Entry<String,String> entry:entrys){3             System.out.println(entry.getKey()+"--"+entry.getValue());4         }

Output result:

Free -- Xiaoye

Jack -- James

Mary -- xiaocao

Zhang -- zhang San

 

Compile a Person class

 1 class Person{ 2     private String name; 3     private int age; 4     public Person(String name, int age) { 5         super(); 6         this.name = name; 7         this.age = age; 8     } 9     public String getName() {10         return name;11     }12     public void setName(String name) {13         this.name = name;14     }15     public int getAge() {16         return age;17     }18     public void setAge(int age) {19         this.age = age;20     }21 }

Create and add elements in the main method, and output

1 TreeMap <Person, String> pdata = new TreeMap <Person, String> (); 2 pdata. put (new Person ("zhangsan", 20), "Zhang San"); 3 pdata. put (new Person ("lisi", 25), "Li Si"); 4 pdata. put (new Person ("wangwu", 30), "Wang Wu"); 5 pdata. put (new Person ("zhangsan", 33), "Zhang San"); 6 System. out. println (pdata );

Running result:

Error because the Person class is not implementedcompareTo(T o)Comparison Method

 

Method 1: implement the Comparable Interface

1 class Person implements Comparable <Person> {2 3}

The rewrite method is as follows: sort by age

1     @Override2     public int compareTo(Person o) {3         if (this.age - o.getAge() > 0) {4             return 1;5         } else if (this.age - o.getAge() < 0) {6             return -1;7         }8         return 0;9     }

Then execute the command and output the result:

{Com. iotek. map. person @ 2a139a55 = James, com. iotek. map. person @ 15db9742 = Li Si, com. iotek. map. person @ 6d06d69c = Wang Wu, com. iotek. map. person @ 7852e922 = James 3}

 

The complete Person class is as follows:

1 class Person implements Comparable <Person> {2 private String name; 3 private int age; 4 5 public Person (String name, int age) {6 super (); 7 this. name = name; 8 this. age = age; 9} 10 11 public String getName () {12 return name; 13} 14 15 public void setName (String name) {16 this. name = name; 17} 18 19 public int getAge () {20 return age; 21} 22 23 public void setAge (int age) {24 this. age = age; 25} 26 27 @ Override28 public int compareTo (Person o) {29 if (this. age-o. getAge ()> 0) {30 return 1; 31} else if (this. age-o. getAge () <0) {32 return-1; 33} 34 return 0; 35} 36}View Code

 

Method 2: Do not use the Comparable interface in Person. Use the anonymous internal class in the main method:

 1 TreeMap<Person, String> pdata = new TreeMap<Person, String>(new Comparator<Person>() { 2  3             @Override 4             public int compare(Person o1, Person o2) { 5                 if(o1.getAge()>o2.getAge()){ 6                     return 1; 7                 } 8                 else if(o1.getAge()<o2.getAge()){ 9                     return -1;10                 }11                 return 0;12             }13         });

The output result is the same as the preceding one.

 

The preceding steps use int age for sorting. The String name sorting method is as follows:

1         @Override2             public int compare(Person o1, Person o2) {3                 return o1.getName().compareTo(o2.getName());4             }

Output result:

{Com. iotek. map. Person @ 2a139a55 = Li Si, com. iotek. map. Person @ 15db9742 = Wang Wu, com. iotek. map. Person @ 6d06d69c = Zhang San}

Because zhangsan is the same, it is replaced. The modification is as follows:

1 @ Override 2 public int compare (Person o1, Person o2) {3 if (o1.getName (). compareTo (o2.getName ()> 0) {4 return 1; 5} 6 else if (o1.getName (). compareTo (o2.getName () <0) {7 return-1; 8} 9 else {10 // The same age will still replace 11 return o1.getAge ()-o2.getAge (); 12} 13}

Comparator and Comparable Interfaces

① The key of TreeMap stores reference data and must meet certain conditions.

A) either reference type to implement the Comparable Interface

B) either provide a Comparator object for the TreeMap container to implement the Comparator Interface