Java TreeMap类

Java TreeMap类是基于红黑树的实现。它提供了一种按排序顺序存储键值对的有效方法。

关于Java TreeMap类的要点是：

• Java TreeMap包含基于键的值。它实现了NavigableMap接口并扩展了AbstractMap类。
• Java TreeMap仅包含唯一元素。
• Java TreeMap不能具有null键, 但是可以具有多个null值。
• Java TreeMap不同步。
• Java TreeMap保持升序。

TreeMap类声明

我们来看一下java.util.TreeMap类的声明。

public class TreeMap<K, V> extends AbstractMap<K, V> implements NavigableMap<K, V>, Cloneable, Serializable

TreeMap类参数

让我们看一下java.util.TreeMap类的参数。

• K：这是此地图维护的键的类型。
• V：这是映射值的类型。

Java TreeMap类的构造方法

Java TreeMap类的方法

Java TreeMap示例

import java.util.*;
class TreeMap1{
 public static void main(String args[]){
   TreeMap<Integer, String> map=new TreeMap<Integer, String>();  
	  map.put(100, "Amit");  
	  map.put(102, "Ravi");  
	  map.put(101, "Vijay");  
	  map.put(103, "Rahul");  
	  
	  for(Map.Entry m:map.entrySet()){  
	   System.out.println(m.getKey()+" "+m.getValue());  
	  }  
 }
}

Output:100 Amit
       101 Vijay
       102 Ravi
       103 Rahul

Java TreeMap示例：remove()

import java.util.*;
public class TreeMap2 {
   public static void main(String args[]) {
	TreeMap<Integer, String> map=new TreeMap<Integer, String>();  
	  map.put(100, "Amit");  
	  map.put(102, "Ravi");  
	  map.put(101, "Vijay");  
	  map.put(103, "Rahul");  
	  System.out.println("Before invoking remove() method");
	  for(Map.Entry m:map.entrySet())
	  {
		  System.out.println(m.getKey()+" "+m.getValue());	  
	  }
	  map.remove(102);	  
	  System.out.println("After invoking remove() method");
	  for(Map.Entry m:map.entrySet())
	  {
		  System.out.println(m.getKey()+" "+m.getValue());	  
	  }
	  }
}

输出：

Before invoking remove() method
100 Amit
101 Vijay
102 Ravi
103 Rahul
After invoking remove() method
100 Amit
101 Vijay
103 Rahul

Java TreeMap示例：NavigableMap

import java.util.*;
class TreeMap3{
 public static void main(String args[]){
   NavigableMap<Integer, String> map=new TreeMap<Integer, String>();  
	  map.put(100, "Amit");  
	  map.put(102, "Ravi");  
	  map.put(101, "Vijay");  
	  map.put(103, "Rahul");  
	  //Maintains descending order
	  System.out.println("descendingMap: "+map.descendingMap());
	  //Returns key-value pairs whose keys are less than or equal to the specified key.
	  System.out.println("headMap: "+map.headMap(102, true));
	  //Returns key-value pairs whose keys are greater than or equal to the specified key.
	  System.out.println("tailMap: "+map.tailMap(102, true));
	  //Returns key-value pairs exists in between the specified key.
	  System.out.println("subMap: "+map.subMap(100, false, 102, true)); 
 }
}

descendingMap: {103=Rahul, 102=Ravi, 101=Vijay, 100=Amit}
headMap: {100=Amit, 101=Vijay, 102=Ravi}
tailMap: {102=Ravi, 103=Rahul}
subMap: {101=Vijay, 102=Ravi}

Java TreeMap示例：SortedMap

import java.util.*;
class TreeMap4{
 public static void main(String args[]){
   SortedMap<Integer, String> map=new TreeMap<Integer, String>();  
	  map.put(100, "Amit");  
	  map.put(102, "Ravi");  
	  map.put(101, "Vijay");  
	  map.put(103, "Rahul");  
	  //Returns key-value pairs whose keys are less than the specified key.
	  System.out.println("headMap: "+map.headMap(102));
	  //Returns key-value pairs whose keys are greater than or equal to the specified key.
	  System.out.println("tailMap: "+map.tailMap(102));
	  //Returns key-value pairs exists in between the specified key.
	  System.out.println("subMap: "+map.subMap(100, 102));  
 }
}

headMap: {100=Amit, 101=Vijay}
tailMap: {102=Ravi, 103=Rahul}
subMap: {100=Amit, 101=Vijay}

HashMap和TreeMap有什么区别？

Java TreeMap示例：书

import java.util.*;  
class Book {  
int id;  
String name, author, publisher;  
int quantity;  
public Book(int id, String name, String author, String publisher, int quantity) {  
    this.id = id;  
    this.name = name;  
    this.author = author;  
    this.publisher = publisher;  
    this.quantity = quantity;  
}  
}  
public class MapExample {  
public static void main(String[] args) {  
    //Creating map of Books  
    Map<Integer, Book> map=new TreeMap<Integer, Book>();  
    //Creating Books  
    Book b1=new Book(101, "Let us C", "Yashwant Kanetkar", "BPB", 8);  
    Book b2=new Book(102, "Data Communications & Networking", "Forouzan", "Mc Graw Hill", 4);  
    Book b3=new Book(103, "Operating System", "Galvin", "Wiley", 6);  
    //Adding Books to map 
    map.put(2, b2);
    map.put(1, b1);
    map.put(3, b3);
    
    //Traversing map
    for(Map.Entry<Integer, Book> entry:map.entrySet()){  
    	int key=entry.getKey();
    	Book b=entry.getValue();
        System.out.println(key+" Details:");
        System.out.println(b.id+" "+b.name+" "+b.author+" "+b.publisher+" "+b.quantity); 
    }  
}  
}

输出：

1 Details:
101 Let us C Yashwant Kanetkar BPB 8
2 Details:
102 Data Communications & Networking Forouzan Mc Graw Hill 4
3 Details:
103 Operating System Galvin Wiley 6