如何在Java中迭代 TreeMap？

给定一个 TreeMap，任务是在Java中迭代这个 TreeMap。 Java中的TreeMap用于实现Map接口和NavigableMap以及抽象类。我们不能直接使用迭代器来迭代 TreeMap，因为 TreeMap 不是一个集合。所以我们将不得不使用 TreeMap.entrySet() 方法。此方法返回包含在此树形图中的映射的集合视图（Set）。所以我们可以使用 Map.Entry 的 getKey() 和 getValue() 方法迭代键值对。此方法最常见，如果您在循环中需要映射键和值，则应使用此方法。

示例 1：

Java

// Java program to iterate over a TreeMap
 
import java.util.Map;
import java.util.TreeMap;
 
class IterationDemo {
    public static void main(String[] arg)
    {
        Map gfg
            = new TreeMap();
 
        // enter name/url pair
        gfg.put("GFG", "geeksforgeeks.org");
        gfg.put("Practice", "practice.geeksforgeeks.org");
        gfg.put("Code", "code.geeksforgeeks.org");
        gfg.put("Quiz", "quiz.geeksforgeeks.org");
 
        // using for-each loop for
        // iteration over TreeMap.entrySet()
        for (Map.Entry
                 entry : gfg.entrySet())
            System.out.println(
                "[" + entry.getKey()
                + ", " + entry.getValue() + "]");
    }
}

输出：

[Code, code.geeksforgeeks.org]
[GFG, geeksforgeeks.org]
[Practice, practice.geeksforgeeks.org]
[Quiz, quiz.geeksforgeeks.org]

现在让我们看看遍历 TreeMap 对象中的条目。为了实现，我们正在考虑非常简单的地图元素关联性，其中我们有三个元素，它们分别是“Geeks”、“for”、“Geeks”，并且是键值 '1'、'2' 和 '3'整数类型。所以只有从这里我们才能得到我们需要创建一个 TreeMap 类的对象。

示例 2：

Java

// Java Program to Iterate Over Entries in a TreeMap
 
import java.util.*;
 
// Importing required
// Main class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
 
        // Creating a TreeMap class object
        // Objects are of key-value pairs (integer and
        // string type)
        TreeMap tm
            = new TreeMap();
 
        // Customly adding elements
        tm.put(1, "Geeks");
        tm.put(2, "For");
        tm.put(3, "Geeks");
 
        //  Get all entries using the entrySet() method
        Set > entries
            = tm.entrySet();
 
        // Way 1
        // Using for loops
        for (Map.Entry entry : entries) {
            System.out.println(entry.getKey() + "->"
                               + entry.getValue());
        }
 
        // New line to differentiate differences in output
        // between for loop and for each loop
        System.out.println();
 
        // Way 2 - getting code shorter and simpler
        // For each loops
 
        entries.forEach(entry -> {
            System.out.println(entry.getKey() + "->"
                               + entry.getValue());
        });
 
        // New line to differentiate differences in output
        // between for each loop and iterator traversal
        System.out.println();
 
        // Way 3 - New way to
        // Getting an iterator
        Iterator > iterator
            = entries.iterator();
 
        // Additional step here
        // To Initialize object holding for
        // key-value pairs to null
        Map.Entry entry = null;
 
        // Holds true till there is no element remaining in
        // the object using hasNExt() method
        while (iterator.hasNext()) {
 
            // Moving onto next pairs using next() method
            entry = iterator.next();
 
            // Printing the key-value pairs
            // using getKet() and getValue() methods
            System.out.println(entry.getKey() + "->"
                               + entry.getValue());
        }
    }
}

输出

1->Geeks
2->For
3->Geeks

1->Geeks
2->For
3->Geeks

1->Geeks
2->For
3->Geeks