代码优化：使用Entry遍历Map

推荐，尤其是容量大时

Map<String,String> map = new HashMap<>();
for (Map.Entry<String,String> entry : map.entrySet()) {
    String key = entry.getKey();
    String value = entry.getValue();
}

避免使用这种方式：

Map<String,String> map = new HashMap<>();
for (String key : map.keySet()) {
    String value = map.get(key);
}

 遍历Map的四种方法：

第一种：通过Map.keySet遍历key和value

for (String key : map.keySet()) {
   System.out.println("key= "+ key + " and value= " + map.get(key));
  }

第二种：通过Map.entrySet使用iterator遍历key和value

Iterator<Map.Entry<String, String>> it = map.entrySet().iterator();
  while (it.hasNext()) {
   Map.Entry<String, String> entry = it.next();
   System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
  }

第三种：通过Map.entrySet遍历key和value，推荐，尤其是容量大时

for (Map.Entry<String, String> entry : map.entrySet()) {
   System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
  }

第四种：通过Map.values()遍历所有的value，但不能遍历key

for (String v : map.values()) {
   System.out.println("value= " + v);
  }

javap比较entrySet（）keySet（）的区别：

public class Test5 {
    public static void main(String[] args) {


        Map<String, String> map = new HashMap<String, String>();
        map.put("1", "value1");
        map.put("2", "value2");
        map.put("3", "value3");

        //第一种：普遍使用，二次取值
        System.out.println("通过Map.keySet遍历key和value：");
        for (String key : map.keySet()) {
            System.out.println("key= "+ key + " and value= " + map.get(key));
        }

        //第三种：推荐，尤其是容量大时
        System.out.println("通过Map.entrySet遍历key和value");
        for (Map.Entry<String, String> entry : map.entrySet()) {
            System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
        }
    }
}

class文件内容如下：

Classfile /D:/project/prism/NMPA/sdjg-rest/target/test-classes/com/ccb/jawa/rest/Test5.class
  Last modified 2022-1-16; size 1735 bytes
  MD5 checksum d0c7a40d5fe9c26751d42005a3ce2c8f
  Compiled from "Test5.java"
public class com.ccb.jawa.rest.Test5
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
    #1 = Methodref          #25.#54       // java/lang/Object."<init>":()V
    #2 = Class              #55           // java/util/HashMap
    #3 = Methodref          #2.#54        // java/util/HashMap."<init>":()V
    #4 = String             #56           // 1
    #5 = String             #57           // value1
    #6 = InterfaceMethodref #58.#59       // java/util/Map.put:(Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
    #7 = String             #60           // 2
    #8 = String             #61           // value2
    #9 = String             #62           // 3
   #10 = String             #63           // value3
   #11 = Fieldref           #64.#65       // java/lang/System.out:Ljava/io/PrintStream;
   #12 = String             #66           // 通过Map.keySet遍历key和value：
   #13 = Methodref          #67.#68       // java/io/PrintStream.println:(Ljava/lang/String;)V
   #14 = InterfaceMethodref #58.#69       // java/util/Map.keySet:()Ljava/util/Set;
   #15 = InterfaceMethodref #70.#71       // java/util/Set.iterator:()Ljava/util/Iterator;
   #16 = InterfaceMethodref #72.#73       // java/util/Iterator.hasNext:()Z
   #17 = InterfaceMethodref #72.#74       // java/util/Iterator.next:()Ljava/lang/Object;
   #18 = Class              #75           // java/lang/String
   #19 = InterfaceMethodref #58.#76       // java/util/Map.get:(Ljava/lang/Object;)Ljava/lang/Object;
   #20 = String             #77           // 通过Map.entrySet遍历key和value
   #21 = InterfaceMethodref #58.#78       // java/util/Map.entrySet:()Ljava/util/Set;
   #22 = Class              #79           // java/util/Map$Entry
   #23 = InterfaceMethodref #22.#80       // java/util/Map$Entry.getValue:()Ljava/lang/Object;
   #24 = Class              #81           // com/ccb/jawa/rest/Test5
   #25 = Class              #82           // java/lang/Object
   #26 = Utf8               <init>
   #27 = Utf8               ()V
   #28 = Utf8               Code
   #29 = Utf8               LineNumberTable
   #30 = Utf8               LocalVariableTable
   #31 = Utf8               this
   #32 = Utf8               Lcom/ccb/jawa/rest/Test5;
   #33 = Utf8               main
   #34 = Utf8               ([Ljava/lang/String;)V
   #35 = Utf8               key
   #36 = Utf8               Ljava/lang/String;
   #37 = Utf8               entry
   #38 = Utf8               Entry
   #39 = Utf8               InnerClasses
   #40 = Utf8               Ljava/util/Map$Entry;
   #41 = Utf8               args
   #42 = Utf8               [Ljava/lang/String;
   #43 = Utf8               map
   #44 = Utf8               Ljava/util/Map;
   #45 = Utf8               LocalVariableTypeTable
   #46 = Utf8               Ljava/util/Map$Entry<Ljava/lang/String;Ljava/lang/String;>;
   #47 = Utf8               Ljava/util/Map<Ljava/lang/String;Ljava/lang/String;>;
   #48 = Utf8               StackMapTable
   #49 = Class              #83           // java/util/Map
   #50 = Class              #84           // java/util/Iterator
   #51 = Utf8               MethodParameters
   #52 = Utf8               SourceFile
   #53 = Utf8               Test5.java
   #54 = NameAndType        #26:#27       // "<init>":()V
   #55 = Utf8               java/util/HashMap
   #56 = Utf8               1
   #57 = Utf8               value1
   #58 = Class              #83           // java/util/Map
   #59 = NameAndType        #85:#86       // put:(Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
   #60 = Utf8               2
   #61 = Utf8               value2
   #62 = Utf8               3
   #63 = Utf8               value3
   #64 = Class              #87           // java/lang/System
   #65 = NameAndType        #88:#89       // out:Ljava/io/PrintStream;
   #66 = Utf8               通过Map.keySet遍历key和value：
   #67 = Class              #90           // java/io/PrintStream
   #68 = NameAndType        #91:#92       // println:(Ljava/lang/String;)V
   #69 = NameAndType        #93:#94       // keySet:()Ljava/util/Set;
   #70 = Class              #95           // java/util/Set
   #71 = NameAndType        #96:#97       // iterator:()Ljava/util/Iterator;
   #72 = Class              #84           // java/util/Iterator
   #73 = NameAndType        #98:#99       // hasNext:()Z
   #74 = NameAndType        #100:#101     // next:()Ljava/lang/Object;
   #75 = Utf8               java/lang/String
   #76 = NameAndType        #102:#103     // get:(Ljava/lang/Object;)Ljava/lang/Object;
   #77 = Utf8               通过Map.entrySet遍历key和value
   #78 = NameAndType        #104:#94      // entrySet:()Ljava/util/Set;
   #79 = Utf8               java/util/Map$Entry
   #80 = NameAndType        #105:#101     // getValue:()Ljava/lang/Object;
   #81 = Utf8               com/ccb/jawa/rest/Test5
   #82 = Utf8               java/lang/Object
   #83 = Utf8               java/util/Map
   #84 = Utf8               java/util/Iterator
   #85 = Utf8               put
   #86 = Utf8               (Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
   #87 = Utf8               java/lang/System
   #88 = Utf8               out
   #89 = Utf8               Ljava/io/PrintStream;
   #90 = Utf8               java/io/PrintStream
   #91 = Utf8               println
   #92 = Utf8               (Ljava/lang/String;)V
   #93 = Utf8               keySet
   #94 = Utf8               ()Ljava/util/Set;
   #95 = Utf8               java/util/Set
   #96 = Utf8               iterator
   #97 = Utf8               ()Ljava/util/Iterator;
   #98 = Utf8               hasNext
   #99 = Utf8               ()Z
  #100 = Utf8               next
  #101 = Utf8               ()Ljava/lang/Object;
  #102 = Utf8               get
  #103 = Utf8               (Ljava/lang/Object;)Ljava/lang/Object;
  #104 = Utf8               entrySet
  #105 = Utf8               getValue
{
  public com.ccb.jawa.rest.Test5();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 11: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/ccb/jawa/rest/Test5;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=3, locals=4, args_size=1
         0: new           #2                  // class java/util/HashMap
         3: dup
         4: invokespecial #3                  // Method java/util/HashMap."<init>":()V
         7: astore_1
         8: aload_1
         9: ldc           #4                  // String 1
        11: ldc           #5                  // String value1
        13: invokeinterface #6,  3            // InterfaceMethod java/util/Map.put:(Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
        18: pop
        19: aload_1
        20: ldc           #7                  // String 2
        22: ldc           #8                  // String value2
        24: invokeinterface #6,  3            // InterfaceMethod java/util/Map.put:(Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
        29: pop
        30: aload_1
        31: ldc           #9                  // String 3
        33: ldc           #10                 // String value3
        35: invokeinterface #6,  3            // InterfaceMethod java/util/Map.put:(Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
        40: pop
        41: getstatic     #11                 // Field java/lang/System.out:Ljava/io/PrintStream;
        44: ldc           #12                 // String 通过Map.keySet遍历key和value：
        46: invokevirtual #13                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        49: aload_1
        50: invokeinterface #14,  1           // InterfaceMethod java/util/Map.keySet:()Ljava/util/Set;
        55: invokeinterface #15,  1           // InterfaceMethod java/util/Set.iterator:()Ljava/util/Iterator;
        60: astore_2
        61: aload_2
        62: invokeinterface #16,  1           // InterfaceMethod java/util/Iterator.hasNext:()Z
        67: ifeq          99
        70: aload_2
        71: invokeinterface #17,  1           // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;
        76: checkcast     #18                 // class java/lang/String
        79: astore_3
        80: getstatic     #11                 // Field java/lang/System.out:Ljava/io/PrintStream;
        83: aload_1
        84: aload_3
        85: invokeinterface #19,  2           // InterfaceMethod java/util/Map.get:(Ljava/lang/Object;)Ljava/lang/Object;
        90: checkcast     #18                 // class java/lang/String
        93: invokevirtual #13                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        96: goto          61
        99: getstatic     #11                 // Field java/lang/System.out:Ljava/io/PrintStream;
       102: ldc           #20                 // String 通过Map.entrySet遍历key和value
       104: invokevirtual #13                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       107: aload_1
       108: invokeinterface #21,  1           // InterfaceMethod java/util/Map.entrySet:()Ljava/util/Set;
       113: invokeinterface #15,  1           // InterfaceMethod java/util/Set.iterator:()Ljava/util/Iterator;
       118: astore_2
       119: aload_2
       120: invokeinterface #16,  1           // InterfaceMethod java/util/Iterator.hasNext:()Z
       125: ifeq          156
       128: aload_2
       129: invokeinterface #17,  1           // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;
       134: checkcast     #22                 // class java/util/Map$Entry
       137: astore_3
       138: getstatic     #11                 // Field java/lang/System.out:Ljava/io/PrintStream;
       141: aload_3
       142: invokeinterface #23,  1           // InterfaceMethod java/util/Map$Entry.getValue:()Ljava/lang/Object;
       147: checkcast     #18                 // class java/lang/String
       150: invokevirtual #13                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       153: goto          119
       156: return
      LineNumberTable:
        line 15: 0
        line 16: 8
        line 17: 19
        line 18: 30
        line 21: 41
        line 22: 49
        line 23: 80
        line 24: 96
        line 27: 99
        line 28: 107
        line 29: 138
        line 30: 153
        line 31: 156
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
           80      16     3   key   Ljava/lang/String;
          138      15     3 entry   Ljava/util/Map$Entry;
            0     157     0  args   [Ljava/lang/String;
            8     149     1   map   Ljava/util/Map;
      LocalVariableTypeTable:
        Start  Length  Slot  Name   Signature
          138      15     3 entry   Ljava/util/Map$Entry<Ljava/lang/String;Ljava/lang/String;>;
            8     149     1   map   Ljava/util/Map<Ljava/lang/String;Ljava/lang/String;>;
      StackMapTable: number_of_entries = 4
        frame_type = 253 /* append */
          offset_delta = 61
          locals = [ class java/util/Map, class java/util/Iterator ]
        frame_type = 250 /* chop */
          offset_delta = 37
        frame_type = 252 /* append */
          offset_delta = 19
          locals = [ class java/util/Iterator ]
        frame_type = 250 /* chop */
          offset_delta = 36
    MethodParameters:
      Name                           Flags
      args
}
SourceFile: "Test5.java"
InnerClasses:
     public static #38= #22 of #58; //Entry=class java/util/Map$Entry of class java/util/Map

 虽然使用keyset及entryset来进行遍历能取得相同的结果，但两者的遍历速度是有差别的，keySet()的速度比entrySet()慢了很多，也就是keySet方式遍历Map的性能不如entrySet性能好，为了提高性能，以后多考虑用entrySet()方式来进行遍历。