代码优化：使用Entry遍历Map
作者：互联网
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。
标签：lang,Ljava,java,Map,Utf8,util,代码优化,Entry 来源： https://www.cnblogs.com/zwh0910/p/15810522.html