是否可以使用特定形式打印哈希图的键?

Question

我正在处理Junit测试，java中的hashmap。 我的代码看起来像：

HashMap <String,String> var = new HashMap <String,String>(); 
var.put("key1","value1");
var.put("key2","value2");
var.put("key3","value3");
var.put("key4","value4");
Iterator<String> itKey = var.keySet().iterator();
String Existing_coaches = "" ;
while(itKey.hasNext()){
String key = (String)itKey.next();
// Existing_coaches = i use concat function 
} 
return  Existing_coaches ;

我想做的是返回itkeys，如下所示：

键 1， 键 2， 键3， 键 4

我们从第一个键+逗号等开始。

所以我需要知道什么是第一个键和最后一个键.

关于我们如何做到这一点的任何想法？

Answer 1

用逗号和空格连接键：

import java.util.*;
public class MyClass {
public static void main(String args[]) {
Map<String, String> m = new LinkedHashMap<>();
m.put("key1", "val1");
m.put("key2", "val2");
m.put("key3", "val3");
m.put("key4", "val4");
System.out.println(String.join(", ", m.keySet()));
}
}

输出：

key1, key2, key3, key4

Answer 2

Map<String, String> map = new LinkedHashMap<>();
// Approach 1 : use advanced for (foreach) loop
StringBuilder keyString = new StringBuilder();
for(String key: map.keySet()){
keyString = keyString.append(key + ", ");
}
String resultStr = keyString.toString().substring(0, keyString.toString().length()-2); // deducting 2 for comma and space
System.out.println(resultStr);
// Approach 2: use for loop
StringBuilder keyString2 = new StringBuilder();
Object[] keySet =  (Object[]) map.keySet().toArray();
for(int i=0; i < keySet.length; i++) {
if(i == keySet.length-1) {
keyString2 = keyString2.append(keySet[i].toString());
} else {
keyString2 = keyString2.append(keySet[i].toString() + ", ");
}
}
System.out.println(keyString2.toString());
// Approach 3: Use String join
System.out.println(String.join(", ", map.keySet()));

Answer 3

如果您不熟悉 String.join，请使用第一个，或者如果您喜欢它，请使用第二个。两种方式都有效：

import java.util.HashMap;
import java.util.Iterator;
public class Hash {
public static void main(String args[]) {
HashMap <String, String> var = new HashMap <String, String>(); 
var.put("A", "1");
var.put("B", "2");
var.put("C", "3");
var.put("D", "4");
System.out.println("method 1: " + myHash(var));
System.out.println("method 2: " + myHash2(var));
}
public static String myHash(HashMap var) {
Iterator<String> itKey = var.keySet().iterator();
String Existing_coaches = "" ;
while(itKey.hasNext()){
String key = (String)itKey.next();
Existing_coaches = Existing_coaches + key + ", ";
}
return  Existing_coaches.substring(0, Existing_coaches.length() -2) ;
}
public static String myHash2(HashMap var) {
return String.join(", ", var.keySet());
}
}

输出为：

method 1: A, B, C, D
method 2: A, B, C, D