Check for duplicate values for ip address (user input) using hashmap in java

Main class to enter all the domain details like Domain name and ip address. I need to check for duplicate values entered for ip address. I need a code for hashmap to check for duplicate values.
Input would be www.gmail.net 195.116.254.154
output would be like - if the ipAddress is 195.116.254.154 the output should be www.gmail.net

public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
DomainBO dBO = new DomainBO();
Map<String, String> domainMap = new HashMap<>();
dBO.setDomainMap(domainMap);
String ip, Dname;
int flag = 0;
int input;
int count = 0;
String DomainName = null;
do {
System.out.println("1. Add DNS details");
System.out.println("2. Find matching Domain Name");
System.out.println("3. Exit");
System.out.println("Enter your choice");
input = sc.nextInt();
sc.nextLine();
if (input == 1) {
count++;
System.out.println("Enter the domain name");
Dname = sc.nextLine();
System.out.println("Enter the IP address");
ip = sc.nextLine();
if (isValid(Dname) || IPAddressValidator(ip))// check for valid ipaddress and name
{
dBO.addDNSDetails(Dname, ip);
flag = 0;
}
else
flag = 0 ;
}
if (input == 2) {
if (count > 0) {
System.out.println("Enter the IP address to find the domain name");
ip = sc.nextLine();
DomainName = dBO.findDomainName(ip);
if (DomainName == null) {
System.out.println("No matching domain name found");
flag = 0;
} else {
System.out.println(DomainName);
flag = 0;
}
}
}
if (input == 3) {
System.out.println("Thank you for using the Application");
flag = 1;
break;
}
} while (flag == 0);
}

The DomainBO.java class is used to enter domain details and retrieve the domain name by inputting Ipaddress.

public class DomainBO {
private Map<String,String> domainMap;
public Map<String,String> getDomainMap() {
return domainMap;
}
public void setDomainMap(Map<String,String> domainMap) {
this.domainMap = domainMap;
}
//This method should add the domainName as key and their ipAddress as value into a Map
public void addDNSDetails  (String domainName,String ipAddress)
{	
if(domainMap.containsKey(ipAddress)) 
{
return;
}
else
{
domainMap.put(domainName, ipAddress); 
}
}
public String findDomainName(String ipAddress) {
String domain=null;
if(domainMap.isEmpty()){
return domain;
}
else
for (Map.Entry<String, String> entry : domainMap.entrySet()) {
String k = entry.getKey();
String v = entry.getValue(); 
if (ipAddress.equals(v)) {
domain = k;                 
}
else
{
domain = "No matching domain name found";
}
}     
return domain;
}

}

You say that you want to map domainName to ip addr. but you check for domainMap.containsKey(ipAddress). That is not right because your keys are domain names, not ip addresses.

You can use a code like this to check for duplicates in values(ip addr.) if that is what you need

for (Map.Entry<String, String> set : domainMap.entrySet()) {
if (set.getValue().equals(currentIp)) 
System.out.println("Duplicate ip"); // Do whatever you want in here
}

In the addDNSDetails method you've written

domainMap.put(domainName, ipAddress); 

The first parameter is the key used to find the data, the second parameter is the value. You're putting the domain name as the key, but by your description want to use the IP address to find the domain, not the other way around. So change this code to:

domainMap.put(ipAddress, domainName);