How to Remove Items/Entries with Specific Values from Map/HashMa

Time：2020-09-07 12:13:31
Class：Weblog
Read：36

Java

In Java, you can usually remove items or entries with some specific value from Map or HashMap object using traditional method:

public Map<String, String> removeItemByValue(Map<String, String> data, String value) {
  for (var key: data.keySet()) {
    if (data.get(key) == null) {
       data.remove(key);
    }
  }
  return data;
}
 
var data = new HashMap<String, String>();
data.put("a", "1");
data.put("b", "2");
data.put("c", "2");
removeItemByValue(data, null); 
// only prints a = 1
for (var key: data.keySet()) {
  System.out.println(key + "=" + data.get(key)); 
}

public Map<String, String> removeItemByValue(Map<String, String> data, String value) {
  for (var key: data.keySet()) {
    if (data.get(key) == null) {
       data.remove(key);
    }
  }
  return data;
}

var data = new HashMap<String, String>();
data.put("a", "1");
data.put("b", "2");
data.put("c", "2");
removeItemByValue(data, null); 
// only prints a = 1
for (var key: data.keySet()) {
  System.out.println(key + "=" + data.get(key)); 
}

A Better Map-Item Removal Approach via using values()

The values() method in Map object actually returns a Collection view of the valujes that are contained in the map object. And the remove object return null when the item is not existent anymore. Thefore, we can use the following modern 1-liner in Java to remove the items in a Map that have the specific value.

public Map<String, String> removeItemByValue(Map<String, String> data, String value) {
  while (data.values().remove(value)) ;
  return data;
}

public Map<String, String> removeItemByValue(Map<String, String> data, String value) {
  while (data.values().remove(value)) ;
  return data;
}

To double check and verify that items are actually removed:

package com.helloacm;
 
import java.util.HashMap;
import java.util.Map;
 
public class Main {
    public static void main(String[] args) {
        var data = new HashMap<String, String>();
        data.put("a", "b");
        data.put("e", "2");
        data.put("f", "2");
        while (data.values().remove("2"));
        // prints "a"        
        for (Map.Entry<String, String> entry: data.entrySet()) {
            System.out.println(entry.getKey());
        }
    }
}

package com.helloacm;

import java.util.HashMap;
import java.util.Map;

public class Main {
    public static void main(String[] args) {
        var data = new HashMap<String, String>();
        data.put("a", "b");
        data.put("e", "2");
        data.put("f", "2");
        while (data.values().remove("2"));
        // prints "a"        
        for (Map.Entry<String, String> entry: data.entrySet()) {
            System.out.println(entry.getKey());
        }
    }
}

–EOF (The Ultimate Computing & Technology Blog) —

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