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Java program to update the value of HashMap using keys

Comprehensive Java Examples

In this example, we will learn how to update the value of Java HashMap using keys.

To understand this example, you should understand the followingJava ProgrammingTopic：

Java HashMap

Example1：Using put() to update the value of HashMap

Example

import java.util.HashMap;
class Main {
　　public static void main(String[] args) {
　　　　HashMap<String, Integer> numbers = new HashMap<>();
　　　　numbers.put("First",　1);
　　　　numbers.put("Second",　2);
　　　　numbers.put("Third",　3);
　　　　System.out.println("HashMap: "　+　numbers);
　　　　//Return the value of the key Second
　　　　int value = numbers.get("Second");
　　　　//Update the value
　　　　value = value　*　value;
　　　　//Insert the updated value into the HashMap
　　　　numbers.put("Second", value);
　　　　System.out.println("Updated HashMap:　"　+　numbers);
　　}
}

Output Result

HashMap:　{Second=2, Third=3,　First=1}
Updated HashMap:　{Second=4, Third=3,　First=1}

In the above example, we usedHashMap put()method to update the value of the key Second. Here, first, we useHashMap get()to access the value of the method　 .

Example2：Using computeIfPresent() method to update the value of HashMap

Example

import java.util.HashMap;
class Main {
　　public static void main(String[] args) {
　　　　HashMap<String, Integer> numbers = new HashMap<>();
　　　　numbers.put("First",　1);
　　　　numbers.put("Second",　2);
　　　　System.out.println("HashMap: "　+　numbers);
　　　　//　to update the value of Second
　　　　//　Using computeIfPresent()
　　　　numbers.computeIfPresent("Second", (key, oldValue)　->　oldValue　*　2);
　　　　System.out.println("Updated HashMap:　"　+　numbers);
　　}
}

Output Result

HashMap:　{Second=2,　First=1}
Updated HashMap:　{Second=4,　First=1}

In the above example, we used the computeIfPresent() method to recalculate the value of the key Second. For more information, please visitHashMap computeIfPresent().

In this case, we willLambda expressionis used as the method parameter.

Example3：Using merge() to update the value of HashMap

Example

import java.util.HashMap;
class Main {
　　public static void main(String[] args) {
　　　　HashMap<String, Integer> numbers = new HashMap<>();
　　　　numbers.put("First",　1);
　　　　numbers.put("Second",　2);
　　　　System.out.println("HashMap: "　+　numbers);
　　　　//Update the value of 'First'
　　　　//Using Merge() Method
　　　　numbers.merge("First",　4,　(oldValue,　newValue)　->　oldValue　+　newValue);
　　　　System.out.println("Updated HashMap:　"　+　numbers);
　　}
}

Output Result

HashMap:　{Second=2,　First=1}
Updated HashMap:　{Second=2,　First=5}

In the above example, the merge() method adds the old value and new value of the key 'First' together. And, the updated value is inserted into the HashMap. For more information, please visitHashMap merge().

Comprehensive Java Examples

Java Serialization Java Ternary Operator