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[ Java HashMap](#)

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The merge() method first checks if the specified key exists. If it does not exist, it adds the key-value pair to the hashMap.

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The syntax of the merge() method is:

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hashmap.merge(key, value, remappingFunction)
\nNote: hashmap is an object of the HashMap class.

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Parameter Description:

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• key - key
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• value - value
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• remappingFunction - remapping function, used to recalculate the value
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Return Value

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If the value corresponding to the key does not exist, it returns that value. If it exists, it returns the value recalculated through the remappingFunction.

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Example

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The following example demonstrates the use of the merge() method:

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Example

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import java.util.HashMap;\n\nclass Main {\n\npublic static void main(String[] args){\n\n//Create a HashMap\n\n HashMap<String, Integer> prices =new HashMap<>();\n\n// Insert a mapping into the HashMap\n\n prices.put("Shoes", 200);\n\n prices.put("Bag", 300);\n\n prices.put("Pant", 150);\n\nSystem.out.println("HashMap: "+ prices);\n\nint returnedValue = prices.merge("Shirt", 100, (oldValue, newValue)-> oldValue + newValue);\n\nSystem.out.println("Price of Shirt: "+ returnedValue);\n\n// Print the updated HashMap\n\nSystem.out.println("Updated HashMap: "+ prices);\n\n}\n\n}

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Output of the above program:

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HashMap: {Pant=150, Bag=300, Shoes=200}\nPrice of Shirt: 100\nUpdated HashMap: {Pant=150, Shirt=100, Bag=300, Shoes=200}

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In the above example, we created a HashMap named prices.

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Note the expression:

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prices.merge("Shirt", 100, (oldValue, newValue) -> oldValue + newValue)

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In the code, we used an anonymous function lambda expression (oldValue, newValue) -> oldValue + newValue) as the remapping function.

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Because the key Shirt is not in prices, the merge() method inserts the mapping Shirt=100 into prices, and the result of the remapping function is ignored.

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To learn more about lambda expressions, visit Java Lambda Expressions.

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HashMap merge() method inserts mapping items with duplicate keys:

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Example

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import java.util.HashMap;\n\nclass Main {\n\npublic static void main(String[] args){\n\n// Create a HashMap\n\n HashMap<String, String> countries =new HashMap<>();\n\n// Insert mapping entries into the HashMap\n\n countries.put("Washington", "America");\n\n countries.put("Canberra", "Australia");\n\n countries.put("Madrid", "Spain");\n\nSystem.out.println("HashMap: "+ countries);\n\n//Merge the mapping for key Washington\n\nString returnedValue = countries.merge("Washington", "USA", (oldValue, newValue)-> oldValue +"/"+ newValue);\n\nSystem.out.println("Washington: "+ returnedValue);\n\n//Print the updated HashMap\n\nSystem.out.println("Updated HashMap: "+ countries);\n\n}\n\n}

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Output of the above program:

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HashMap: {Madrid=Spain, Canberra=Australia, Washington=America}\nWashington: America/USA \nUpdated HashMap: {Madrid=Spain, Canberra=Australia, Washington=America/USA}

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In the above example, we created a HashMap named countries.

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Note the expression:

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countries.merge("Washington", "USA", (oldValue, newValue) -> oldValue + "/" + newValue)

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Here, we used the lambda expression (oldValue, newValue) -> oldValue + "/" + newValue) as the remapping function.

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Because the key washington already exists in countries, the old value is replaced by the value returned by the remapping function. Therefore, the mapping for Washington contains America/USA.

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[ Java HashMap](#)