HashMap in Java

Learn Java HashMap — storing key-value pairs, all essential methods, iterating entries, handling duplicate keys, getOrDefault, merge, computeIfAbsent, and common mistakes.

HashMap stores key-value pairs. Every key maps to exactly one value. Keys must be unique — adding the same key again replaces the old value. Values can repeat. Order is not guaranteed.

This is the most commonly used Map in Java. Use it whenever you need to look up a value by a key — word counts, user lookup tables, configuration, caching.

Creating and Using HashMap

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Lesson example

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Output

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Line by line:

scores.put("Asha", 95) with existing key — updates the value. Returns the old value (92).
scores.get("Dev") — key does not exist → returns null.
remove("Ravi") — removes the key-value pair. Returns the removed value (78).

Iterating a HashMap

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Lesson example

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Output

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Useful Methods: getOrDefault, putIfAbsent, merge, computeIfAbsent

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Lesson example

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Output

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getOrDefault(word, 0) — returns 0 if word is not in the map (instead of null). Common pattern for frequency counting.
putIfAbsent("apple", 99) — apple already exists with 3, so 99 is ignored.
merge("Asha", 5, Integer::sum) — old value (10) + new value (5) using sum function = 15.

HashMap Allows One null Key

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Lesson example

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Output

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HashMap allows one null key and multiple null values. Hashtable and TreeMap do not allow null keys.

Common Mistake 1: Assuming Order

java example

Map<String, Integer> map = new HashMap<>();
map.put("c", 3);
map.put("a", 1);
map.put("b", 2);

System.out.println(map);  // order NOT guaranteed — could be {a=1, b=2, c=3} or anything

// If you need sorted keys:
// Use TreeMap
// If you need insertion order:
// Use LinkedHashMap

Common Mistake 2: Using get() Without Null Check

java example

Map<String, Integer> map = new HashMap<>();
int count = map.get("apple");  // NullPointerException — key not found, returns null
                               // null cannot be unboxed to int

// CORRECT
int count = map.getOrDefault("apple", 0);
// or
Integer val = map.get("apple");
if (val != null) { int count2 = val; }

Common Mistake 3: Modifying While Iterating

java example

Map<String, Integer> map = new HashMap<>();
map.put("a", 1); map.put("b", 2); map.put("c", 3);

// WRONG — ConcurrentModificationException
// for (String key : map.keySet()) {
//     if (map.get(key) < 2) map.remove(key);
// }

// CORRECT — remove using iterator
map.entrySet().removeIf(e -> e.getValue() < 2);
System.out.println(map);  // {b=2, c=3}

Practice Task

Task: Given an array of exam scores: {"Asha", 92, "Ravi", 78, "Priya", 92, "Dev", 65, "Asha", 88} — alternating name and score (treat Asha's second score as an update). Use a HashMap<String, Integer> to:

Store each student's latest score
Find the student with the highest score
Count how many students scored above 80

Hint: Iterate the array in steps of 2. For max, loop over entrySet(). For count, use values() with a counter.

Solution:

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Lesson example

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Output

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What to observe: Asha's first score (92) gets replaced by 88. The final map has 4 entries (not 5). Iterating values() gives only the scores without keys — useful when you only need the values.

HashMap stores key-value pairs. Every key maps to exactly one value. Keys must be unique — adding the same key again replaces the old value. Values can repeat. Order is not guaranteed.

This is the most commonly used Map in Java. Use it whenever you need to look up a value by a key — word counts, user lookup tables, configuration, caching.

Creating and Using HashMap

Try it in WoHoCode

Edit the program, then run it here.

Lesson example

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Ln 1, Col 1java | Ctrl+Enter: run | Ctrl+S: save

Output

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Line by line:

scores.put("Asha", 95) with existing key — updates the value. Returns the old value (92).
scores.get("Dev") — key does not exist → returns null.
remove("Ravi") — removes the key-value pair. Returns the removed value (78).

Iterating a HashMap

Try it in WoHoCode

Edit the program, then run it here.

Lesson example

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Output

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Useful Methods: getOrDefault, putIfAbsent, merge, computeIfAbsent

Try it in WoHoCode

Edit the program, then run it here.

Lesson example

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Ln 1, Col 1java | Ctrl+Enter: run | Ctrl+S: save

Output

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Line by line:

getOrDefault(word, 0) — returns 0 if word is not in the map (instead of null). Common pattern for frequency counting.
putIfAbsent("apple", 99) — apple already exists with 3, so 99 is ignored.
merge("Asha", 5, Integer::sum) — old value (10) + new value (5) using sum function = 15.

HashMap Allows One null Key

Try it in WoHoCode

Edit the program, then run it here.

Lesson example

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Ln 1, Col 1java | Ctrl+Enter: run | Ctrl+S: save

Output

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HashMap allows one null key and multiple null values. Hashtable and TreeMap do not allow null keys.

Common Mistake 1: Assuming Order

java example

Map<String, Integer> map = new HashMap<>();
map.put("c", 3);
map.put("a", 1);
map.put("b", 2);

System.out.println(map);  // order NOT guaranteed — could be {a=1, b=2, c=3} or anything

// If you need sorted keys:
// Use TreeMap
// If you need insertion order:
// Use LinkedHashMap

Common Mistake 2: Using get() Without Null Check

java example

Map<String, Integer> map = new HashMap<>();
int count = map.get("apple");  // NullPointerException — key not found, returns null
                               // null cannot be unboxed to int

// CORRECT
int count = map.getOrDefault("apple", 0);
// or
Integer val = map.get("apple");
if (val != null) { int count2 = val; }

Common Mistake 3: Modifying While Iterating

java example

Map<String, Integer> map = new HashMap<>();
map.put("a", 1); map.put("b", 2); map.put("c", 3);

// WRONG — ConcurrentModificationException
// for (String key : map.keySet()) {
//     if (map.get(key) < 2) map.remove(key);
// }

// CORRECT — remove using iterator
map.entrySet().removeIf(e -> e.getValue() < 2);
System.out.println(map);  // {b=2, c=3}

Practice Task

Store each student's latest score
Find the student with the highest score
Count how many students scored above 80

Hint: Iterate the array in steps of 2. For max, loop over entrySet(). For count, use values() with a counter.

Solution:

Try it in WoHoCode

Edit the program, then run it here.

Lesson example

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Ln 1, Col 1java | Ctrl+Enter: run | Ctrl+S: save

Output

Run this example to see its output.

HashMap in Java

Creating and Using HashMap

Iterating a HashMap

Useful Methods: getOrDefault, putIfAbsent, merge, computeIfAbsent

HashMap Allows One null Key

Common Mistake 1: Assuming Order

Common Mistake 2: Using get() Without Null Check

Common Mistake 3: Modifying While Iterating

Practice Task

Study next in this module

HashMap in Java

Creating and Using HashMap

Iterating a HashMap

Useful Methods: getOrDefault, putIfAbsent, merge, computeIfAbsent

HashMap Allows One null Key

Common Mistake 1: Assuming Order

Common Mistake 2: Using get() Without Null Check

Common Mistake 3: Modifying While Iterating

Practice Task

Study next in this module