Java | HashMap

HashMap associates unique keys with values and provides fast average lookup by key. Use it when your problem asks for retrieval by an identifier, not when order or duplicate keys are the primary concern. A correct map design starts with a stable key rule, explicit missing-key behavior and ownership of mutable values.

Keys Identify Values

A key must represent a stable identity for the value you want to retrieve. Reusing mutable or poorly defined key objects can make a stored value impossible to find later.

Decide Missing-Key Behavior

get returns null when no mapping exists, but null can also be a stored value. Use containsKey, getOrDefault or a domain result to distinguish absence from a deliberately mapped null.

Choose the Right Update Method

put replaces any existing value, while putIfAbsent preserves the first one. Pick the method that states the duplicate-key policy rather than silently overwriting data.

Do Not Depend on Iteration Order

HashMap does not promise a stable iteration order. Use LinkedHashMap or a sorted structure only when ordering is a real user-visible or algorithmic requirement.

Worked Examples

Example 1: Store and Retrieve by Key

The map makes the relationship between an immutable identifier and a title explicit. Lookup is by the key rather than by scanning every stored value.

Java

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

Map<Long, String> titles = new HashMap<>();
titles.put(42L, "Java Collections");
System.out.println(titles.get(42L));

Expected Result

Java Collections

Example 2: Handle an Absent Mapping

getOrDefault defines the missing-key result in one expression. Callers receive an intentional label instead of having to infer the meaning of null.

Java

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

Map<String, Integer> counts = new HashMap<>();
counts.put("java", 3);
System.out.println(counts.getOrDefault("rust", 0));

Expected Result

Example 3: Preserve the First Value

putIfAbsent records the first owner for a key and ignores a later duplicate. This makes the duplicate policy clear at the point where data enters the map.

Java

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

Map<String, String> owners = new HashMap<>();
owners.putIfAbsent("course-1", "Ada");
owners.putIfAbsent("course-1", "Lin");
System.out.println(owners.get("course-1"));

Expected Result

Ada

Build and Practice Plan

Build confidence by changing one rule at a time. Start with the normal case, make the expected value visible, then add the nearest boundary and a rejected input. Keep Java syntax in service of the contract: a program is useful only when its behavior remains understandable as inputs, scale and failure conditions change.

Step 1: create one normal case, one boundary case and one rejected or failure case for keys identify values before expanding the implementation.
Step 2: create one normal case, one boundary case and one rejected or failure case for decide missing-key behavior before expanding the implementation.
Step 3: create one normal case, one boundary case and one rejected or failure case for choose the right update method before expanding the implementation.
Step 4: create one normal case, one boundary case and one rejected or failure case for do not depend on iteration order before expanding the implementation.

Design Review Questions

Before accepting a change, identify the valid values, the owner of mutable state, the behavior for missing or invalid input and the test that proves the important boundary. Keep I/O, storage and framework wiring outside the small Java rule until the local behavior is proven.

A clear result is better than a hidden side effect. Name the condition, return the calculation where possible and let the outer layer decide whether to print, store or transmit it.

Example 4: Make the input visible

Start by naming the value the next line depends on. This keeps the lesson input explicit before we apply a transformation or call an external boundary. We are applying that habit while revisiting Java HashMap.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Java

public class Main {
    public static void main(String[] args) {
        String lesson = "Java HashMap";
        System.out.println(lesson.toUpperCase());
    }
}

Expected Result

JAVA HASHMAP

Try next: Replace the sample value with one from your own notes, predict the result, and then run it.

Example 5: Make the input visible

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Java

public class Main {
    public static void main(String[] args) {
        String lesson = "Java HashMap";
        System.out.println(lesson.toUpperCase());
    }
}

Expected Result

JAVA HASHMAP

Try next: Replace the sample value with one from your own notes, predict the result, and then run it.

Example 6: Make the input visible

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Java

public class Main {
    public static void main(String[] args) {
        String lesson = "Java HashMap";
        System.out.println(lesson.toUpperCase());
    }
}

Expected Result

JAVA HASHMAP

Try next: Replace the sample value with one from your own notes, predict the result, and then run it.

Example 7: Guard an empty boundary

A small guard makes the allowed and rejected path visible. We want a caller to receive an intentional result instead of discovering an unexplained failure later. We are applying that habit while revisiting Java HashMap.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Java

public class Main {
    static boolean accepted(String value) {
        return value != null && !value.isBlank();
    }

    public static void main(String[] args) {
        System.out.println(accepted("Java HashMap"));
    }
}

Expected Result

true

Try next: Try an empty value as well as the normal value and describe why the two outcomes differ.

Example 8: Keep the transformation pure

This step returns a value instead of mixing the rule with display or storage work. That separation gives us a small unit we can run, test, and reuse. We are applying that habit while revisiting Java HashMap.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Java

public class Main {
    static int doubleValue(int value) { return value * 2; }

    public static void main(String[] args) {
        System.out.println(doubleValue(6));
    }
}

Expected Result

Try next: Call the same rule twice with different inputs and confirm that each result depends only on its input.

Example 9: Make a branch deliberate

Use a named branch where the business rule changes. A future reader should be able to point at the condition and state the result for both sides. We are applying that habit while revisiting Java HashMap.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Java

public class Main {
    public static void main(String[] args) {
        int score = 7;
        String result = score >= 5 ? "ready" : "review";
        System.out.println(result);
    }
}

Expected Result

ready

Try next: Change the boundary value by one and explain exactly which branch becomes active.

Example 10: Use a collection without hidden mutation

Collections become safer when we can see which value is read, which value is derived, and whether the original collection remains owned by the caller. We are applying that habit while revisiting Java HashMap.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Java

import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<String> lessons = List.of("Java HashMap", "boundary");
        System.out.println(String.join(" | ", lessons));
    }
}

Expected Result

Java HashMap | boundary

Try next: Add one more item and predict the derived result before executing the code.

Example 11: Return a named outcome

A named outcome is easier to test than an implied side effect. We can now distinguish a successful path from a rejected path without guessing from console noise. We are applying that habit while revisiting Java HashMap.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Java

public class Main {
    record Outcome(String status, String lesson) {}

    public static void main(String[] args) {
        Outcome outcome = new Outcome("ready", "Java HashMap");
        System.out.println(outcome.status());
    }
}

Expected Result

ready

Try next: Rename the outcome in your own words while preserving the same success and failure rule.

Example 12: Trace a small loop

Work through one value at a time. The point is not to memorize syntax; it is to see how the accumulator or output changes after each repeatable step. We are applying that habit while revisiting Java HashMap.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Java

public class Main {
    public static void main(String[] args) {
        int total = 0;
        for (int value : new int[] {1, 2, 3}) total += value;
        System.out.println(total);
    }
}

Expected Result

Try next: Use a shorter collection first, then add one value and trace the extra iteration aloud.

Example 13: Keep display at the edge

The core rule prepares a value, while the outer line decides how to show it. That keeps the result usable by a UI, test, API, or command-line caller. We are applying that habit while revisiting Java HashMap.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Java

public class Main {
    static String display(String value) { return "Lesson: " + value; }

    public static void main(String[] args) {
        System.out.println(display("Java HashMap"));
    }
}

Expected Result

Lesson: Java HashMap

Try next: Change only the display text and verify that the underlying rule and result stay unchanged.

Example 14: Test the closest boundary

A useful example includes the nearest boundary, not only the happy path. We are making the behavior inspectable before a real caller relies on it. We are applying that habit while revisiting Java HashMap.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Java

public class Main {
    static int quantity(int value) {
        if (value < 1) throw new IllegalArgumentException("quantity must be positive");
        return value;
    }

    public static void main(String[] args) {
        System.out.println(quantity(1));
    }
}

Expected Result

Try next: Try the lowest supported value and then one value just outside the accepted range.

Example 15: Write a tiny regression check

A compact assertion turns the lesson into a repeatable promise. It helps us catch a changed rule before a larger program hides the source of the mistake. We are applying that habit while revisiting Java HashMap.

Keep the example small enough to trace. First predict the normal result; then change one input and explain the boundary result in your own words.

Java

public class Main {
    public static void main(String[] args) {
        String result = "Java HashMap".toUpperCase();
        if (!result.equals("JAVA HASHMAP")) throw new AssertionError("contract failed");
        System.out.println("contract holds");
    }
}

Expected Result

contract holds

Try next: Deliberately change the expected value once, observe the failed check, and then restore the correct contract.

Common Mistakes

Avoid: treating keys identify values as a syntax trick while leaving the domain rule, ownership or error behavior implicit.
Avoid: treating decide missing-key behavior as a syntax trick while leaving the domain rule, ownership or error behavior implicit.
Avoid: treating choose the right update method as a syntax trick while leaving the domain rule, ownership or error behavior implicit.
Avoid: treating do not depend on iteration order as a syntax trick while leaving the domain rule, ownership or error behavior implicit.

Interview Checklist

State the input domain and the boundary value before explaining the syntax.
Explain the failure or rejected-input outcome rather than assuming the happy path.
Identify mutation, ownership and any value that must not be changed by a caller.
Name a focused test that would prove the rule still holds after a future change.
Separate the local Java decision from printing, persistence, networking or framework code.