How to Create a Shallow Copy of a Map in Java: Methods and Best Practices

What are the best methods to create a shallow copy of a Map in Java, and is there a preferred approach?

Map<String, Object> data = new HashMap<>();

// Method 1: Using the constructor for shallow copy
Map<String, Object> shallowCopy1 = new HashMap<>(data);

// Method 2: Using the clone method for shallow copy
@SuppressWarnings("unchecked")
Map<String, Object> shallowCopy2 = (Map<String, Object>) ((HashMap<String, Object>) data).clone();

Creating a shallow copy of a Map in Java can be achieved through a couple of straightforward methods. Each method has its own implications regarding type safety and readability. Here’s a comprehensive breakdown of both techniques and recommendations for their use.

// Method 1: Recommended approach
Map<String, Object> shallowCopy1 = new HashMap<>(data);

// Method 2: Using clone with suppression of warnings
@SuppressWarnings("unchecked")
Map<String, Object> shallowCopy2 = (Map<String, Object>) ((HashMap<String, Object>) data).clone();

Causes

• Understanding how shallow copies work in Java is essential for proper data manipulation.
• Avoiding issues related to type safety and unchecked casts is important in Java.

Solutions

• Use `new HashMap<>(data)` for a clear and type-safe approach to create a shallow copy.
• If you prefer or require a clone, ensure to use `@SuppressWarnings("unchecked")` to avoid compile-time warnings.

Mistake: Using the clone method without suppressing unchecked warnings can lead to compiler errors.

Solution: Always use @SuppressWarnings("unchecked") when using the clone method if casting.

Mistake: Assuming that cloned HashMap is a deep copy.

Solution: Understand that clone creates a shallow copy, and nested objects will reference the same instances in memory.