What is the diamond operator in Java

Q: What is the diamond operator in Java?

The diamond operator (<>) allows the compiler to infer the generic type from the context, reducing verbosity and improving readability.

Q: Can diamond operator be used with anonymous classes in Java 7?

No, it cannot be used with anonymous classes in Java 7. This limitation was removed in Java 9.

💡 Concept: Diamond Operator

The diamond operator <> was introduced in Java 7 to simplify the instantiation of generic types by inferring the type parameters from the context.

📘 Quick Intro

Instead of repeating generic type information on both sides of the assignment, the diamond operator allows you to omit it on the right-hand side. The compiler infers it.

🧠 Analogy

Think of the diamond operator like using a smart contact list: once you type a name, the phone app fills in the number. Similarly, once the compiler sees the left-hand side, it fills in the types on the right automatically.

🔧 Technical Explanation

Before Java 7: List<String> list = new ArrayList<String>();
After Java 7: List<String> list = new ArrayList<>();
The compiler infers the type String from the declaration on the left-hand side.
The diamond operator improves readability and reduces redundancy.
Cannot be used with anonymous inner classes (prior to Java 9).

🎯 Use Cases

✅ Simplifying generic object instantiation.
✅ Making code cleaner and less verbose.
✅ Automatically inferring types without loss of type safety.

💻 Example: Diamond Operator


import java.util.*;

public class DiamondOperatorExample {
    public static void main(String[] args) {
        // Before Java 7
        Map<Integer, String> oldMap = new HashMap<Integer, String>();

        // After Java 7 using diamond operator
        Map<Integer, String> newMap = new HashMap<>();

        newMap.put(1, "One");
        newMap.put(2, "Two");
        System.out.println(newMap);
    }
}

❓ Interview Q&A

Q1: What does the diamond operator do?
A: Infers generic types at instantiation.

Q2: When was the diamond operator introduced?
A: Java 7.

Q3: Does it improve type safety?
A: Yes, without losing generic type checks.

Q4: Can it be used with anonymous classes in Java 7?
A: No.

Q5: Is <> required on the left-hand side?
A: No, just on the right side for inference.

Q6: Can you mix explicit and inferred generics?
A: Yes, but it’s not recommended.

Q7: Does it reduce code verbosity?
A: Yes.

Q8: Does type inference work at runtime?
A: No, it’s a compile-time feature.

Q9: Is diamond operator backward compatible?
A: No, only works in Java 7 and above.

Q10: What happens if you omit <> entirely?
A: Raw type warning.

📝 MCQs

Q1. What is the purpose of the diamond operator?

Type casting
Type inference
Looping
Compilation

Q2. When was the diamond operator introduced?

Java 5
Java 6
Java 7
Java 8

Q3. What happens without the diamond operator?

No compilation
Verbose syntax
No generics
More speed

Q4. Can diamond operator be used with anonymous classes in Java 7?

Yes
No
Sometimes
Only in Java 6

Q5. Does diamond operator impact runtime?

Yes
No
Maybe
Depends on compiler

Q6. Is diamond operator a compile-time feature?

No
Yes
Partly
Not always

Q7. What is the syntax of the diamond operator?

{}
()
<>
[]

Q8. Does diamond operator eliminate redundancy?

No
Yes
Sometimes
Only with arrays

Q9. What happens if diamond operator is omitted in Java 7?

Nothing
Error
Null pointer
Raw type warning

Q10. Which side is the diamond used on?

Left-hand side
Right-hand side
Both
Only return type

💡 Bonus Insight

Using the diamond operator makes your generic code easier to read and helps avoid repeating type information. It's especially useful in deeply nested or map/list structures.

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