Java Interview Questions – Part 5
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21. What is a String in Java?

A String represents a sequence of characters. Java String objects are immutable, meaning their contents cannot be changed after creation.

Code Example

String name = "Java";
System.out.println(name);

Output

Java

22. What is the Difference Between StringBuilder and StringBuffer?

Both classes allow mutable string operations. StringBuilder is generally preferred for single-threaded operations, while StringBuffer synchronizes its methods.

Key Points
- StringBuilder is generally faster without synchronization
- StringBuffer provides synchronized methods
- Both allow string modification

23. What is ArrayList in Java?

ArrayList is a resizable collection that implements the List interface and supports ordered elements and duplicates.

Code Example

import java.util.ArrayList;

ArrayList<String> names = new ArrayList<>();
names.add("Amit");
names.add("Rahul");

System.out.println(names);

Output

[Amit, Rahul]

24. What is HashMap in Java?

HashMap stores values as key-value pairs. Keys are unique, while values can be duplicated.

Code Example

import java.util.HashMap;

HashMap<String, Integer> marks = new HashMap<>();
marks.put("Amit", 85);
marks.put("Rahul", 90);

System.out.println(marks.get("Amit"));

Output

85

25. What is the Difference Between ArrayList and HashMap?

ArrayList stores elements in an indexed sequence, whereas HashMap stores values associated with keys.

Key Points
- ArrayList uses indexes
- HashMap uses keys
- ArrayList permits duplicate elements
- HashMap keys must be unique

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