String java interview questions

Outlines

• What is String in java
• String pool in Java
• difference between String and StringBuffer in java

What is String in Java

In Java, a String is a class that represents a sequence of characters. It is one of the most commonly used classes in Java and is part of the java.lang package, which means it is automatically imported into every Java program. Strings in Java are immutable, meaning once a String object is created, it cannot be changed. If you want to modify a string, you create a new string object.

Creating Strings in Java:

1. Using String Literal:

You can create a string using double quotes. Java automatically converts string literals to String objects.

String greeting = "Hello, World!";

2. Using the new Keyword:

You can create a string object using the new keyword, which explicitly creates a new instance of the String class.

String greeting = new String("Hello, World!");

String Methods and Operations:

Strings in Java come with a variety of built-in methods for manipulating and analyzing the content. Here are some common operations and methods related to strings:

1. Concatenation:

Strings can be concatenated using the + operator.

String firstName = "John";
String lastName = "Doe";
String fullName = firstName + " " + lastName; // Results in "John Doe"

2. Length:

You can find the length of a string using the length() method.

String text = "Java Programming";
int length = text.length(); // length is 17

3. Substring:

You can extract a substring from a string using the substring() method.

String text = "Hello, World!";
String substring = text.substring(7); // Results in "World!"

4. Equality:

Strings should be compared using the equals() method, not ==.

String str1 = "hello";
String str2 = "hello";
boolean isEqual = str1.equals(str2); // true

5. Immutable Nature:

Strings are immutable; their values cannot be changed after creation. When you modify a string, a new object is created.

String original = "Hello";
String modified = original.concat(", World!"); // Creates a new string object

In summary, a String in Java is a sequence of characters, and it is an important and versatile data type that finds widespread use in Java applications, from simple text manipulation to complex operations involving regular expressions and internationalization.

What is string pool in Java

In Java, the String pool (or String literal pool) is a special area in the JVM (Java Virtual Machine) where String objects are stored. When you create a String object using a string literal (i.e., within double quotes), the JVM checks the String pool first. If the string already exists in the pool, a reference to the existing String object is returned. If not, a new String object is created in the pool, and its reference is returned.

String pooling is done to conserve memory by reusing String objects whenever possible, which is especially useful since strings are immutable in Java (i.e., their values cannot be changed once they are created).

Here’s an example to illustrate how the String pool works:

/*
 * Author: Zameer Ali Mohil
 * */
public class StringPoolExample {
    public static void main(String[] args) {
        String str1 = "hello";
        String str2 = "hello";
        String str3 = new String("hello");

        // true (Both refer to the same String object in the pool)
        System.out.println("str1 == str2: " + (str1 == str2)); 
        // false (str3 is a new String object created in the heap)
        System.out.println("str1 == str3: " + (str1 == str3)); 
        
        // Interning the string creates a reference to the String object in the pool
        str3 = str3.intern();
        // true (After interning, both refer to the same String object in the pool)
        System.out.println("str1 == str3.intern(): " + (str1 == str3)); 
    }
}

In this example:

• str1 and str2 both refer to the same String object in the String pool because they are created using string literals.
• str3 is created using the new keyword, so it creates a new String object in the heap memory.
• By calling intern() on str3, it is interned, meaning a reference to the String object in the pool is returned. After interning, str1 and str3 both refer to the same String object in the pool.

String pooling is particularly useful when you’re dealing with a large number of strings, as it helps conserve memory by reusing common strings instead of creating new objects for every occurrence of the same string value.

difference between StringBuffer and StringBuilder in Java

Both StringBuffer and StringBuilder in Java are used for the same purpose: to create and manipulate mutable sequences of characters. The main difference between them lies in their synchronization behavior:

StringBuffer:

1. Thread-Safe:
   • StringBuffer is synchronized, which means it is thread-safe. Multiple threads can safely use StringBuffer without external synchronization.
2. Slower Performance in Multithreaded Environments:
   • Because of synchronization, StringBuffer can be slower than StringBuilder, especially in multithreaded environments, where synchronization can lead to overhead.
3. Usage:
   • Use StringBuffer when you need a mutable sequence of characters, and your code is running in a multithreaded environment.

Example using StringBuffer:

/*
 * Author: Zameer Ali Mohil
 * */
public class StringBufferExample {
    public static void main(String[] args) {
        StringBuffer buffer = new StringBuffer("Hello");
        buffer.append(" World"); // Modifies the same StringBuffer object
        System.out.println(buffer);
    }
}

StringBuilder:

1. Not Thread-Safe:
   • StringBuilder is not synchronized, which means it is not thread-safe. Multiple threads should not access a StringBuilder instance concurrently without proper synchronization.
2. Faster Performance in Single-Threaded Environments:
   • Because it lacks synchronization, StringBuilder can be faster than StringBuffer, especially in single-threaded environments.
3. Usage:
   • Use StringBuilder when you need a mutable sequence of characters and you are sure that your code will be used in a single-threaded environment, or you will handle synchronization explicitly.

Example using StringBuilder:

/*
 * Author: Zameer Ali Mohil
 * */
public class StringBuilderExample {
    public static void main(String[] args) {
        StringBuilder builder = new StringBuilder("Hello");
        builder.append(" World"); // Modifies the same StringBuilder object
        System.out.println(builder);
    }
}

In both examples, append() is used to modify the content of the StringBuffer and StringBuilder objects. However, it’s important to note that if these examples were run concurrently by multiple threads, you might encounter issues with StringBuffer if synchronization is not handled properly. In such cases, using StringBuilder in a single-threaded environment or handling synchronization manually might be a better choice for improved performance.

Both StringBuffer and StringBuilder are classes in Java that allow you to create mutable sequences of characters. The choice between them depends on your specific use case: use StringBuffer when thread safety is necessary, and use StringBuilder when you are working in a single-threaded environment or can handle synchronization manually.

Difference between String, StringBuffer and StringBuilder in Java

Certainly! Below is a comprehensive comparison between String, StringBuffer, and StringBuilder in Java, highlighting their differences and providing examples for each.

1. String:

• Immutability:
  • String objects are immutable, meaning their values cannot be changed after creation.
• Performance:
  • Concatenating strings using the + operator creates new objects, which can be inefficient for frequent modifications in loops.

Example:

String str1 = "Hello";
String str2 = " World";
// Creates new objects, not efficient in loops
String result = str1 + str2; 

2. StringBuffer:

• Mutability:
  • StringBuffer objects are mutable, allowing modifications of their content.
• Performance:
  • Efficient for concatenating strings in loops due to its mutable nature.

Example:

StringBuffer buffer = new StringBuffer("Hello");
// Modifies the same StringBuffer object, efficient in loops
buffer.append(" World"); 

3. StringBuilder:

• Mutability:
  • StringBuilder objects are mutable, similar to StringBuffer.
• Performance:
  • Faster than StringBuffer in single-threaded environments due to lack of synchronization.

Example:

StringBuilder builder = new StringBuilder("Hello");
// Modifies the same StringBuilder object, efficient in loops (faster than StringBuffer)
builder.append(" World"); 

Comparison Table:

Example Demonstrating Differences:

/*
 * Author: Zameer Ali Mohil
 * */
public class StringExample {
    public static void main(String[] args) {
        // String Example
        String str1 = "Hello";
        String str2 = " World";
        // Inefficient for frequent modifications
        String result = str1 + str2; 

        // StringBuffer Example
        StringBuffer buffer = new StringBuffer("Hello");
        // Efficient for frequent modifications in loops
        buffer.append(" World"); 

        // StringBuilder Example
        StringBuilder builder = new StringBuilder("Hello");
        // Efficient for frequent modifications in loops (faster than StringBuffer)
        builder.append(" World"); 
    }
}

In the example above, String concatenation results in the creation of new objects, which can be inefficient in loops. On the other hand, StringBuffer and StringBuilder are efficient for frequent modifications, with StringBuilder being faster in single-threaded scenarios due to lack of synchronization. The choice between them depends on the specific requirements of the application.