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
andstr2
both refer to the same String object in the String pool because they are created using string literals.str3
is created using thenew
keyword, so it creates a new String object in the heap memory.- By calling
intern()
onstr3
, it is interned, meaning a reference to the String object in the pool is returned. After interning,str1
andstr3
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
:
- Thread-Safe:
StringBuffer
is synchronized, which means it is thread-safe. Multiple threads can safely useStringBuffer
without external synchronization.
- Slower Performance in Multithreaded Environments:
- Because of synchronization,
StringBuffer
can be slower thanStringBuilder
, especially in multithreaded environments, where synchronization can lead to overhead.
- Because of synchronization,
- Usage:
- Use
StringBuffer
when you need a mutable sequence of characters, and your code is running in a multithreaded environment.
- Use
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
:
- Not Thread-Safe:
StringBuilder
is not synchronized, which means it is not thread-safe. Multiple threads should not access aStringBuilder
instance concurrently without proper synchronization.
- Faster Performance in Single-Threaded Environments:
- Because it lacks synchronization,
StringBuilder
can be faster thanStringBuffer
, especially in single-threaded environments.
- Because it lacks synchronization,
- 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.
- Use
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.
Aspect | StringBuffer | StringBuilder |
---|---|---|
Thread Safety | Synchronized, thread-safe. Multiple threads can safely access StringBuffer methods simultaneously. | Not synchronized, not thread-safe. Not suitable for concurrent use by multiple threads. |
Performance | Slower compared to StringBuilder due to synchronization. | Faster compared to StringBuffer in single-threaded environments as it lacks synchronization. |
Mutability | Mutable. Can be modified after creation. | Mutable. Can be modified after creation. |
Usage | Suitable for multithreaded applications where thread safety is required. | Suitable for single-threaded environments or when handling synchronization manually. |
Introduced In | Introduced in Java 1.0. | Introduced in Java 5.0. |
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.
- Concatenating strings using the
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 toStringBuffer
.
- Performance:
- Faster than
StringBuffer
in single-threaded environments due to lack of synchronization.
- Faster than
Example:
StringBuilder builder = new StringBuilder("Hello");
// Modifies the same StringBuilder object, efficient in loops (faster than StringBuffer)
builder.append(" World");
Comparison Table:
Aspect | String | StringBuffer | StringBuilder |
---|---|---|---|
Immutability | Immutable | Mutable | Mutable |
Thread Safety | n/a | Synchronized, thread-safe | Not synchronized, not thread-safe |
Performance | Moderate (concatenation creates new objects) | Efficient for single-threaded use, but slower due to synchronization | Very efficient, especially in single-threaded use |
Usage | When content should not change frequently | When thread safety is required | When thread safety is not required, and performance is crucial |
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.