Java Strings

In Java, strings are a core feature, used to represent sequences of characters. Java provides the String class to create and manipulate strings.

1. Creating Strings

Strings in Java can be created in several ways:

• Using String Literals:

  String str1 = "Hello, World!";

  This creates a string object in the string pool if it doesn't already exist.
• Using the new Keyword:

  String str2 = new String("Hello, World!");

  This creates a new string object in the heap memory, even if the string already exists in the string pool.

2. String Characteristics

Immutability: Strings in Java are immutable, meaning once a String object is created, its value cannot be changed. Any modification to a string results in the creation of a new string.

String Pool: Java maintains a special memory region called the 'string pool' to store string literals. If a string is created using a literal and already exists in the pool, Java will reuse the reference rather than creating a new object.

3. Common String Methods

Java provides many useful methods to manipulate and work with strings:

• Length of String:

  int len = str1.length();

  Returns the length of the string.
• Concatenation:

  String str3 = str1 + " How are you?";

  Using the + operator.

  String str4 = str1.concat(" How are you?");

  Using the concat method.
• Character Access:

  char c = str1.charAt(0);

  Returns the character at the specified index.
• Substring:

  String sub = str1.substring(7);

  Returns a substring from index 7 to the end.

  String sub2 = str1.substring(7, 12);

  Returns a substring from index 7 to 11.
• String Comparison:

  boolean equals = str1.equals(str2);

  Compares strings for equality.

  boolean equalsIgnoreCase = str1.equalsIgnoreCase("hello, world!");

  Ignores case.

  int cmp = str1.compareTo(str2);

  Lexicographically compares two strings.
• String Search:

  int index = str1.indexOf("World");

  Returns the index of the first occurrence of "World".

  int lastIndex = str1.lastIndexOf('o');

  Returns the last occurrence of 'o'.
• String Replace:

  String replaced = str1.replace("World", "Java");

  Replaces "World" with "Java".
• Trim:

  String trimmed = str1.trim();

  Removes leading and trailing whitespace.
• Split:

  String[] parts = str1.split(",");

  Splits the string into an array based on the delimiter.
• Convert to Upper or Lower Case:

  String upper = str1.toUpperCase();

  Converts the string to uppercase.

  String lower = str1.toLowerCase();

  Converts the string to lowercase.

4. StringBuffer and StringBuilder

For scenarios requiring frequent modification of strings, Java provides StringBuffer and StringBuilder classes:

• StringBuffer:
  Mutable sequence of characters, thread-safe.

  StringBuffer sb = new StringBuffer("Hello");
  sb.append(" World!");

  Modifies the same object.
• StringBuilder:
  Similar to StringBuffer, but not thread-safe (faster in single-threaded environments).

  StringBuilder sb = new StringBuilder("Hello");
  sb.append(" World!");

  Modifies the same object.

5. String Conversion

Strings can be easily converted to other data types and vice versa:

• String to Primitive Types:

  int num = Integer.parseInt("123");
  double d = Double.parseDouble("3.14");

• Primitive Types to String:

  String str = String.valueOf(123);
  String str2 = Integer.toString(456);

6. String Interning

The intern() method can be used to return a canonical representation of the string, ensuring that all equal strings share the same memory reference.

String interned = str1.intern();

7. String Formatting

Java provides a String.format() method to create formatted strings:

String formatted = String.format("Hello, %s! You are %d years old.", "John", 25);

8. Checking if a String is Empty

boolean isEmpty = str1.isEmpty();

Checks if the string is empty.