My Java Notes: Understanding String

Source: Dev.to

What Is a String in Java?

A String in Java is a sequence of characters and is an object of the java.lang.String class.

Example

String name = "Java";

Here, "Java" is a string literal.

Ways to Create a String

Using a String Literal

String s1 = "Hello";

• Stored in the String Constant Pool (SCP)
• Memory‑efficient

Using the new Keyword

String s2 = new String("Hello");

• Creates a new object on the heap, independent of the SCP

Why Strings Are Immutable?

In Java, String objects cannot be changed after they are created. Any operation that appears to modify a string actually creates a new String object.

Example

String s = "Java";
s = s.concat(" Programming"); // a new String is created

String Constant Pool (SCP)

The SCP is a special area of the heap that stores string literals. When a literal is created, the JVM checks:

• If an identical string already exists in the SCP → reuse it
• Otherwise → create a new object in the SCP

Example

String s1 = "Java";
String s2 = "Java"; // s1 and s2 reference the same object

String s3 = new String("Java"); // a distinct object on the heap

Why SCP Is Important?

• Saves memory by avoiding duplicate objects
• Improves performance through reuse of existing strings

Advantages of the String Constant Pool

• Stores only one copy of identical literals
• Reduces memory consumption
• Faster comparison of string literals (==)
• Provides better security due to immutability
• Managed automatically by the JVM

Disadvantages of the String Constant Pool

• Strings are immutable, so frequent modifications are costly
• Excessive literals can increase heap usage
• Slight overhead for JVM checks when creating strings
• Less flexible than StringBuilder or StringBuffer for mutable operations

Commonly Used String Methods in Java

length()

Returns the length of the string.

String s = "Java";
System.out.println(s.length()); // 4

charAt(int index)

Returns the character at the specified index.

System.out.println(s.charAt(1)); // a

equals(String another)

Compares the content of two strings.

String a = "Java";
String b = "Java";
System.out.println(a.equals(b)); // true

equalsIgnoreCase(String another)

Compares strings ignoring case.

System.out.println(a.equalsIgnoreCase("java")); // true

compareTo(String another)

Compares strings lexicographically.

System.out.println("Java".compareTo("Java")); // 0

toUpperCase()

Converts the string to uppercase.

System.out.println("java".toUpperCase()); // JAVA

toLowerCase()

Converts the string to lowercase.

System.out.println("JAVA".toLowerCase()); // java

trim()

Removes leading and trailing whitespace.

String s = "  Java  ";
System.out.println(s.trim()); // "Java"

contains(CharSequence s)

Checks if the string contains a given sequence.

System.out.println("Java Programming".contains("Java")); // true

startsWith(String prefix)

Checks whether the string starts with the specified prefix.

System.out.println("Java".startsWith("Ja")); // true

endsWith(String suffix)

Checks whether the string ends with the specified suffix.

System.out.println("Java".endsWith("va")); // true

substring(int beginIndex)

Returns a substring starting from beginIndex.

System.out.println("Java".substring(1)); // "ava"

substring(int beginIndex, int endIndex)

Returns a substring between the specified indexes.

System.out.println("Java".substring(1, 3)); // "av"

replace(char oldChar, char newChar)

Replaces occurrences of a character.

System.out.println("Java".replace('a', 'o')); // "Jovo"

split(String regex)

Splits the string into an array based on the given regular expression.

String[] words = "Java is easy".split(" ");