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Junior

What do you know about the String class in Java?

sobes.tech AI

Answer from AI

String in Java is an immutable class representing a sequence of characters.

Key features:

  • Immutability: Once a String object is created, its contents cannot be changed. Any operation that appears to modify the string actually returns a new String object. This makes String objects thread-safe.
  • String Pool: Java maintains a pool of String literals. When a string literal is created (e.g., "hello"), JVM first checks the pool. If such a string already exists, a reference to the existing object is returned. If not, a new object is created in the pool. Using the constructor new String("hello") always creates a new object outside the pool.
  • Operator + Overloading: The + operator is overloaded for string concatenation. The compiler can optimize concatenation using StringBuilder or StringBuffer for better performance.
  • Interface Implementations: String implements Serializable, Comparable<String>, and CharSequence.
  • Methods: The String class provides many useful methods such as length(), charAt(), substring(), indexOf(), equals(), equalsIgnoreCase(), startsWith(), endsWith(), trim(), replace(), split(), and others.

Examples of creating strings:

// Creation via literal (uses string pool)
String str1 = "Hello";

// Creation via constructor (creates a new object outside the pool)
String str2 = new String("World");

// Creation from a character array
char[] chars = {'J', 'a', 'v', 'a'};
String str3 = new String(chars);

String comparison:

  • equals(): Compares string contents.
  • ==: Compares references. For string literals with the same content, it usually means the same reference from the pool, but for objects created with new String(), == is false even if contents are the same.
String s1 = "java";
String s2 = "java";
String s3 = new String("java");

System.out.println(s1 == s2); // true (both from pool)
System.out.println(s1 == s3); // false (s3 is a new object)
System.out.println(s1.equals(s3)); // true (compares contents)

To modify strings in Java, classes StringBuilder (not synchronized, for single-threaded environments) and StringBuffer (synchronized, for multi-threaded environments) are used, which are mutable.

// Example with StringBuilder
StringBuilder sb = new StringBuilder("Hello");
sb.append(" World");
String result = sb.toString(); // Convert back to String
System.out.println(result); // Output: Hello World
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