Mis on stringide bassein Java-s?

String pool is a special memory area in Java's heap where string literals and interned strings are stored. Its main purpose is to save memory by reusing identical string values.

How it works:

When a string literal (e.g., "hello") is created, the JVM first checks if a string with the same value already exists in the string pool.

If it exists: JVM returns a reference to the existing object in the pool.
If it does not exist: JVM creates a new String object in the pool and returns a reference to it.

Creating strings and the pool:

Using a string literal:

// "hello" is a string literal
String s1 = "hello";
String s2 = "hello";
// s1 and s2 refer to the same object in the pool

Using the new String() constructor:

String s3 = new String("world");
String s4 = new String("world");
// s3 and s4 refer to different objects, even if the value is the same.
// The string literal "world" may be in the pool, but s3 and s4 are not.

Using the intern() method:

String s5 = new String("java");
// The intern() method checks if a string with the same value exists in the pool
String s6 = s5.intern();
// If it exists, returns a reference to the pooled object
// If not, adds the current string to the pool and returns a reference to it

Advantages of the pool:

Memory savings: Reduces the number of duplicated string objects.
Performance improvement: Comparing string literals (==) can be faster than comparing String objects with equals(), if both operands refer to the same object in the pool.

Important:

The == operator checks if two variables refer to the same object in memory. To compare string contents, always use the equals() method:

String str1 = "abc"; // in the pool
String str2 = "abc"; // in the pool, same reference
String str3 = new String("abc"); // outside the pool
String str4 = str3.intern(); // in the pool, same reference as `str1` and `str2`

System.out.println(str1 == str2); // true
System.out.println(str1 == str3); // false
System.out.println(str1 == str4); // true
System.out.println(str1.equals(str3)); // true