What concepts underlie the use of reference data types?

Reference data types in Java are objects that variables refer to, rather than directly storing the value. Key concepts:

• Object reference: A variable stores the address of the object in memory, not the object itself.
• Shared access: Multiple variables can refer to the same object, affecting the object's state.
• Pass-by-reference: When passing an object to a method, a copy of the reference is passed, not a copy of the object.
• Null references: A reference type variable can be null, which requires checking before use.

Example:

String a = new String("hello");
String b = a; // b refers to the same object as a
b = "world"; // b now refers to a new object, a remains unchanged

Understanding reference types is important for memory management and avoiding errors related to object modification.