Study guide: Java Constructors and Object Lifetime

📋 Course Outline

1. Object Creation and Constructors
2. Constructor Rules and Types
3. Overloading and Constructor Chaining
4. Object References and Memory
5. Garbage Collection and Resource Handling

📖 1. Object Creation and Constructors

🔑 Key Concepts & Definitions

• Constructor: Prepares a new object by initializing its fields or performing setup, and runs when the program creates the object with new and a matching constructor call.

📝 Essential Points

• In class Student { String name; Student() { name = "New Student"; } }, calling new Student() runs Student() and assigns New Student to name.

📖 2. Constructor Rules and Types

🔑 Key Concepts & Definitions

• No-argument constructor: Has no parameters, as in Student() { }.
• Parameterized constructor: Accepts input values, as in Student(int id, String name) { ... }.

★ Must-know

📌 A constructor's name must be identical to its class name, and a constructor has no return type.

Further detail

• Constructors cannot be declared:
  • static
  • final
  • abstract
  • synchronized

📖 3. Overloading and Constructor Chaining

★ Must-know

• Constructor overloading means creating constructors with different parameter lists.

• Constructor chaining uses this() to chain constructors within a class and super() to chain across inheritance; either call must appear before any other statement in the constructor.

Further detail

• Declaring a custom constructor means a programmer should not expect a default constructor to be provided.

📌 A constructor call must match the number and types of arguments accepted by the constructor.

📖 4. Object References and Memory

🔑 Key Concepts & Definitions

• Scope: The part of a program where a variable name can be accessed.

★ Must-know

📌 A class describes structure, while each object created from the class has its own state.

• 🔄 An object life cycle proceeds through: class loading, object creation with new and a constructor, object use through a reference, possible eligibility for garbage collection when no live path reaches it

📌 Method-call frames and local variables or references are commonly associated with the stack, whereas objects are generally allocated in the heap.

Further detail

• In Student s = new Student();, the variable s refers to the Student object created by new Student().

📌 Assigning null to a reference means that reference no longer points to the object, but another reference may still reach it.

📖 5. Garbage Collection and Resource Handling

★ Must-know

📌 An object is reachable when a live reference path leads to it; an object is unreachable when no live reference path reaches it and may then be eligible for garbage collection.

• Eligibility for garbage collection does not mean that the object will be collected immediately.

📌 Java manages object memory automatically, but files, sockets, and database connections still need to be closed properly.

Further detail

• Calling System.gc() does not force the JVM to collect a specific object immediately.

• Calling a method through a null reference is a common programming error.

💡 Memory Hook

Reachable, unreachable, eligible, then possibly collected—not necessarily immediately.