Constructor vs Method in Java

Direct Answer

A constructor initializes a new instance. It has the class name and no declared return type, and Java invokes it as part of object creation. A method has its own name and return type and can be invoked after construction. One object-creation operation may run several constructors through this(...) and super(...) chaining, so 'a constructor runs once' is not the full model.

Code Example

class Account {
    private double balance;

    // CONSTRUCTOR: same name as class, NO return type
    public Account(double opening) {
        this.balance = opening;         // initialize state on creation
    }

    // METHOD: any name, must have a return type (void here)
    public void deposit(double amt) {
        balance += amt;                 // behavior called on an existing object
    }

    // WARNING: this is a METHOD, not a constructor!
    // public void Account() { }       // <-- 'void' makes it an ordinary method
}

Account a = new Account(100);           // constructor runs once, automatically
a.deposit(50);                          // method called explicitly, balance = 150
a.deposit(25);                          // called again — methods have no limit

Common Mistakes & Pitfalls

• Giving a constructor a return type — adding void (or any type) silently demotes it to a regular method, so your initialization never runs and the bug compiles cleanly without a single warning.
• Expecting constructors to be inherited — a subclass does not inherit the parent's constructors; it must define its own and reach the parent's via super().
• Calling a constructor like a method — writing obj.Student() won't compile; a constructor fires only through new, never on an object that already exists.
• Assuming a no-arg constructor always exists — the moment you write any constructor of your own, the free default disappears and new Student() stops compiling.