How primitive types and reference types are passed to a method

• When a primitive type is passed to a method, it is passed by value. That means the method can't change the value of the variable itself. Instead, the method must return a new value that gets stored in the variable.
• When a reference type (an object) is passed to a method, it is passed by reference. That means that the method can change the data in the object itself, so a new value doesn't need to be returned by the method.

Primitive types are passed by value.

A method that changes the value of a double type

static double increasePrice(double price)
             // returns a double
{
    return price *= 1.1;
}

Code that calls the method

double price = 49.5;
price = increasePrice(price);   // reassignment statement
System.out.println("price: " + price);
Result
price: 54.45

Objects are passed by reference.

A method that changes a value stored in a Product object

static void increasePrice(Product product)
                          // no return value
{
    double price = product.getPrice();
    product.setPrice(price *= 1.1);
}

Code that calls the method

Product product = ProductDB.getProduct("java");
System.out.println("product.getPrice(): " + product.getPrice());
increasePrice(product);   // no reassignment necessary
System.out.println("product.getPrice(): " + product.getPrice());
Result
product.getPrice(): 49.5
product.getPrice(): 54.45