Method Parameters
The Java programming language always uses call by value. That means that the method
gets a copy of all parameter values. In particular, the method cannot modify the con-
tents of any parameter variables that are passed to it.
There are, however, two kinds of method parameters:
• Primitive types (numbers, boolean values)
• Object references
You have seen that it is impossible for a method to change a primitive type parameter.
The situation is different for object parameters（The method gets a copy of the object reference, and both the original and the copy refer to the same object.） .
summary:
• A method cannot modify a parameter of primitive type (that is, numbers orboolean values).
• A method can change the state of an object parameter.
• A method cannot make an object parameter refer to a new object.
Object Construction
Here is what happens in detail when a constructor is called:
1. All data fields are initialized to their default value (0, false, or null).
2. All field initializers and initialization blocks are executed, in the order in which they occur in the class declaration.
3. If the first line of the constructor calls a second constructor, then the body of the second constructor is executed.
4. The body of the constructor is executed.