Thinking in Java
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Fields and methods

When you define a class (and all you do in Java is define classes, make objects of those classes, and send messages to those objects), you can put two types of elements in your class: fields (sometimes called data members), and methods (sometimes called member functions). A field is an object of any type that you can communicate with via its reference. It can also be one of the primitive types (which isn’t a reference). If it is a reference to an object, you must initialize that reference to connect it to an actual object (using new, as seen earlier) in a special method called a constructor (described fully in Chapter 4). If it is a primitive type, you can initialize it directly at the point of definition in the class. (As you’ll see later, references can also be initialized at the point of definition.)

Each object keeps its own storage for its fields; the fields are not shared among objects. Here is an example of a class with some fields:

class DataOnly {
  int i;
  float f;
  boolean b;
}

This class doesn’t do anything, but you can create an object:

DataOnly d = new DataOnly();

You can assign values to the fields, but you must first know how to refer to a member of an object. This is accomplished by stating the name of the object reference, followed by a period (dot), followed by the name of the member inside the object:

objectReference.member

For example:

d.i = 47;
d.f = 1.1f; // ‘f’ after number indicates float constant
d.b = false;

It is also possible that your object might contain other objects that contain data you’d like to modify. For this, you just keep “connecting the dots.” For example:

myPlane.leftTank.capacity = 100;

The DataOnly class cannot do much of anything except hold data, because it has no methods. To understand how those work, you must first understand arguments and return values, which will be described shortly.

Default values for primitive members

When a primitive data type is a member of a class, it is guaranteed to get a default value if you do not initialize it:

Note carefully that the default values are what Java guarantees when the variable is used as a member of a class. This ensures that member variables of primitive types will always be initialized (something C++ doesn’t do), reducing a source of bugs. However, this initial value may not be correct or even legal for the program you are writing. It’s best to always explicitly initialize your variables.

This guarantee doesn’t apply to “local” variables—those that are not fields of a class. Thus, if within a method definition you have:

int x;

Then x will get some arbitrary value (as in C and C++); it will not automatically be initialized to zero. You are responsible for assigning an appropriate value before you use x. If you forget, Java definitely improves on C++: you get a compile-time error telling you the variable might not have been initialized. (Many C++ compilers will warn you about uninitialized variables, but in Java these are errors.)

Thinking in Java
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