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Java: Variables

Quick summary

  • Variables are referred to as fields in Javadoc
  • Variables include the data type (int, String) before the variable name
  • Variables are only valid within the block {} in which they appear (their scope)

Eclipse example: variables

There are three parts to declaring a variable: data type, variable name, initial value.

Example:

int newVariable = 10;

If you’re declaring a primitive, the data type is spelled in all lower-case: int, double, float, boolean, char.

If you’re creating an complex object, the data type is uppercase.

example:

Date newDate = new Date();

You don’t have to initialize the value here, though. You could separate the two activities, just like you do with primitives.

Date newDate;

new Date();

The Date() part is the constructor for this class. The new keyword precedes this constructor.

Some constructors require parameters; others do not. The API documentation specifies.

If you don’t initialize it, the variable will have a value of null.

Data newDate;

Variables have validity only within the scope they are declared. If you declare the variable inside a code block, the variable will only be valid inside the code block.

If you want variables to be available to multiple methods or functions, you can use a class variable, aka, a field. A variable declared outside a function will belong to the entire class. in that case, it’s called a field (aka class variable). The Javadocs refers to this as a field.

But if the variable is just declared within the scope of a method, between the block {}, it’s a local variable.

If the variable is declared for an object instantiated from a class, it’s called an “instance variable.”

Numeric data types

There are a set number of data types for numeric primitives: byte, short, int, long, float, double, BigDecimal.

int i = 10;
float f = 150.5f;
double d= 150.5d;

For the float and double, you include f and d at the end of the number’s value.

BigDecimal

The BicDecimal class is used when you need precise values. You use strings for the values of Big Decimals.

double d = 1115.27;
String ds =

Variable types

  • fields - variable that is in scope for the whole class
  • instance variables - variables available to objects of a class – they differ based on the instance; the opposite is a class variable
  • class variables - variables available to the class, not to any objects
  • local variables - a variable declared inside the body of a method – unavailable outside the scope of the method

When you have variables within a class, they’re referred to as “fields” or “instance variables.”

public static final

Suppose we have the following setter and getter methods:

class Child {
private double childAge;
public void setAge (double newAge) {
childAge = newAge;
}
public double getAge() {
return childAge;
}
}

In this example, here’s what the following are called:

  • childAge: instance variable or object field
  • newAge: local variable

“A field is valid (in scope) throughout the whole class/object. A field is in fact a kind of variable, but it applies to the whole class/object.”

“A class variable is what you call a static field. That has scope throughout the whole class, and is accessible from any method of that class, static or instance (non-static).

An instance field is not accessible from any static parts of the class.”

public class Kettle
{
private int temperature;

public void becomeHotter(int degrees)
{
int heat = temperature + degrees;
temperature = heat;
}

public int getTemperature()
{
return temperature;
}
. . .
}

“The variable temperature is declared inside the class and outside the methods; you call it a field. It is accessible and in scope in the two methods shown. The “variable” degrees is “declared” inside the () for the becomeHotter() method; it is a parameter, and is not a field. Its scope is that method. The variable heat is declared inside the becomeHotter method. It is a local variable and its scope is from “int heat . . .” to the end of the method. It is not a field.”

Access modifiers

Best practice is to make the fields of a class private and use getter and setter methods for objects to access the field values.

Best practices with access modifiers for fields

You usually want to avoid using public before string names. This protects you from users changing the variable names and screwing up your code.

For example, suppose your class were the following:

public class Hamster {
public int furCount = "2000";
}

Then someone theoretically could change it by doing this:

Hamster.furCount = '3000';

Therefore, Java requires you to create an object from Hamster first and then set furCount, like this:

Hamster hammy = new Hamster();
hammy.furCount = 30;

This way I’m only changing the instance variable for hammy, not for Hamster.

[I’m actually not sure if I got this right – need to follow up and check.]

If you do make your variables public, you often use public static final to make them public but lock them down in a specific state. This is a constant. The constant is made in uppercase.

public final static in ID = 8;

Suppose I have a class called Plant. And in that class, I declare an instance variable called name.

public class Plant {

public String name;

}

In my main method, I can access that variable through dot notation like this: class.variable:

public class App {

public static void main(String[] args) {
Plant plant = new Plant();
plant.name = "mark";
System.out.println("plant name: " + plant.name);

}

}

Now suppose I create a constructor in my class that sets the value of the instance variable. A constructor, remember, is named the same as the class:

public class Plant {

public String name;

public Plant () {
this.name = "Freddy";
}
}

Now when I instantiate the class, it will use the value set for the instance variable:

public class App {

public static void main(String[] args) {
Plant plant = new Plant();
plant.name = "mark";
System.out.println("plant name: " + plant.name); //
Mark
}

}

With instance variables, you usually want to encapsulate the variables so that people can’t directly change their values. Instead, you use methods that control how people can access and use/define the values.

this is just a shortcut for the name.

About Tom Johnson

Tom Johnson

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