Guide to Object-oriented Programming With Java (University at Buffalo Version)

Chapter 4: Java Language Structures

In addition to the previously mentioned Java structures (projects, packages, classes, and comments), Java's other structures include: identifiers, keywords, variables, and methods. Most Java programmings use camelCase to name their variables, identifiers, and methods. For example, a variable to store the first name would be: firstName.

Classes

As previously mentioned, a class is a "container" for code. Each Java program can contain only one (1) class method. The name of the class must begin with an Uppercase letter. The name of your .java file should be the same as the name of your class. For example, the file HelloWorld would have a public class HelloWorld { } statement. All code in a class in enclosed in braces: { }. A class can have different access modes Valid Access modifiers are:

public - the class is available to any other class in the any package
protected - ony available to the classes in the packages and its subclasses
no modifier - only available to the classes in the same package
private - a member of a c lass can only access that class

Figure 4-1: Access Modifiers

Identifiers

An identifier is a sequence of one or more characters. The first character must be a valid first character (letter, $, _). Each subsequent character in the sequence must be a valid nonfirst character (letter, digit, $, _).

BMIcalculator
w
h
weight
height

Figure 4-2: Java Identifiers

Keywords

A keyword is a reserved word.

boolean
break
case
if
int
String

Figure 4-3: Some Common Java Keywords

Variables

In Java variables are devices that are used to store data, such as a number, an array, an object, or a string of character data. Java is considered as a strongly typed programming language so all variables should be declared as a type before they can be used. Variables follow the same naming conventions as other Java structures except variable start with a lowercase letter.

// declare variables
double w;
double h;
double BMI;

Figure 4-4: Sample Java Variables

Methods

A method is a similiar to functions in C++ or PHP.

public BMIcalculator(double w, double h) {
weight = w;
height = h;
}

Figure 4-5: A Sample Java Method

package edu.buffalostate.cis425.sp24..put-your-username-here;

/**
* File: BMIcalculator.java
*
* Calculate Body Mass Index (BMI)
* It prints a the BMI to standard out
*
* @author Your Name Here
*
*/

public class BMIcalculator {

// declare variables
double w;
double h;
double BMI;

public BMIcalculator(double w, double h) {
weight = w;
height = h;
}

public calculateBMI() {
return weight / (height * height);
}

/**
* The main() is the entry point for a Java program when executed
*
* @param string array - command line arguments passed to program
* @return none
*
*/
public static void main(String[] args) {
BMIcalculator calculator = new BMIcalculator(60, 1.70);
double bmi = calculator.calculateBMI();
// print to the console
System.out.println("Your BMI is " + bmi);
}
}

Figure 4-6: BMIcalculator.java Code

Console — Figure 4-7: `BMIcalculator.java` Output

Let's get started with a variables/methods program!