Introduction, Primitive Java

Computer programming is an essential skill not only in the field of computing, but also engineering, science, mathematics, and every discipline involving quantitative data. For example: engineers build models of systems before building the systems themselves, in order to understand the important issues. The computer is the ultimate tool for modeling and simulation.

In this course, we will learn about the techniques needed to write complex programs and to understand their behavior.

Review of Primitive Java

“Primitive Java” - the subset of Java that is essentially C.

A Java program is a collection of Java classes. A Java class is a user-defined data type, very much like a C struct type. However, in addition to having fields (member variables), a Java class also has methods (member functions). A class is a way to add “behavior” to instances of a user-defined data type. This is the key idea in object-oriented programming, which will be a major theme of this course.

Example Java class (type and run in within Eclipse):

public class Hello {
  public static void main(String[] args) {
    System.out.println("Hello, world!");
  }
}

Things to note:

• A class is a sequence of fields and methods. In the Hello class, there is a single method, called main.
• The main method returns void, is public and static. public means it’s accessible by other classes. static means it is not an instance method. When you see static on a method, you should think of it as being like a C function.
• System.out.println is a method call which prints text to the console.

Getting information from the keyboard

Here’s a slightly more interesting Java program:

import java.util.Scanner;

public class Hello {
  public static void main(String[] args) {
    Scanner keyboard = new Scanner(System.in);  // (1)

    System.out.println("What is your name? ");  // (2)
    String name = keyboard.next();              // (3)

    System.out.println("Hello, " + name);       // (4)
  }
}

The import directive tells the Java compiler that we want to use the java.util.Scanner class. This class is very useful for reading information, such as text strings, numbers, etc. from the keyboard,

Line 1 of the main method creates an instance of the java.util.Scanner class. In Java, an instance of a class is called an object. We’ll talk more about this idea next time, but for now, just think of this line as causing a Scanner reading from the keyboard to come into existence. We use the variable called keyboard to refer to the newly-created Scanner.

Line 3 uses the Scanner object to read a string value from the keyboard, and saves the string in a variable called name. This is an example of calling an instance method. Specifically, we’re calling the next method on the Scanner object referred-to by the variable keyboard. A method call is a request for an object to do something. We’ll talk more about method calls next time. For now, remember that a method call is the most important way to get something done in an object-oriented program.

Line 4 prints some text to the console. It uses string concatenation to combine the fixed string value “Hello, “ with whatever string the user typed when line 3 was executed. In Java, the plus (+) operator performs string concatenation whenever at least one of its operands is a string.

System.out.printf

The built-in System.out.printf method in Java works almost exactly the same way as the printf function in C. Here’s a variation on the program above:

import java.util.Scanner;

public class Hello {
  public static void main(String[] args) {
    Scanner keyboard = new Scanner(System.in);

    System.out.println("What is your name? ");
    String name = keyboard.next();

    System.out.printf("Hello, %s\n", name);
  }

Arithmetic

Java supports many familiar numeric data types and operators that are present in C. For example:

C data type	Java data type
int	int
short	short
char	char
float	float
double	double

Pretty easy to remember, huh?

Here’s a simple program which does a computation on two integer values:

import java.util.Scanner;

public class Hello {
  public static void main(String[] args) {
    Scanner keyboard = new Scanner(System.in);  // (1)

    System.out.println("Enter two integers:");  // (2)
    int a = keyboard.nextInt();                 // (3)
    int b = keyboard.nextInt();                 // (4)

    int sum = a + b;                            // (5)
    System.out.println("The sum is " + sum);    // (6)
  }
}

Lines 3 and 4 use the Scanner class’s nextInt method to read an integer value.

Line 6 demonstrates that string concatenation works for combining a string value with a numeric value.

Summary

• Java is object-oriented
• A class is a user-defined data type, like a struct type in C, but you can add behavior to it
• A method is a function that belongs to a class and can perform operations on instances of the class (which are called “objects”)
• “Primitive” Java is a subset of Java that is very similar to C
• Use System.out.println and System.out.printf to print output
• String concatenation creates a new string of characters by appending a value to an existing string
• Use a Scanner object to get input from the user