Why are you here?

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:

Printing information to the console

Notice that in the above method, we printed text to the screen/console using System.out.println() with the literal text enclosed in quotation marks. This method automatically appends a newline character to the end such that subsequent print statements will begin on new lines, i.e. you do not need to include \n.

If you do not want the newline character automatically appended, i.e. you want subsequent print statements to continue on the same line, use System.out.print().

Unlike C where we needed to use printf() with various placeholders in order to include values from variables/expressions, for System.out.println() we simply concatenate literal text with the variables using the + operator. Java will convert the numeric types to their corresponding String representation (or use the .toString() method for objects which will be discussed later). For example, assuming a person’s age is stored in an int variable called age, we could print their age using

  System.out.println("Your age is " + age);

System.out.printf

Although discouraged, alternatively, we can still use the C type printf() statement with formatting placeholders that are identical to C. Note: You will need to include explicit newline \n characters with this method. Thus the above statement would be

  System.out.printf("Your age is %d\n",age);

Getting information from the keyboard

To get user input from the keyboard, we need to create a Scanner object that reads data from System.in. The Scanner object contains methods for obtaining inputs of different datatypes.

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 which will obtain the user input as a String. 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.

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