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Abstract Data Types

• Definition: A variable can be declared using an abstract data type (ADT), one common form being a class.

• Classes in Java: Many predefined classes in Java, as well as user-defined classes, encapsulate data and methods.

  • Object: A variable declared with a class is referred to as an object, which stores a reference to the object's data and methods.

  • Instantiation: The process of creating a new object is termed instantiation.

• Creating an Object:

  • Syntax: <class> <variable name> = new <class>(<arguments>);

  • Example: Circle spot = new Circle(4); creates a Circle object initialized with a radius of 4.

• Method Access: Use the dot operator to access members of a class. Example: spot.getRadius() retrieves the radius of the spot object.

Java Packages

• Java Runtime Environment (JRE): Comes with several packages (collections of classes).

  • Key Packages:

    • java.lang: Fundamental classes.

    • java.util: Utility classes (e.g., reading input, random number generation).

• Package Documentation: Summarizes available classes and respective data/methods, often with code examples.

• Naming Convention:

  • Packages start with java. followed by dots and the package name.

  • Example: import java.util.Scanner; imports the Scanner class.

• Accessing Packages: The import statement allows access to package members. Example: import java.util.*; imports all classes in the util package.

Obtaining User Input

• Input Stream: Sequence of characters from an input device (e.g., keyboard).

• Scanner Class: Utilized to read integers, floats, and strings from an input stream.

  • Instantiation with Input Stream:

    • Scanner input = new Scanner(System.in);

• Scanner Methods:

  • next() - Returns a string from input until a space.

  • nextLine() - Returns the whole line.

  • nextInt() - Reads an integer.

  • nextDouble() - Reads a double.

  • nextBoolean() - Reads a boolean.

  • close() - Closes the input stream.

• Important Note: Use nextLine() carefully after nextInt() to avoid reading empty strings due to end-of-line character.

• Example Code: An application that calculates the area of a rectangle:

public class RectangleArea2 {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.print("Enter length: ");
        int length = input.nextInt();
        System.out.print("Enter width: ");
        int width = input.nextInt();
        input.close();
        int area = length * width;
        System.out.println("Area: " + area);
    }
}

Numeric Expressions

• Operators:

  • Arithmetic: +, -, *, /, %.

• Numeric Expression: Contains at least one operand and may include operators.

  • Example: 6 + 5.

• Division Behavior:

  • Integer division when both operands are int.

  • Real division returns the entire quotient including the decimals and is performed if one or more operator is a double.

• Modulus Operator: Modulus division returns the remainder resulting from division. The % operator truncates the operands, if necessary, to return an integer:this means that when using the modulus operator with integer values, any decimal portion of the operands is discarded, ensuring that the result is always an integer.

• Modulus division is useful for retrieving digits of a number.

• Operator Precedence:

  • Order: Multiplication/Division > Addition/Subtraction.

  • Parentheses can override default precedence. Example: (6 + 4) * (2 - 1).

Assignment Operators

• Basic Assignment: = assigns the evaluated expression on the right to the variable on the left.

• Compound Assignment: +=, -=, *=, /=, %= perform operations before assigning.

  • Example: numPlayers += 2; increases numPlayers by 2.

• Decimal Formatting: The DecimalFormat class allows formatting of numbers with specified decimal places.

Constants

• Definition: A constant is a variable declared with final keyword whose value cannot be changed after assignment.

  • Syntax: final <type> <identifier>.

  • Example: final double PI = 3.14;

• Naming Convention: Constant identifiers are typically uppercase with underscores.

• Usage: Used for values that maintain consistency throughout the program, adding clarity.

Identifiers and Keywords

• Identifying Rules: Start with a letter; case-sensitive.

• Keywords: Special reserved words in Java that cannot be used as identifiers. Examples include class, int, final.

Error Types in Programming

• Syntax Error: Rules of Java are violated, identified by the compiler.

• Logic Error: Program executes but produces incorrect results; must be diagnosed by the programmer.

• Run-time Error (Exception): Occurs during execution, throwing an error (e.g., division by zero).

Case Study: Birthday Game

• Objective: Create a game that determines a player’s birthday based on a mathematical calculation.

• Algorithm: Steps for the player to compute a number from their birth month and day.

• Code Structure: Includes input handling, calculations, and displaying results.

• Example: The player calculates a modified number and inputs it; the program outputs their birthday based on the calculations.