Chapter 5

  • Chapter 5 - Using Pre-Built Methods
    • Understanding how to utilize pre-built classes from the Java API Library can streamline development and avoid redundant code.
  • The API Library
    • Pre-built classes should be leveraged to avoid unnecessary complexities.
    • Example: Use the Scanner class for user input instead of creating custom input methods.
    • The Math class simplifies complex mathematical calculations.
    • Java's API Library (Application Programming Interface) holds these ready-to-use classes.
    • Classes must be imported to be used in program, e.g., import java.util.Scanner;.
    • Packages: Groups of classes, e.g., java.util includes various utility classes like Scanner.
    • Some classes (like Math) are auto-imported from the java.lang package.
  • API Headings
    • Interaction with API classes requires an understanding of their methods and how to call them.
    • Example: To perform input, it’s necessary to know methods like next, nextLine, nextInt, etc.
    • Source Code Headings: Describe method signatures that indicate return types and parameters.
    • Example: public int nextInt() - returns an integer and takes no parameters.
  • Math Class
    • Key methods from the Math class include:
    • public static int abs(int num): Returns absolute value of num.
    • public static double max(double x, double y): Returns larger of x and y.
    • public static double random(): Generates a random double between 0.0 and 1.0.
    • Methods are static (class methods) and called like Math.abs().
    • Passing an int to methods expecting a double is permitted.
    • Named Constants: Math class contains PI, a constant value for the ratio of a circle's circumference to its diameter, accessed as Math.PI.
  • Wrapper Classes for Primitive Types
    • Wrapper classes encapsulate primitive data types to add functionalities.
    • Examples include: Integer for int, Double for double.
    • The java.lang package automatically imports these classes, so explicit import is unnecessary.
    • In GUI applications, numbers must be converted to strings for display and read back as strings for input.
    • Conversion methods include:
    • Integer.parseInt(<string>) for string to int conversion,
    • Double.toString(<#>) for num to string conversion.
  • Lottery Example
    • Example of a lottery program using Scanner for input:
      java import java.util.Scanner; public class Lottery { public static void main(String[] args) { Scanner stdIn = new Scanner(System.in); // Program logic to guess a number } }
    • Users guess a randomly generated number, and program evaluates and prompts based on input.
  • String Class and Methods
    • The String class includes methods for string manipulation such as:
    • substring(int beginIndex): Returns substring from specified index.
    • indexOf(char ch): Finds first occurrence of a character.
    • lastIndexOf(char ch): Finds last occurrence from right to left.
    • Example of string usage:
      java String example = "Hello World"; int index = example.indexOf('o'); String sub = example.substring(6);
  • Formatted Output with printf Method
    • Use System.out.printf for formatted console output.
    • Example format:
      java System.out.printf("Total: $%.2f\n", value);
    • Format specifiers include:
    • %s: String
    • %d: Decimal integer
    • %f: Floating-point number
    • %e: Scientific notation
    • Control precision and width with additional format specifiers.
  • Advanced Formatting with printf
    • Width and precision specifiers control output layout.
    • Example:
      java System.out.printf("Value: %7.2f\n", 123.456);
    • Flags include:
    • -: left justify output,
    • 0: leading zeros,
    • ,: locale-specific grouping.
  • Final Example: Budget Report
    • A sample program employing printf for a budget report layout, showcasing functionality through structured output and manipulation of financial figures.
      java System.out.printf("Account: %-25s%13.2f\n", "Supplies", amount);