Copyright © 2013
Robert W. Hasker

Prebuilt methods

• Ch. 5
• Covering the Java API: Application Programmer Interface
  • System programming: Windows, machine; mostly I/O and resources
  • API: where we work

Math

• Math library
• Operation: Math.abs

          int population = in.nextInt();
          // echo positive version:
          System.out.println(Math.abs(popultation));
          // set population to positive version:
          population = Math.abs(population);
  

• Another operation: Math.sqrt

          double square_area = ...;
          double side = Math.sqrt(square_area);
  

• What are Math.abs, Math.sqrt?
  • functions: code which takes a value as an argument and returns a new value
  • Similar to f(x) = x² + 3
• These are documented in the Oracle's Java Math API documentation
  • Look at documentation for sqrt: note double argument, double result
  • See Math.round: float argument, int result
• More on types
  • Types discussed up to this point: int, double
    • Range for int: -2147483648 to +2147483647 (32 bits)
    • When 2 billion is not enough: long (64 bits)
    • Range for long: approximately -10²⁰ to +10²⁰
    • Also: short (16 bits): -32768 to 32767
    • Range for double: approximately ±10³⁰⁸ down to ±5 * 10^-324 (64 bits)
    • When don't need the extra precision and space is an issue: float (32 bits)
    • Range for float: ±3 * 10³⁸ down to ±10^-45
    • [How to convert to different types?]
      • Eg: have int x, want long value
      • [What do you think will happen if try converting long to int?]
  • How to write constants for the different types?
    • Integers: default int, add L (upper or lower case) to create long value: 5L uses 64 bits
    • Floating point: default double, add F to force float: 1.5F
    • Very large or small floating point values: use e: 1.989e33 (mass of sun in grams to 4 significant places), 9.109e-28 (mass of electron in grams also to 4 significant places)
  • Note Math.round has several forms: Math.round(1.0D) gives a long, Math.round(1.0F) gives an int

        int ave_population = Math.round((float)(pop1 + pop2 + pop3) / 3.0);
    

• Other Math operations of note:
  • Math.min, Math.max
    • Example: ensure no student gets below a 70 on the exam:

              double computed_score = ...
              double exam_score = Math.max(                  );
      

    • Change so all scores between 70 and 100
• Math.sin, Math.cos, Math.tan, Math.asin, Math.acos, Math.atan
• Math.PI - double constant approximating PI to 17 digits
• Useful for lab: Math.atan2
  • See formula for bearing between points (on cartesian grid)
• double Math.pow(double base, double exponent)
  • Math.pow(1.5, -2.3)
  • Careful: Math.pow(x, 2) and Math.pow(x, 0.5) are very inefficient compared to x * x and Math.sqrt(x)
• double Math.random()

Other libraries

• See Java8 API
• Note over 4000 classes, each with at least dozens of operations
• Cannot memorize them all!
• One we've used already: Scanner
  • Scanner in = new Scanner(System.in);
    • Creates a value called "in" of type Scanner
    • Sets up scanner to read from system input (keyboard, generally)
  • in.nextDouble() - returns next double value
  • in.next(), in.nextInt()
  • import java.util.Scanner;
    • Declare using Scanner class
    • Don't have to import it; can write

              java.util.Scanner in = new java.util.Scanner(System.in);
      

• Wrappers for primitive types
  • Integer, Float, Double, etc
  • Exercise:
    • Read double value - note doubles have a max of 20 digits
    • Use toString from Double to convert the double into a string
    • Write the characters of the string, one per line
    • Read a word, print the digits of the number interspersed with the letters of the word (up to the size of the smallest of the pair)
  • Exercise: use Character operations to print a word in all lower case.

Review

• Java API
• Math
• wrapper classes: Double, Integer, Character, etc.