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.
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);