Lesson 7: Prewritten Classes and Methods · Object-Oriented Programming with Java

Lesson Overview

This lesson explains how to use Java's built-in classes, constants, imported packages, mathematical methods, and date/time-related classes.

Automatically Available Classes

Java provides many ready-made classes. Some are available automatically because every Java program imports the java.lang package by default.

Examples of commonly used classes from java.lang include:

System
String
Math
Integer
Double
Character
Boolean

A package is a group of related classes. Packages help organize Java's class library.

Math Constants and Methods

The Math class provides useful constants and methods for numeric calculations. Its members are static, so they are called using the class name.

Math.PI
Math.sqrt(25)
Math.max(5, 3)

Common Math members:

Member	Purpose
`Math.PI`	Value of pi
`Math.abs(x)`	Absolute value
`Math.sqrt(x)`	Square root
`Math.pow(x, y)`	x raised to the y power
`Math.round(x)`	Nearest whole number
`Math.ceil(x)`	Smallest whole number not less than x
`Math.floor(x)`	Largest whole number not greater than x
`Math.max(x, y)`	Larger of two values
`Math.min(x, y)`	Smaller of two values
`Math.sin(x)`	Sine, using radians
`Math.cos(x)`	Cosine, using radians
`Math.random()`	Random decimal from 0.0 up to but not including 1.0

Activity: Math Class Demonstration

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Importing Prewritten Classes

Not every Java class is imported automatically. To use classes outside java.lang, you can:

Use the full package path.
Import a specific class.
Import all classes from a package using *.

Examples:

java.util.Date today = new java.util.Date();

import java.util.Date;
Date today = new Date();

import java.util.*;
Date today = new Date();

Import statements are placed before the class declaration.

Date and Time Classes

Older Java programs may use java.util.Date, but many of its methods are now considered legacy. For modern Java, prefer the java.time package, such as LocalDate and LocalDateTime.

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Try It: Prewritten Classes

Experiment with other Math methods like Math.floor(), Math.ceil(), or Math.random().

public class Main {
    public static void main(String[] args) {
        // Math class
        System.out.println("PI: " + Math.PI);
        System.out.println("sqrt(144): " + Math.sqrt(144));
        System.out.println("pow(2, 10): " + Math.pow(2, 10));
        System.out.println("abs(-42): " + Math.abs(-42));
        System.out.println("max(18, 21): " + Math.max(18, 21));

        // String methods
        String s = "  Hello, BSIT!  ";
        System.out.println(s.trim());
        System.out.println(s.trim().replace("BSIT", "World"));

        // Integer wrapper
        System.out.println("Max int: " + Integer.MAX_VALUE);
        System.out.println("Parse: " + Integer.parseInt("42"));
    }
}

Member

Purpose

Math.PI

Value of pi

Math.abs(x)

Absolute value

Math.sqrt(x)

Square root

Math.pow(x, y)

x raised to the y power

Math.round(x)

Nearest whole number

Math.ceil(x)

Smallest whole number not less than x

Math.floor(x)

Largest whole number not greater than x

Math.max(x, y)

Larger of two values

Math.min(x, y)

Smaller of two values

Math.sin(x)

Sine, using radians

Math.cos(x)

Cosine, using radians

Math.random()

Random decimal from 0.0 up to but not including 1.0

public class Main { public static void main(String[] args) { // Math class System.out.println("PI: " + Math.PI); System.out.println("sqrt(144): " + Math.sqrt(144)); System.out.println("pow(2, 10): " + Math.pow(2, 10)); System.out.println("abs(-42): " + Math.abs(-42)); System.out.println("max(18, 21): " + Math.max(18, 21)); // String methods String s = " Hello, BSIT! "; System.out.println(s.trim()); System.out.println(s.trim().replace("BSIT", "World")); // Integer wrapper System.out.println("Max int: " + Integer.MAX_VALUE); System.out.println("Parse: " + Integer.parseInt("42")); } }