Class_01_RectangleArea.java

import java.util.Scanner;

public class Class_01_RectangleArea {

	public static void main(String[] args) {
		Scanner console = new Scanner(System.in);
		System.out.print("a = ");
		int a = Integer.parseInt(console.next());
		System.out.print("b = ");
		int b = Integer.parseInt(console.next());
		int area = a*b;
		System.out.println(String.format("area is: %d", area));
		console.close();
	}

}

Class_02_TriangleArea.java

import java.util.Locale;
import java.util.Scanner;

public class Class_02_TriangleArea {
	
	public static void main(String[] args) {
		
		Locale.setDefault(Locale.ROOT);
		
		Scanner console = new Scanner(System.in);
		Point[] triangle = new Point[3];

		for(int i=0; i<triangle.length; i++){
			System.out.print(String.format("x%d = ", i+1));
			double x = console.nextDouble();
			console.nextLine();
			System.out.print(String.format("y%d = ", i+1));
			double y = console.nextDouble();
			console.nextLine();
			triangle[i] = new Point(x,y);
		}
		double dist1 = triangle[0].distanceTo(triangle[1]);
		double dist2 = triangle[1].distanceTo(triangle[2]);
		double dist3 = triangle[2].distanceTo(triangle[0]);
		double p = 0.5*(dist1+dist2+dist3);
		double area = Math.sqrt(p*(p-dist1)*(p-dist2)*(p-dist3));
		System.out.println(String.format("%d",Math.round(area)));
		console.close();
	}
}

Class_03_PointInsideFigure.java

import java.util.Locale;
import java.util.Scanner;

public class Class_03_PointInsideFigure {

	public static void main(String[] args) {
		Locale.setDefault(Locale.ROOT);
		Scanner console = new Scanner(System.in);
		
		Rectangle r1 = new Rectangle(new Point(12.5, 13.5), new Point(17.5, 8.5));
		Rectangle r2 = new Rectangle(new Point(12.5,  8.5), new Point(22.5, 6.0));
		Rectangle r3 = new Rectangle(new Point(20.0, 13.5), new Point(22.5, 8.5));
		
		System.out.print("Enter x and y, separated by a space: ");
		String line = console.nextLine();
		while(line.length()>0){
			String[] items = line.split(" ");
			double x = Double.parseDouble(items[0]);
			double y = Double.parseDouble(items[1]);
			Point p = new Point(x,y);
			if(r1.pointInside(p) || r2.pointInside(p) || r3.pointInside(p)){
				System.out.println("Inside");
			}else{
				System.out.println("Outside");
			}
			System.out.print("Enter x and y, separated by a space: ");
			line = console.nextLine();
		}
		console.close();
	}
}