Area of any type of triangle can be calculated using the formula, Area = (b*h)/2, where b is the base of the triangle and h is the vertical height. The following diagram shows how to define base and vertical height of a triangle.

The following sample Java program calculates the area of a triangle given its base and vertical height. Note that this approach works for any triangle.

import java.util.Scanner;

/**
 * Calculates area of triangle in Java given its base and vertical height.
 * @author jj
 */
public class AreaOfTriangle1 {
    
    public static void main(String[] args) {
        
        Scanner sn = new Scanner(System.in);
        System.out.println("Enter base of triangle:");
        double base = sn.nextDouble();
        
        System.out.println("Enter vertical height of triangle:");
        double verticalHeight = sn.nextDouble();
        
        AreaOfTriangle1 at   = new AreaOfTriangle1();
        double area = at.calculateArea(base,verticalHeight);
        
        System.out.println("Area = "+area);
        
    }

    /**
     * Calculates area of a triangle using base and vertical height
     * @param base
     * @param verticalHeight
     * @return 
     */
    private double calculateArea(double base, double verticalHeight) {
        return base*verticalHeight/2;
    }
}

However if you don't know base and vertical height, you can also calculate area of a triangle using Heron's formula. This requires the length of all three sides of the triangle. According to heron's formula, the area of a triangle with 3 sides a, b and c is,

Area = square root of (p*(p-a)*(p-b)*(p-c)) (where p = (a+b+c)/2).

The following sample Java program calculates the area of a triangle given the length of its 3 sides. The following Java program also checks whether the given 3 sides can form part of a triangle. For a triangle, the sum of its 2 sides will always be greater than the third side.

import java.util.Scanner;

/**
 * Calculates area of triangle in Java given its 3 sides.
 * @author jj
 */
public class AreaOfTriangle2 {
    
    public static void main(String[] args) {
        
        Scanner sn = new Scanner(System.in);
        System.out.println("Enter length of first side of triangle:");
        double a = sn.nextDouble();
        
        System.out.println("Enter length of second side of triangle:");
        double b = sn.nextDouble();
        
        System.out.println("Enter length of third side of triangle:");
        double c = sn.nextDouble();
        
        AreaOfTriangle2 at   = new AreaOfTriangle2();
        if(at.isTriangleValid(a, b, c)) {
            double area = at.calculateArea(a,b,c);        
            System.out.println("Area = "+area);
        }else {
            System.out.println("Sides entered cannot form a triangle!");
        }
        
    }

    /**
     * Calculates area of a triangle using length of its 3 sides
     */
    private double calculateArea(double a, double b, double c) {
        double p = (a+b+c)/2;       
        return Math.sqrt(p*(p-a)*(p-b)*(p-c));
    }
    
    /**
     * Checks whether the 3 sides can form a triangle
     * The sum of any two sides must be greater than the other side
     * @param a
     * @param b
     * @param c
     * @return 
     */
    private boolean isTriangleValid(double a, double b, double c) {
        if((a+b)>c && (a+c)>b && (b+c)>a) {
            return true;
        }else {
            return false;
        }
    }
}