Assume that the total area is never beyond the maximum possible value of int.

public class Solution {
    public int computeArea(int A, int B, int C, int D, int E, int F, int G, int H) {
        int overlaparea = 0;
        int width = 0;
        int height = 0;
        if(C < E && A < E || C > G && A > G || B > H && D > H || D < F && B < F)
             overlaparea = 0;
        else{
            int overlapW = Math.min(C, G) - Math.max(A, E);
            int overlapH = Math.min(D, H) - Math.max(B,F);
            overlaparea = overlapW * overlapH;
        }
        return (C - A) * (D - B) + (G - E) *(H -F) - overlaparea;
    }
}