import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

import org.apache.poi.xslf.usermodel.XMLSlideShow;
import org.apache.poi.xslf.usermodel.XSLFShape;
import org.apache.poi.xslf.usermodel.XSLFSlide;

public class ReadingShapes {
   
   public static void main(String args[]) throws IOException {
      
      //creating a slideshow 
      File file = new File("shapes.pptx");
      XMLSlideShow ppt = new XMLSlideShow(new FileInputStream(file));
      
      //get slides 
      XSLFSlide[] slide = ppt.getSlides();
      
      //getting the shapes in the presentation
      System.out.println("Shapes in the presentation:");
      for (int i = 0; i < slide.length; i++){
         
         XSLFShape[] sh = slide[i].getShapes();
         for (int j = 0; j < sh.length; j++){
            
            //name of the shape
            System.out.println(sh[j].getShapeName());
         }
      }
      
      FileOutputStream out = new FileOutputStream(file);
      ppt.write(out);
      out.close();    
   }
}

$javac ReadingShapes.java
$java ReadingShapes

Shapes in the presentation: 
Rectangle 1
Oval 1
Isosceles Triangle 1