Q: Why aren't my variables using the input value For a project I have to use a constructor  to set default values for a rectangle height and width and also be able to input numbers. however when I input the program continues to use default values any way to get it to only use default values when there is no input?
package geometry;
// file name Rectangle.java
/**
 *
 * @author 12484
 */
public class Rectangle {

 // **********************instance variables (properties)
 private double myWidth;
 private double myLength;
// ********************************* constructors
 // default constructor

 public Rectangle() {
  myWidth = 1;
  myLength = 1;
 }
 // "other" constructor

 public Rectangle (double width, double length) {
  myWidth = width;
  myLength = length;
 }

 // ********************************* accessor methods
 public double getWidth() {
  return myWidth;
 }

 public double getLength() {
  return myLength;
 }

 // ********************************* modifier methods
 public void setWidth(double width) {
  myWidth = width;
 }

 public void setLength(double length) {
  myLength = length;
 }

 // ********************************* interesting methods
 public double computeArea() {
  getLength();
  getWidth();
  double area = Math.pow(getLength(), getWidth());
  return area;
 }

 public double computePerimeter() {
  getLength();
  getWidth();
  double perimeter = Double.sum(getLength(), getWidth());
  return perimeter;
 }
}


package geometry;
// filename Rectest.java
public class Rectest {
    public static void main(String[]args){
        Rectangle len = new Rectangle();
        Rectangle wid = new Rectangle();
        Rectangle a = new Rectangle();
        len.setLength(5.2);
        wid.setWidth(7.5);
        System.out.println(a.computeArea());
    }
}

default values are 1 for length and 1 for width

A: len,wid and a are all different objects(rectangles in this case) with their own instance variables(width and length).
What you want to do is create a Rectangle object, set its length and width and then execute the computeArea on that object.
public class Rectest {
    public static void main(String[]args){
        Rectangle a = new Rectangle();
        a.setLength(5.2);
        a.setWidth(7.5);
        System.out.println(a.computeArea());
    }
}


A: What you actually want to do is the following:
public class Rectest {
    public static void main(String[]args){
        Rectangle rectangle = new Rectangle();
        rectangle.setLength(5.2);
        rectangle.setWidth(7.5);
        System.out.println(rectangle.computeArea());
    }
}

Or even better since you have a constructor for both length and width:
public class Rectest {
    public static void main(String[]args){
        Rectangle rectangle = new Rectangle(7.5, 5.2);
        System.out.println(rectangle.computeArea());
    }
}

In your example you were setting length in one of the Rectangle objects, then the width in another and finally, computing the are of a third one that was untouched and thus with the default values for both length and width.
Finally, as already mentioned by @harold and @JimRhodes (thanks to you both), your computeArea() and computePerimeter() methods are wrong. They should be:
public double computeArea() {
  return myLength * myWidth;
}

public double computePerimeter() {
  return 2 * (myLength + myWidth);
}

The calls to getLength() and getWidth() had no meaning or effect and thus are not needed.

A: Because when you are create Rectangle a. you are using the default constructor.
Rectangle a = New Rectangle();
it should be
Rectangle a = New Rectangle(7.5,5.2)
assuming that you are trying to make a rectangle 7.5 by 5.2.
Your code is actually making three rectangles. I have commented your code below to help you better understand what each line is doing.
//This creates a new rectangle using the default constructor        
Rectangle len = new Rectangle();

//This creates a new rectangle using the default constructor        
Rectangle wid = new Rectangle();

//This creates a new rectangle using the default constructor
Rectangle a = new Rectangle();
//This sets Rectangle len length to 5.2
len.setLength(5.2);

//this set Rectangle wid width to 7.5
wid.setWidth(7.5);
//This computes the area of rectangle a.
System.out.println(a.computeArea());

to better understand try adding this to your code.
System.out.println(len.computeArea());  // 5.2
System.out.println(wid.computeArea());  //7.5
