Class Rectangle that computes the Area

I need help to create a class named rectangle that contains
-fields for length and width
-setters and getters for the fields
-a constructor that accepts length and width as parameters
-an overloaded 0-arg constructor that sets field values to 0
-a method named computeArea that accepts no parameters and returns the area

This is what I had so far:
Rectangle.java

public class Rectangle {

  private double length;
  private double width;

  public void getLength(){
    return length;
  }

  public void getWidth(){
    return width;
  }

  public void setLength(double newLength){
    this.length = newLength;
  }

  public void setWidth(double newWidth){
    this.width = newWidth;
  }

  public Rectangle(double length, double width){
    this.length = length;
    this.width = width;
  }


  public Rectangle(){
    length = 0;
    width = 0;
  }

  double area = length * width;

  public double computeArea(){
    return area;
  }
}

TestRectangle.java

public class TestRectangle{
  public static void main(String[] arg){

    Rectangle rect1 = new Rectangle();

    rect1.setLength = 4.0;
    rect1.setWidth = 5.0;

    System.out.println("Length: " + rect1.getLength());
  }
}