How should I pass the "type" parameter of in a single constructor in an if/else statement?

How should I pass the "type" parameter of a constructor in an if/else statement? For eg - cal(2,2,0,rectangle). So if the type=rectangle then calculate area of a rectangle. If type=circle, calculate the area of a circle.

I am using a single constructor. My issue is that I know the logic but I can't write it in syntax. I am using Java or Apex.

I want to use if-else statement. How should I pass the type parameter in the code?

My program is like this -

1. if "type"=square, the compiler will call calculate area of the square.

2. if "type"=circle, the compiler will call calculate area of the circle.

public class Area {
private String type;
private Integer length;
private Integer breadth;
private Integer height;
private Integer area;
public void setType(String t){
type=t;    
}
public void setLength(Integer l){
length=l;    
}
public void setbreadth(Integer b){
breadth=b;    
}
public void setheight(Integer h){
height=h;    
}
/* public void setArea(Integer a){
area=a;    
} */
public Integer getLength(){
return length;  
}
public Integer getbreadth(){
return breadth;  
}
public Integer getheight(){
return height;  
}
public string gettype(){
return type;  
}
public Integer AreaRectangle(){
return area=length*breadth;
}
public Integer AreaSquare(){
return area=length*length;
}
public integer AreaTriangle(){
return area=1/2 *(breadth*height);
}
public Area(){ // default constructor
length=9;
breadth=2;
height=7;       
}
public Area(String t,Integer l ,Integer b,Integer h ){         // parameterised constructor
type=t;
length=l;
breadth=b;
height=h;
}  
}

You don't. You create an abstract class called shape.

public abstract class Shape {
abstract double area();  
}

And then two other classes that extend Shape and each provides the proper implementation

public class Square extends Shape {
private int side;
public Square(int side) {
this.side = side;
}
public double area() {
return (double) side * side;
}
}

Now at the place you want to call it:

Shape shape = new Square(5);
double area = shape.area();
Int radius = 4;
shape = new Circle(radius);
double circle area = shape.area();