Why can't this() and super() both be used together in a constructor?

Question

Why can\'t this() and super() both be used together in a constructor?

What is the reason for incorporating such a thing?

Answer 1

Compare example below. Class FirstChild sets instance variable name in 2 constructors as calling the second constructor from the first one is excluded by need to call super().

In class SecondChild there is introduced third private constructor that takes 2 parameters – first one passed to supper() and the second one used to set name. First 2 constructors are calling the third one. Super() is called exactly once, also instance variable is set only in one constructor. Code produces the same result without need to call super() and this() in same constructor.

class FirstChild extends ConstructorTest{
    private String name = null;
    public FirstChild(){
        super("super text 1");
        //this("Unknown"); //UNCOMMENTED DOES NOT COMPILE
        name = "Unknown";
    }
    public FirstChild(String name){
        super("super text 2");
        this.name = name;
    }
    public String getName(){
        return name;
    }
}

class SecondChild extends ConstructorTest{
    private String name = null;
    public SecondChild(){
        this("super text 1", "Unknown");
    }
    public SecondChild(String name){
        this("super text 2", name);
    }
    private SecondChild(String superStr, String name)
    {
        super(superStr);
        this.name = name;
    }
    public String getName(){
        return name;
    }
}

public class ConstructorTest{
    public ConstructorTest(String str){
        System.out.println("ConstructorTest constructor called with parameter \"" + str + "\"");
    }
    public static void main(String... args)
    {
        System.out.println("Hello from main, FirstChild results:");
        FirstChild fc1 = new FirstChild();
        FirstChild fc2 = new FirstChild("John");
        System.out.println("           child fc1 name: " + fc1.getName());
        System.out.println("           child fc2 name: " + fc2.getName());
        System.out.println("Hello from main, SecondChild results:");
        SecondChild sc1 = new SecondChild();
        SecondChild sc2 = new SecondChild("John");
        System.out.println("           child sc1 name: " + sc1.getName());
        System.out.println("           child sc2 name: " + sc2.getName());
    }
}