来自构造函数的Java调用构造函数

Question

I have a constructor

private Double mA;
private Double mB;

Foo(Double a) {
  mA = a;
  mB = a + 10;
}

Foo(Double a, Double b) {
  mA = a;
  mB = b;
  // some logic here
}

if I make a call to second constructor like this:

Foo(Double a) {
  Double b = a + 10;
  this(a, b);
}

than compiler tells me, that constructor should be the first statement. So do I need to copy all logic from the second constructor to first one?

Answer 1

Why don't you just do this(a, a+10) instead?

Note that this() or super() must be the first statement in a constructor, if present. You can, however, still do logic in the arguments. If you need to do complex logic, you can do it by calling a class method in an argument:

static double calculateArgument(double val) {
    return val + 10; // or some really complex logic
}

Foo(double a) {
    this(a, calculateArgument(a));
}

Foo(double a, double b) {
    mA = a;
    mB = b;
}

Answer 2

If you use this() or super() call in your constructor to invoke the other constructor, it should always be the first statement in your constructor.

That is why your below code does not compile: -

Foo(Double a) {
  Double b = a + 10;
  this(a, b);
}

You can modify it to follow the above rule: -

Foo(Double a) {
  this(a, a + 10);  //This will work.
}

Answer 3

Invocation of another constructor must be the first line in the constructor.

You can call explicit constructor invocation like -

Foo(Double a) {
  this(a, a+10);
}