Java。 在构造函数中初始化列表

Question

I have a problem of initializing List in constructor.

I am trying to assign parameters to variable "tr" and tr.getVertices() is a List.

SimpleTriangle tr = new SimpleTriangle(tr.getVertices(), tr.getColour(), tr.getThickness(), ShapeType.TRIANGLE);
trList.add(tr);

It higlights tr.getVertices() and says "Variable tr might not been initialized"

Where SimpleTriangle is a child class.

public SimpleTriangle(List<Point> vertices, Color colour, int thickness, ShapeType shapeType) {
super(vertices, colour, thickness, shapeType); }

and parent class has method

    public List<Point> getVertices() {
    return vertices;
}

Answer 1

You are trying to call a method on a variable you haven't initialized yet:

//              v--this variable        v--is not yet initialized here
SimpleTriangle tr = new SimpleTriangle(tr.getVertices(), tr.getColour(), tr.getThickness(), ShapeType.TRIANGLE);

You are trying to initialize a SimpleTriangle with it's own vertices, which doesn't make any sense. You'll have to pass some List of Point s to the constructor. The same is true for all other parameters of the constructor.

Answer 2

The error is correct, you cannot use a variable before it has been initialised.

You need to provide it real values, not ones which will only make sense in the future.

Note: Ruby does allow you to do what you suggest and could be considered a bug in the design of the language.

Answer 3

tr is a local variable which you want to instantiate. You need to instantiate an object before using its methods. Its like you are asking null to get a process done. And in-fact you can not even call it null as local variables in java are not auto initialized.