Java中的对象arraylist初始化

Question

I have an object called Carriage . When I initialize it I have to send a number of seats to the constructor:

Carriage newCarriage = new Carriage(30); //30=num of seats

Now I created a new class called Train , and this is its constructor:

public class Train {
    ArrayList<Carriage> trainCarriageList;

    Train(int baseTrainSize,int baseCarriageSize){
        trainCarriageList= new ArrayList<Carriage>(baseTrainSize);
    }
}

Now the question is: How do I initiate a train arraylist with the size of "baseTrainSize" and each Carriage with the size of baseCarriageSize correctly?

Answer 1

When you pass the size (x) to the constructor of an ArrayList you DON'T initialize it with x new elements, you initialize it with exactly 0 elements. The capacity is x. The inner implementation of ArrayList is a regular array which will start with size x . When the array is filled, the array list initializes a new array of size 2x and copies the array of size x to it. This is a costly step. By declaring the size, you can spare this step if you know exactly how many elements will be in the ArrayList .

Answer 2

You can use a loop for that. But logically speaking, just looping to add elements all initialized with the same value does not make sense to me. Rather add one more constructor to have the default value , whatever you need. Although if you really need to do that then do something like this:

for(int i=0 ; i < baseTrainSize; i++) {
Carriage carriage = new Carriage(30);
trainCarriageList.add(carriage);
}

Answer 3

An ArrayList<Carriage> does not contain any Carriage until you don't add them to the collection.

This means that you shouldn't care about initialising how many seats a Carriage has when you initialise the container for them but just when you initialise them.

trainCarriageList = new ArrayList<Carriage>(baseTrainSize);
...
Carriage carriage = new Carriage(30);
trainCarriageList.add(carriage);

Answer 4

You could have Class Train implement List. This way, your train object would literally be a list of Carriage s.

Answer 5

Use this:

public class Train {
  ArrayList<Carriage> trainCarriageList;

  Train(int baseTrainSize,int baseCarriageSize){
    // initialize the List with the base size:
    trainCarriageList= new ArrayList<Carriage>(baseTrainSize);
    // initialize each list element with it's own base size:
    for(int i=0;i<baseTrainSize;i++){
      trainCarriageList.set(i, new Carriage(baseCarriageSize));
    }
  }
}