java - 决定集合实例变量的延迟初始化

Question

问题嵌套在代码中。我有 2 个代码选项。每个列出的优点和缺点。想知道一个首选的方法。

选项1：

public class Graph {
    private final List<ArrayList<Integer>> adjList;

    /**
     * DISADVANTAGE:
     * 1. Slow to create an object.
     * 2. Memory allocated prematurely, lazy initialization could be used to speed up object creation.
     */
    public Graph(int vertexCount) {
        adjList = new ArrayList<ArrayList<Integer>>(vertexCount);
        for (int i = 0; i < vertexCount; i++) {
            adjList.add(new ArrayList<Integer>());
        }
        this.vertexCount = vertexCount;
    }

    /**
     * ADVANTAGE:
     * 1. No synchronization issues, reducing complexity and overhead of sync.
     * 2. No need to null check, since constructor has taken care of lot .  
     */
    public void addEdge(int vertexFrom, int vertexTo) {
        adjList.get(vertexFrom).add(vertexTo);
        adjList.get(vertexTo).add(vertexFrom);
    }

    /**
     * DISADVANTAGE:
     * 
     * Inspite of knowing that none of the List<Integers> are null, an extra check for size == 0, needed to return  Collections.EMPTY_LIST
     * Eg:
     * if (adjList.get(vertex).size == 0) { return  Collections.EMPTY_LIST; }
     * return adjList.get(vertex).size
     */
    public List<Integer> adj(int vertex) {
        return adjList.get(vertex); 

    }
}

选项 2

public class Graph {
    private final List<ArrayList<Integer>> adjList;
    private final int vertexCount;
    private int edgeCount;

    /**
     * ADVANTAGE: quick construction
     * 
     */
    public Graph(int vertexCount) {
        adjList = new ArrayList<ArrayList<Integer>>(vertexCount);
        this.vertexCount = vertexCount;
    }

    /**
     * DISADVANTAGE:
     * 1. Effort spent on null checks.
     * 
     * 2. Thread safety complications.
     */
    public void addEdge(int vertexFrom, int vertexTo) {
        if ( adjList.get(vertexFrom) == null ) {
            adjList.add(vertexFrom, new ArrayList<Integer>())
        }
        if ( adjList.get(vertexTo) == null ) {
            adjList.add(vertexTo, new ArrayList<Integer>())
        }
        adjList.get(vertexFrom).add(vertexTo);
        adjList.get(vertexTo).add(vertexFrom);
        edgeCount++;
    }

    public List<Integer> adj(int vertex) {
        if (adjList.get(vertex) == 0) return Collections.EMPTY_LIST; 
        return adjList.get(vertex);
    }


    public List<ArrayList<Integer>> getGraph() {
        return adjList;
    }
}