java - toArray() 与 toArray(new Object[0])

Question

我有一个ArrayList<Clause>叫listtable 。由于某种原因Clause[] whatever = listtable.toArray()给出不兼容的类型错误，但 Clause[] whatever = listtable.toArray(new Clause[0])工作得很好。为什么会这样呢？这两个调用有什么区别？ javadoc 说它们“功能相同”。

这是我的完整代码(相关语句就在结束之前):

public static Clause[] readCNF(String name,Boolean print) throws IOException 
    {
        BufferedReader file = new BufferedReader(new FileReader("./" + name));

        ArrayList<Clause> listtable = new ArrayList<Clause>();
        String line = null; 
        while ((line = file.readLine()) != null) {

            if(line.charAt(0) == 'p')
            {
                 Scanner scanner = new Scanner(line); 
                 scanner.next(); scanner.next(); Clause.NumVars = scanner.nextInt(); Clause.NumClauses = scanner.nextInt(); 

            } else if(line.charAt(0) != 'c') {  

                 ArrayList<Integer> lits = new ArrayList<Integer>();
                 Scanner scanner = new Scanner(line);

                 while(scanner.hasNext())
                 {
                     int var = scanner.nextInt();
                     if(var != 0){ lits.add(var);}
                 }

                 listtable.add(new Clause(lits));

            } 
        }

        if(print) {
            for(Clause clause : listtable)
            {
                clause.print();
            }
        }

       return(listtable.toArray(new Clause[0])); //since the return type is Clause[] this is the same as the statements in the question
    }

Answer 1

toArray() 返回一个 Object 数组。您必须将数组的每个元素转换为您想要的类型。

toArray(T[]) 接受泛型类型并返回特定类型的数组。无需转换返回值和/或数组元素。

正如上面的评论所述，toArray() 方法是预泛型的。

    List<String> list = new ArrayList<String>();
    list.add("Alice");
    list.add("Bob");

    String[] strArray = list.toArray(new String[0]);
    for (String str : strArray) {
        System.out.println(str);
    }

    Object[] objArray = list.toArray();
    for (Object obj : objArray) {
        String str = (String) obj;
        System.out.println(str);
    }