如何使用花括号在Java中定义多维数组？

Question

I don't understand the behaviour of Java about arrays. It forbids to define an array in one case but allows the same definition in another.

The example from tutorial:

String[][] names = {
        {"Mr. ", "Mrs. ", "Ms. "},
        {"Smith", "Jones"}
    };
System.out.println(names[0][0] + names[1][0]);    // the output is "Mr. Smith";

My example:

public class User {
   private static String[][] users;
   private static int UC = 0;

   public void addUser (String email, String name, String pass) {
      int i = 0;

      // Here, when I define an array this way, it has no errors in NetBeans
      String[][] u = { {email, name, pass}, {"one@one.com", "jack sparrow", "12345"} };

      // But when I try to define like this, using static variable users declared above, NetBeans throws errors
      if (users == null) {
         users = { { email, name, pass }, {"one", "two", "three"} };    // NetBeans even doesn't recognize arguments 'email', 'name', 'pass' here. Why?

         // only this way works
         users = new String[3][3];
         users[i][i] = email;
         users[i][i+1] = name;
         users[i][i+2] = pass;
         UC = UC + 1;
      }
    }

The mistakes thrown by NetBeans are:

illegal start of expression,

";" expected ,

not a statement .

And also it doesn't recognize arguments email , name , pass in the definition of the users array. But recognizes them when I define u array.

What is the difference between these two definitions? Why the one works but the another one defined the same way doesn't?

Answer 1

您需要在数组聚合之前添加new String[][] ：

users = new String[][] { { email, name, pass }, {"one", "two", "three"} };

Answer 2

You can use this syntax:

String[][] u = {{email, name, pass}, {"one@one.com", "jack sparrow", "12345"}};

Only when you're declaring the matrix for the first time. It won't work for assigning values to a String[][] variable after you've declared it elsewhere, that's why users = ... fails. For assigning values to an already-declared String[][] (or a matrix of any other type for that matter), use

users = new String[][] { { email, name, pass }, {"one", "two", "three"} };

Answer 3

The first case is an initialization statement, while the second is only an assignment. That kind of filling arrays is only supported when defining a new array.

Answer 4

要重新分配矩阵，您必须使用new ：

users = new String[][] {{email, name, pass }, {"one", "two", "three"}};