二維陣列上的錯誤

Question

這是一個二維數組，從用戶的價值把它給了我NullPointerException錯誤！ btw我是初學者，所以如果你能讓它更加詳細。感謝

public static void main(String[] args) { 
    java.util.Scanner scan = new Scanner(System.in); 
    int[][] matrix = null; 
    System.out.println("Enter " + matrix.length + " rows and " + 
      matrix[0].length + " columns: "); 

    for (int row = 0; row < matrix.length; row++) { 

     for (int column = 0; column < matrix[row].length; column++) { 

      matrix[row][column] = scan.nextInt(); 

     } 
    } 
} 

錯誤

Exception in thread "main" java.lang.NullPointerException 

Answer 1

你應該來定義矩陣的大小一樣，對於爲例：

public static void main(String[] args) { 
    java.util.Scanner scan = new Scanner(System.in); 
    int n, m; 
    n = scan.nextInt(); 
    m = scan.nextInt(); 
    int[][] matrix = new int[n][m]; 
    System.out.println("Enter " + matrix.length + " rows and " 
      + matrix[0].length + " columns: "); 

    for (int row = 0; row < matrix.length; row++) { 

     for (int column = 0; column < matrix[row].length; column++) { 

      matrix[row][column] = scan.nextInt(); 

     } 
    } 
}