如何显示一个字符数组？爪哇

Question

我正在尝试显示一个多维 char 数组，但在尝试显示它时出现错误。有没有更好的方法来显示这个？我在其他地方做错了吗？我无法弄清楚我的代码有什么问题。

public class recBinTree {
private int left, right, columns, rows;
private char [][]binTree;

public recBinTree(int size){
    left = 0;
    right = size-1;
    columns = size;
    double n = Math.log(size)/Math.log(2); //convert to base 2
    rows = (int)n + 1; // # of rows = n
    char[][] binTree = new char[rows][columns];
    //populate entire array with hyphens
    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < columns; j++)
            binTree[i][j] = '-';
    }//end outer for
}//end constructor

public void display(){
    System.out.print("Columns: " + columns + " ");
    System.out.print("Rows: " + rows);
    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < columns; j++)
            System.out.print(binTree[i][j] + " "); //error on this line
        System.out.print("\n");
    }//end outer for
}//end display()


public class Driver {
    public static void main(String[] args) {
        int size = 16;
        recBinTree tree = new recBinTree(size);
        tree.display();  //error on this call
    }
}

编辑：

对不起，这是错误！

Exception in thread "main" java.lang.NullPointerException
at ProjectThreeC.recBinTree.display(recBinTree.java:38)
at ProjectThreeC.Driver.main(Driver.java:14)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

Answer 1

尝试初始化这个 private char [][]binTree; 而不是在 con 结构 (char[][] binTree = new char[rows][columns];) 中创建一个新数组。因此，每次您尝试打印时，private char [][]binTree; 都是 null。对于打印，您可以使用以下内容：

   char[][] c = new char[2][10];
   // read the values
  //now display
    for(int i=0;i<c.length;i++){
        System.out.println(String.valueOf(c[i]));
    }

Answer 2

public class recBinTree {
private int left, right, columns, rows;
private char [][]binTree;

public recBinTree(int size){
    left = 0;
    right = size-1;
    columns = size;
    double n = Math.log(size)/Math.log(2); //convert to base 2
    rows = (int)n + 1; // # of rows = n
    binTree = new char[rows][columns];
    //populate entire array with hyphens
    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < columns; j++)
            binTree[i][j] = '-';
    }//end outer for
}//end constructor

public void display(){
    System.out.print("Columns: " + columns + " ");
    System.out.print("Rows: " + rows);
    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < columns; j++)
            System.out.print(binTree[i][j] + " "); //error on this line
        System.out.print("\n");
    }//end outer for
}//end display()

    public static void main(String[] args) {
        int size = 16;
        recBinTree tree = new recBinTree(size);
        tree.display();  //error on this call
    }
}

Answer 3

您将char [][]binTree; 声明为实例，并再次在构造函数中声明。从构造函数中删除声明。

private char [][]binTree;

public recBinTree(int size){
     ......
   binTree = new char[rows][columns]; //Remove  char[][]  from here
    //populate entire array with hyphens
    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < columns; j++)
            binTree[i][j] = '-';
    }
}

另外，您不需要两个 for 循环来用 - 填充 binTree 数组。只需简单地使用Arrays.fill 操作。

 public recBinTree(int size){
     ......
   binTree = new char[rows][columns];
   Arrays.fill('-');       
}

Answer 4

这样做

    char [][]binTree={{'A','B','C'},{'D','E','F'},{'G','H','I'}};
    System.out.println(Arrays.deepToString(binTree));

Answer 5

通过char[][] binTree = new char[rows][columns];，您将在函数范围内创建一个新变量。你得到一个空指针异常，因为当应用程序稍后在 binTree 变量中查找整个类的范围时，它会发现它是空的。

改成：

binTree = new char[rows][columns];

这将为整个类的范围初始化 binTree 变量。