对象类和数组 - 为什么它返回 'null' ？ [爪哇]

Question

我写了一个小类来创建一个包含 3 个数组的报表对象。在创建对象时，这些数组用值初始化。但是，当我测试该类以查看部门数组中的内容时，它会打印出数组元素为空。为什么？

 class  Report
 {
        // declare instance variables (arrays)
        public String[] departments = new String[4] ;
        public double[] grossTotals = new double[4] ;
        public double[] taxTotals = new double[4]  ;


        // constructor
        public Report(){
            // declare, create and initialise all in one statement
            String[] departments = {"Accounting", "Sales", "HR", +
                                              "Administration"} ;
            double[] grossTotals = {0.0, 0.0, 0.0, 0.0} ;
            double[] taxTotals = {0.0, 0.0, 0.0, 0.0} ;

    } // END constructor
 } // class  Report

测试类：

 class TestReport 
 {
        public static void main(String[] args) {
            // create report object
            Report r = new Report();

                for (int i = 0; i <= 3 ; i++ )
                {
                System.out.println(r.departments[i]) ;
                }

        } //end main
 } // end test class

谢谢

爸爸

Answer 1

变成这样

public Report(){
            // declare, create and initialise all in one statement
            this.departments = {"Accounting", "Sales", "HR", +
                                              "Administration"} ;
            this.grossTotals = {0.0, 0.0, 0.0, 0.0} ;
            this.taxTotals = {0.0, 0.0, 0.0, 0.0} ;

    } // END constru  

实际上，您正在创建构造函数本地的新数组对象，这些对象在构造函数中被初始化。

您的类字段将使用上面的代码进行初始化。

If you have not done it by mistake than please refer to this doc also it will better clear your understandings

**

更新

:** 上面的代码会给你非法的表达式开始

这是工作代码

 class  Report
 {
        // declare instance variables (arrays)
        public String[] departments = null;
        public double[] grossTotals = null;
        public double[] taxTotals = null;


        // constructor
        public Report(){
              this.departments = new String[]{"Accounting", "Sales", "HR", "Administration"} ;
         this.grossTotals = new double[]{0.0, 0.0, 0.0, 0.0} ;
         this.taxTotals = new double[]{0.0, 0.0, 0.0, 0.0} ;
    } // END constructor
 } 

Answer 2

正如其他答案所指出的，您的构造函数创建了新的局部变量“隐藏”实例变量，而不是用数据填充实例变量。

但是，如果您将声明与填充分开，则填充代码会有所不同，并且它们并没有完全正确。您还有一个不属于您的“+”字符。

这段代码可以编译和工作（经过测试），基本上可以满足您的需求。

class  Report
{
       // declare instance variables (arrays)
       public String[] departments;
       public double[] grossTotals;
       public double[] taxTotals;


       // constructor
       public Report(){
           // populate instance variables
           departments = new String[]{"Accounting", "Sales", "HR",
                                             "Administration"} ;
           grossTotals = new double[]{0.0, 0.0, 0.0, 0.0} ;
           taxTotals = new double[]{0.0, 0.0, 0.0, 0.0} ;

   } // END constructor
} // class  Report

您也可以在声明中创建数组，然后在构造函数或其他地方填充数组条目。

使用这种方法，代码可能类似于：

class  Report
{
       // declare instance variables (arrays)
       public String[] departments = new String[4];
       public double[] grossTotals = new double[4];
       public double[] taxTotals = new double[4];


       // constructor
       public Report(){
           // populate instance variable entries
           departments[0] = "Accounting";
           departments[1] = "Sales";
           departments[2] = "HR";
           departments[3] = "Administration";

           for (int i = 0; i < 4; i++) {
               grossTotals[i] = 0.0;
               taxTotals[i] = 0.0;

           }
   } // END constructor
} // class  Report

作为第三种选择，您可以在字段声明中进行所有初始化，如下所示：

class  Report
{
       // declare and populate instance variables (arrays)
       public String[] departments = new String[]{"Accounting", "Sales", "HR",
                                             "Administration"} ;
       public double[] grossTotals = new double[]{0.0, 0.0, 0.0, 0.0} ;
       public double[] taxTotals = new double[]{0.0, 0.0, 0.0, 0.0} ;

} // class  Report

在这种情况下，您根本不需要定义构造函数，因为它什么都不做。 Java 将提供一个空的。