检查字符串数组元素是否为空

Question

所以我查看了有关此问题的其他一些线程，似乎我是否应该能够使用常规比较运算符来检查它。

How to check if my string is equal to null?

Java, check whether a string is not null and not empty?

然而，即使我的程序说该字符串为空，它后来通过执行条件为该字符串不为空的 if 语句来反驳这一点。为了更清楚，这是我的完整程序：

package bank;

public class HowCheckForNull {

    static void showDates(String[] dates){
        for(int i = 0; i < dates.length; i++){
            System.out.println(dates[i]);
            System.out.println(dates[i] == null);
            System.out.println(dates[i] == (String) null);
            System.out.println(dates[i] != null);
            if(dates[i] != null);{ //This should not execute!?
                System.out.print("A transaction of X$ was made on the " + dates[i] + "\n");
            }
        }
        System.out.println("");
    }

    public static void main(String args[]){
        String[] dates = new String[3];
        showDates(dates);
    }
    }

输出：

null
true
true
false
A transaction of X$ was made on the null
null
true
true
false
A transaction of X$ was made on the null
null
true
true
false
A transaction of X$ was made on the null

这里有几件事让我感到困惑，为什么 if 语句执行，即使日志记录另有说明，dates[i] 怎么会等于 null和 (String) null ?

Answer 1

if(dates[i] != null);
                    ^

额外的；导致以下块始终执行（无论 if 语句的评估如何），因为它结束了 if 语句。删除它。

Answer 2

问题是';'在 if(condition); 之后结束语句并以正常方式处理剩余的代码段，而不管任何条件。

代码

    package bank;

    public class HowCheckForNull {

        static void showDates(String[] dates){
            for(int i = 0; i < dates.length; i++){
                System.out.println(dates[i]);
                System.out.println(dates[i] == null);
                System.out.println(dates[i] == (String) null);
                System.out.println(dates[i] != null);
                if(dates[i] != null){ //Now it will not execute.
                    System.out.print("A transaction of X$ was made on the " + dates[i] + "\n");
                }
            }
            System.out.println("");
        }

        public static void main(String args[]){
            String[] dates = new String[3];
            showDates(dates);
        }
    }

输出

null
true
true
false
null
true
true
false
null
true
true
false

检查字符串数组元素是否为空

Check if String array element is null

java

arrays

string

null