java - 它如何返回 NullPointerException？

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 8 年前。

public class TestDogs {

    public static void main(String [] args) {

        Dog [][] theDogs = new Dog[3][];
        System.out.println(theDogs[2][0].toString());
    }
}

class Dog{ }

Answer 1

theDogs[2] 数组为空，因为您没有初始化它。即使您初始化了它，theDogs[2][0] 仍将为 null，因此对其调用 toString 仍会抛出 NullPointerException。

如何初始化数组和 Dog 实例的示例:

Dog [][] theDogs = new Dog[3][];
theDogs[2] = new Dog[7]; // initialize the 3rd row of theDogs 2D array
theDogs[2][0] = new Dog (); // initialize the Dog instance at the 1st column of the 3rd row
System.out.println(theDogs[2][0].toString()); // now you can execute methods of theDogs[2][0]