java - 它如何返回NullPointerException。

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What is a NullPointerException, and how do I fix it?
                                
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public class TestDogs {

    public static void main(String [] args) {

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

class Dog{ }

最佳答案

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]仍然为空，因此在其上调用toString仍然会抛出NullPointerException。

示例如何初始化数组和Dog实例：

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]