java - 调用 getter 方法时出现 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 4 年前。

我有三个类(class) StudentMain、StudentService、Student。 Student 类包含 getters 和 setters 方法，并且从 StudentMain 我将 Student 对象传递给 StudentService。下面是代码:

StudentMain 代码:

public class StudentMain {
static Student data [] = new Student[4];
static { for (int i = 0; i < data.length; i++) 
    data [i] =new Student();
    data [0] = new Student ("Sekar", new int [] {35, 35, 35}); 
    data [1] = new Student(null,new int[]{11,22,33});
    data [2] = null; 
    data [3] = new Student ("Manoj", null);

}
public static void main(String[] args) {
    // TODO Auto-generated method stub
    StudentService studentService = new StudentService ();
    System.out.println ("Number of Objects with Marks array as null =" + studentService.findNumberOfNullMarks (data));
    System.out.println ("Number of Objects with Name as null="+ studentService.findNumberOfNullNames(data));
    System.out.println ("Number of Objects that are entirely null="+ studentService.findNumberOfNullObjects(data));
    }

}

学生代码:

public class Student {
private String name;
private int marks[];
public void setName(String name) {
    this.name=name;
}
public String getName() {
    return name;
}
public void setMarks(int [] marks) {
    this.marks=marks;
}
public int[] getMarks() {
    return marks;
}
public Student() {

}
public Student(String name,int[] marks) {
    setName(name);
    setMarks(marks);
}

}

StudentService 代码:

public class StudentService{
Student[] data;
public int findNumberOfNullMarks(Student data[]) {
    this.data=data;
    int count=0;
    int i=0;
    while(i!=data.length) {
        if(data[i].getMarks()==null)
            count++;
        i++;
    }
    return count;
}
public int findNumberOfNullNames(Student data[]) {
    int count=0;
    int i=0;
    while(i!=data.length) {
        if(data[i].getName()==null)
            count++;
        i++;
    }
    return count;
}
public int findNumberOfNullObjects(Student data[]) {
    int count=0;
    int i=0;
    while(i!=data.length) {
        if(data[i]==null)
            count++;
        i++;
    }
    return count;
}

}

我在 if(data[i].getMarks()==null) 处遇到异常和if(data[i].getMarks()=null)在学生服务课上。

Answer 1

data [0] = new Student ("Sekar", new int [] {35, 35, 35}); 
data [1] = new Student(null,new int[]{11,22,33});
data [2] = null; 
data [3] = new Student ("Manoj", null);

对于 i = 0，data[0] 具有对象并在对象中具有数组标记，因此您不会得到 java.lang.NullPointerException

对于 i = 1，data[1] 具有对象并在对象中具有数组标记，因此您不会得到 java.lang.NullPointerException

对于 i = 2，data[2] 为 null，因此您将得到 java.lang.NullPointerException

对于 i = 3，data[3] 有对象但数组标记为 null，因此您将得到 java.lang.NullPointerException

所以你需要确保你应该有对象和数组。