我的ArrayList无法识别我添加到列表中的内容

Question

Here is the code itself

import java.util.ArrayList;

public class Student {
    private String name;
    private int age;

    public Student (String n, int a) {
        name = n;
        age = a;

    }

    public String toString() {
        return name + " is " + age + " years old";
    }

    ArrayList<Student> rayList = new ArrayList<Student>();
    rayList.add(new Student("Sam", 17));
    rayList.add(new Student("Sandra", 18));
    rayList.add(new Student("Billy", 16));
    rayList.add(new Student("Greg", 17));
    rayList.add(new Student("Jill", 18));

    public static void main(String[] args) {
        System.out.println(rayList.get(0));
    }

}

Some println commands are missing in the main method. But when I try to add the 5 students to my ArrayList, I get the error "Cannot make a static reference to the non-static field rayList"

Answer 1

You're trying to execute code outside of executable context. Code can only be executed from a method, static initialiser or instance initialiser (Thanks NickC) context.

Try moving it into the main method to start with...

public static void main(String[] args) {
    ArrayList<Student> rayList = new ArrayList<Student>();
    rayList.add(new Student("Sam", 17));
    rayList.add(new Student("Sandra", 18));
    rayList.add(new Student("Billy", 16));
    rayList.add(new Student("Greg", 17));
    rayList.add(new Student("Jill", 18));
    System.out.println(rayList.get(0));
}

Updated based on feedback

Your first error Cannot make a static reference to the non-static field rayList was generated because the rayList was not declared static , but you were trying to reference it from a static context.

// Not static
ArrayList<Student> rayList = new ArrayList<Student>();
// Is static
public static void main(String[] args) {
    // Can not be resolved.
    System.out.println(rayList.get(0));
}

rayList is declared as a "instance" field/variable, meaning that it requires an instance of the declaring class ( Student ) before it has meaning.

This could be resolved by...

Creating an instance of Student in main and accessing it via that instance, for example...

public static void main(String[] args) {
    Student student = new Student(...);
    //...
    System.out.println(student.rayList.get(0));
}

Personally, I don't like this, rayList doesn't really belong with Student , it adds no value. Can you imagine having to create an instance of Student before you add any to the List ?

I also don't like accessing instance fields directly, but that's a personal preference.

Making rayList static

static ArrayList<Student> rayList = new ArrayList<Student>();
// Is static
public static void main(String[] args) {
    //...
    System.out.println(rayList.get(0));
}

This is a viable option, but would require more context before it could be deemed good or bad. I personally feel that static fields can cause more problems than they solve, but again, that's a personal opinion and your context may deem a reasonable solution.

Or, you could create a local instance of the List within the context of the static method as seen in the first example.

public static void main(String[] args) {
    ArrayList<Student> rayList = new ArrayList<Student>();
    //...
    System.out.println(rayList.get(0));
}

As I said, which one you choice to do will come down to you, I would personally prefer the last two, but that's me.

Answer 2

This is because you are accessing non static field inside static function . In your case.

ArrayList<Student> rayList = new ArrayList<Student>(); // declare instance variable.

And you cannot access instance variable w/o creating instance of a class.

Instead use,

public static void main(String[] args) {
    ArrayList<Student> rayList = new ArrayList<Student>(); // creates local instance of rayList
    rayList.add(new Student("Sam", 17));
    rayList.add(new Student("Sandra", 18));
    rayList.add(new Student("Billy", 16));
    rayList.add(new Student("Greg", 17));
    rayList.add(new Student("Jill", 18));

    System.out.println(rayList.get(0));
}

Answer 3

That error means that rayList is not static (in other words, it is a member of class Student ) but your main method is:

/** THIS IS NOT STATIC **/
ArrayList<Student> rayList = new ArrayList<Student>();
...

public static void main(String[] args) {
    /** THIS SCOPE IS STATIC **/
    System.out.println(rayList.get(0));
}

You actually can't make a list of Student s as a member of Student because you will either get a stack overflow or run out of memory, depending on how you create it, so I'm guessing you wanted rayList to be static. That would make rayList no longer a member of Student .

You also cannot add to your ArrayList the way you are outside of an initializer block or method.

See here for more on initializer blocks:

• What is an initialization block?
• What is the advantage of using initialization blocks?

Or you could move everything referring to rayList inside your main method.

Answer 4

Make the rayList variable static to use it in main method like this :

Then add the objects in main method and use it :

static ArrayList<Student> rayList = new ArrayList<Student>();
public static void main(String[] arg){

    rayList.add(new Student("Sam", 17));
    rayList.add(new Student("Sandra", 18));
    rayList.add(new Student("Billy", 16));
    rayList.add(new Student("Greg", 17));
    rayList.add(new Student("Jill", 18));

    //Do whatever you want.
}