“在数组列表构造函数中没有找到适合 add(java.lang.String)"的方法? [英] "no suitable method found for add(java.lang.String)"in arraylist constructor?

问题描述

import java.util.ArrayList;
import java.util.Random;

public class College
{
    // instance variables - replace the example below with your own
    private ArrayList<Student> students;

    public College()
    {
        // initialise instance variables
        ArrayList<Student> students = new ArrayList<Student>();
        students.add("Student 1");
        students.add("Student 2");
        students.add("Student 3");

    }
}

基本上它突出显示了错误消息java.lang.IllegalArgumentException:bound must be positive"的.add，我不明白我在这里做错了什么?我在这里查看了很多此类问题线程，但我完全按照他们的要求做了

basically it highlights the .add showing the error message "java.lang.IllegalArgumentException: bound must be positive", I don't understand what I did wrong here? I looked at a lot of these kinds of problem threads here but I did exactly what they did

推荐答案

您正在将 Strings 添加到参数化的 List 以获取 Students.

You are adding Strings to a List parametrized to take Students.

当然这不会编译.

• 向您的 Student 类添加一个构造函数，采用一个 String 参数(以及其中的相关逻辑).
• 然后，使用以下习语:students.add(new Student("Student 1"));

• Add a constructor to your Student class, taking a String argument (and the relevant logic within).
• Then, use the following idiom: students.add(new Student("Student 1"));

需要注意的是，泛型正是在那个阶段编译失败.

To be noted, generics are there precisely to fail compilation at that stage.

如果你使用了一个原始的 List(Java 4 风格)，你的代码会被编译，但是在运行时会发生各种各样的坏事，因为你期望 Student 对象要包含在您的 List 中，但您会得到 Strings.

If you had used a raw List (Java 4-style), your code would have compiled, but all sorts of evil would have happened at runtime, since you'd expect Student objects to be contained in your List, but you'd get Strings instead.

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