为什么我不能创建通用 class 的通用 arraylist 成员？

Question

I want to add methods to return the list or particular element and I want it to work with any generic class. Why isnt Java allowing this?

class ArrayListBuilder<E>{
    private ArrayList<E> a_list=new ArrayList<>();
}

Answer 1

I tried to compile your code and got these errors

ArrayListBuilder.java:2: error: cannot find symbol
    private ArrayList<E> a_list=new ArrayList<>();
            ^
  symbol:   class ArrayList
  location: class ArrayListBuilder<E>
  where E is a type-variable:
    E extends Object declared in class ArrayListBuilder
ArrayListBuilder.java:2: error: cannot find symbol
    private ArrayList<E> a_list=new ArrayList<>();
                                    ^
  symbol:   class ArrayList
  location: class ArrayListBuilder<E>
  where E is a type-variable:
    E extends Object declared in class ArrayListBuilder
2 errors

As you can see there actually are two errors, the first one meaning that you need to import java.util.ArrayList . Once you do that, your code compiles fine. So put in the first line of your code

import java.util.ArrayList;

TL;DR: the solution to your problem is reading all of the error messages. Not just the last one.