public class Main{
    private MyQueue queue = null;
    private MyStack stack = null;
    private MyList list = null;

    public static void main(String args[]){
        Main m = new Main();
        m.run();
    }

    public void run(){
        // only one data structure is not null.
        // for example, let's suppose that it's queue
        this.queue = new MyQueue();

        int intToAdd = 6;   // I want to add this number
        this.selectStruct().insert(intToAdd);       // I want to invoke the "insert" method of the current data structure in use
    }

    private <T> T selectStruct(){       // this should check with data structure is in use (so which of them is not null) and then return the reference
        if (this.queue!=null){
            return this.queue;
        }
        if (this.stack!=null){
            return this.stack;
        }
        if (this.list!=null){
            return this.list;
        }
    }
}

我想用这个叫做 selectStruct 选择当前使用的数据结构并返回引用。从这个参考号打电话给 insert 调用正确类的正确insert方法。
问题是我有几个错误。我从未使用过泛型，所以我有点困惑。

$ javac Main.java
Main.java:22: error: incompatible types: MyQueue cannot be converted to T
                        return this.queue;
                                   ^
  where T is a type-variable:
    T extends Object declared in method <T>selectStruct()
Main.java:25: error: incompatible types: MyStack cannot be converted to T
                        return this.stack;
                                   ^
  where T is a type-variable:
    T extends Object declared in method <T>selectStruct()
Main.java:28: error: incompatible types: MyList cannot be converted to T
                        return this.list;
                                   ^
  where T is a type-variable:
    T extends Object declared in method <T>selectStruct()
Main.java:17: error: cannot find symbol
                this.selectStruct().insert(intToAdd);
                                   ^
  symbol:   method insert(int)
  location: class Object
4 errors