java - 避免使用具有双扩展类的泛型进行未经检查的强制转换？

Question

我有以下代码，我今天刚刚将其重构，因为我了解了 <T extends Buffer> 的含义。真正的意思是，这是简化版本:

public class Buffer {
    protected final int bufferType;
    protected final int bufferDataType;

    protected int bufferId;
    protected boolean created;

    public Buffer(final int bufferType, final int bufferDataType) {
        this.bufferType = bufferType;
        this.bufferDataType = bufferDataType;
    }

    public <T extends Buffer> T create() {
        assertNotCreated();
        bufferId = GL15.glGenBuffers();

        created = true;
        return (T)this;
    }

    public boolean hasBeenCreated() {
        return created;
    }

    private void assertNotCreated() {
        if (hasBeenCreated()) {
            throw new RuntimeException("Buffer has been created already.");
        }
    }
}

<小时/>

public class ArrayBuffer extends Buffer {
    public ArrayBuffer(final int bufferDataType) {
        super(GL15.GL_ARRAY_BUFFER, bufferDataType);
    }    
}

<小时/>

public class DynamicDrawArrayBuffer extends ArrayBuffer {
    public DynamicDrawArrayBuffer() {
        super(GL15.GL_DYNAMIC_DRAW);
    }
}

警告发生在 Buffer.create() ，抑制警告是否安全？有什么办法可以让它更安全吗？

另一个要求是该API的调用/使用代码中不应添加困惑的内容，具体来说，这意味着DynamicDrawArrayBuffer可能无法附加泛型。

Answer 1

这显然不是类型安全的。你可以这样写

ArrayBuffer a = new ArrayBuffer(0);
DynamicDrawArrayBuffer d = a.create();

这将导致 ClassCastException。 create 方法的返回类型是从调用站点推断出来的。此处调用站点提供的唯一信息是返回值必须是 DynamicDrawArrayBuffer - 因此 ArrayBuffer 被粗暴地转换为一个。

我认为您可以在这里轻松利用协方差:create 方法始终可以返回类的类型。然后，create 方法的返回类型由您调用该方法时所拥有的类型信息决定:

public class BufferTest
{
    public static void main(String[] args)
    {
        ArrayBuffer a = new ArrayBuffer();
        ArrayBuffer aa = a.create(); // Yes
        // DynamicDrawArrayBuffer ad = a.create(); // No

        DynamicDrawArrayBuffer d = new DynamicDrawArrayBuffer();
        ArrayBuffer da = a.create(); // Yes
        DynamicDrawArrayBuffer dd = d.create(); // Yes

        Buffer b = a;
        Buffer bb = b.create(); // Yes
        //ArrayBuffer ba = b.create(); // No
    }
}

class Buffer 
{
    public Buffer create() 
    {
        // create etc...
        return this;
    }

}

class DynamicDrawArrayBuffer extends ArrayBuffer 
{
    @Override
    public DynamicDrawArrayBuffer create()
    {
        super.create();
        return this;
    }
}

class ArrayBuffer extends Buffer 
{
    @Override
    public ArrayBuffer create()
    {
        super.create();
        return this;
    }
}