这是我的情况
public abstract class Actions {
public static Actions STAND;
public static Actions ATTACK;
public static Actions COLONIZE;
public static Actions DEFEND;
public static Actions TURN_CW;
public static Actions TURN_CCW;
public static Actions DIE;
public abstract long[] getFramesDurations();
public abstract int[] getBaseTiles();
}
public class SimpleActions extends Actions{
public static Actions STAND = new SimpleActions( new long[]{120,120,120,120,120,120,120}, new int[]{0,1,2,3,4,5,6});
public static Actions ATTACK = new SimpleActions( new long[]{120,120,120,120,120,120,120,120,120}, new int[]{7,8,9,10,11,12,13,14,15});
public static Actions COLONIZE = new SimpleActions( new long[]{120,120,120,120,120,120,120}, new int[]{7,8,9,10,11,12,13,14,15});
public static Actions DEFEND = new SimpleActions(new long[]{1}, new int[]{1});
public static Actions TURN_CW = new SimpleActions( new long[]{1}, new int[]{1});
public static Actions TURN_CCW = new SimpleActions( new long[]{1}, new int[]{1});
public static Actions DIE = new SimpleActions( new long[]{1}, new int[]{1});
private final long[] mActionFramesDurations;
private final int[] mActionBaseTiles;
SimpleActions(long[] pActionFramesDurations, int[] pActionBaseTiles) {
mActionFramesDurations = pActionFramesDurations;
mActionBaseTiles = pActionBaseTiles;
}
public long[] getFramesDurations()
{
return mActionFramesDurations;
}
public int[] getBaseTiles()
{
return mActionBaseTiles;
}
}
public abstract class A<T extends Actions> {
A() {
doSomething(T.STAND);
}
protected void doSomething(Actions action) { use action somewhere}
}
public class B extends A<SimpleActions> {
B() {
super();
}
}
当 A 的构造函数调用 doSomething 时,我总是得到 nullPointerException,因为 action 为 null..
由于 B 扩展了 A,我希望它使用 SimpleActions.STAND,而不是 Actions.STAND。
我做错了什么?我应该怎么做?
最佳答案
泛型的类型参数在运行时是未知的。换句话说,在运行时,A<Actions>
之间没有区别。和一个 A<SimpleActions>
.因此 jvm 无法判断您想要 SimpleActions.STAND
, 而不是 Actions.STAND
.如果您需要在运行时知道类型参数,则需要将它们放在一个单独的变量中。
如果不清楚,请阅读“运行时类型删除”。
根据您的评论进行编辑 - 如果您只在构造函数中执行此逻辑,则可以使构造函数看起来像
A( Class<? extends Action> actionType ){
if( SimpleActions.class.isAssignableFrom( actionType )){
doSomething( SimpleActions.STAND );
}
else{
doSomething( Actions.STAND );
}
}
如果在构造函数之外需要相同的逻辑,那么创建一个类型为 Class<? extends Action>
的成员变量里面A
存储 actionType
.
关于java - 通用 <T extends A> 类使用静态字段而不是 T 的静态字段,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/8936038/