我有父类Entity
:
package incubator;
import java.lang.annotation.Annotation;
import java.lang.reflect.Field;
public class Entity {
private String getTableName() {
String result = null;
Class<?> cl = getClass();
System.out.println(cl.getName());
for (Annotation a : cl.getAnnotations()) {
if (a instanceof EntityTable) {
EntityTable ent = (EntityTable) a;
result = ent.name();
break;
}
}
return result;
}
private String getKeyName() {
String result = null;
Class<?> cl = getClass();
System.out.println(cl.getName());
for (Field f : cl.getDeclaredFields()) {
for (Annotation a : f.getAnnotations()) {
if (a instanceof PrimaryKey) {
PrimaryKey ann = (PrimaryKey) a;
result = ann.name();
}
}
}
return result;
}
public Entity get(int id) throws IllegalAccessException, InstantiationException {
System.out.println("SELECT * FROM "
+ getTableName() + " WHERE (" + getKeyName() + "=?);");
return getClass().newInstance();
}
public void delete() {
System.out.println("DELETE FROM "
+ getTableName() + " WHERE (" + getKeyName() + "=?);");
}
}
儿童班Child
:
package incubator;
@EntityTable(name="table")
public class Child extends Entity {
@PrimaryKey(name="tbl_pcode")
private int id;
@DataField(name="tbl_text")
public String text;
@DataField(name = "tbl_data")
public String data;
public Child() {
id = 0;
}
}
所有注释都像
package incubator;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface EntityTable {
String name();
}
所以,我有一个问题:有没有办法让类get(final int id)
的静态方法Entity
返回Child
的实例?如何在父类中指定子类[es]的结果类型?
感谢您浪费时间陪伴我。最好的祝福。
答案 0 :(得分:3)
由于类型擦除,无法在运行时获取静态上下文中的实际类型。在调用方法时,您必须显式声明该类。使用通用方法,它看起来像这样:
public static <T extends Entity> T get(int id, Class<T> clazz) {
return clazz.newInstance();
}
我不确定这对你的情况是否有用,但如果你想要静止,这是唯一的方法。
答案 1 :(得分:1)
使用泛型:
// Subclasses will have to pass their type as a generic type argument
// Which you will use to declare the return type of get()
class Entity<T extends Entity<T>> {
T get(int id) {
T value = ...; // Load the value from db or whatever
return value;
}
}
// Child tells its parent that the dynamic type is Child
class Child extends Entity<Child> {
}