You may get this type of error “no such column” when you change the models.
Try this one
$python manage.py reset thenameofyourapp
$python manage.py syncdb
2012年10月31日 星期三
2012年3月3日 星期六
instanceof
instanceof
Java提供了instanceof作為判別類別間繼承關係之用，如果要判定的物件為該類別或該類別的子類別則傳回true，否則傳回false。需特別注意的是，物件與類別間要有繼承關係，否則會有compile error，如錯誤狀況1、2。
在Java裡有個特別規定，就是任何陣列都繼承Object，所以類別陣列將繼承Object和Object[]，一般型別陣列則會繼承Object，但是一般型別不是物件，所以一般型別陣列不會繼承Object[]。
範例程式:
class myBase1 { }
class myBase2 extends myBase1 { }
public class myInstanceof extends myBase2 {
public static void main(String[] args) {
myBase1 b1 = new myBase1();
myBase2 b2 = new myBase2();
myInstanceof i1 = new myInstanceof();
if (i1 instanceof myBase1) {
System.out.println("i1是myBase1的後代");
}
else {
System.out.println("i1不是myBase1的後代");
}
if (b1 instanceof myInstanceof) {
System.out.println("b1是myInstanceof的後代");
}
else {
System.out.println("b1不是myInstanceof的後代");
}
if (null instanceof myBase1) {
System.out.println("null是myBase1的後代");
}
else {
System.out.println("null不是myBase1的後代");
}
myBase2[] b2array = new myBase2[10];
if (b2array instanceof Object) {
System.out.println("b2array是Object的後代");
}
else {
System.out.println("b2array不是Object的後代");
}
if (b2array instanceof Object[]) {
System.out.println("b2array是Object[]的後代");
}
else {
System.out.println("b2array不是Object[]的後代");
}
int[] i = new int[10];
if (i instanceof Object) {
System.out.println("i是Object的後代");
}
else {
System.out.println("i不是Object的後代");
}
/*
//錯誤狀況1
if (i instanceof Object[]) {
System.out.println("i是Object[]的後代");
}
else {
System.out.println("i不是Object[]的後代");
}
//錯誤狀況2
Long lng = new Long(10);
if (lng instanceof String) {
System.out.println("lng是String的後代");
}
else {
System.out.println("lng不是String的後代");
}
*/
}
}
執行結果:
i1是myBase1的後代
b1不是myInstanceof的後代
null不是myBase1的後代
b2array是Object的後代
b2array是Object[]的後代
i是Object的後代
錯誤狀況1:
"myInstanceof.java": Error #: 365 : cannot compare int[] with java.lang.Object[]
錯誤狀況2:
"myInstanceof.java": Error #: 365 : cannot compare java.lang.Long with java.lang.String
Conditional Operator ? :
The conditional operator
? : uses the boolean value of one expression to decide which of two other expressions should be evaluated.ConditionalExpression:
ConditionalOrExpression
ConditionalOrExpression
? Expression : ConditionalExpression
The conditional operator is syntactically right-associative (it groups right-to-left). Thus,
a?b:c?d:e?f:g means the same as a?b:(c?d:(e?f:g)).
The conditional operator has three operand expressions.
? appears between the first and second expressions, and : appears between the second and third expressions.
It is a compile-time error for either the second or the third operand expression to be an invocation of a
void method.
In fact, by the grammar of expression statements (§14.8), it is not permitted for a conditional expression to appear in any context where an invocation of a
void method could appear.- If the second and third operands have the same type (which may be the null type), then that is the type of the conditional expression.
- If one of the second and third operands is of primitive type T, and the type of the other is the result of applying boxing conversion (§5.1.7) to T, then the type of the conditional expression isT.
- If one of the second and third operands is of the null type and the type of the other is a reference type, then the type of the conditional expression is that reference type.
- Otherwise, if the second and third operands have types that are convertible (§5.1.8) to numeric types, then there are several cases:
- If one of the operands is of type
byteorByteand the other is of typeshortorShort, then the type of the conditional expression isshort. - If one of the operands is of type T where T is
byte,short, orchar, and the other operand is a constant expression (§15.28) of typeintwhose value is representable in type T, then the type of the conditional expression is T. - If one of the operands is of type T, where T is
Byte,Short, orCharacter, and the other operand is a constant expression (§15.28) of typeintwhose value is representable in the typeU which is the result of applying unboxing conversion to T, then the type of the conditional expression is U. - Otherwise, binary numeric promotion (§5.6.2) is applied to the operand types, and the type of the conditional expression is the promoted type of the second and third operands.Note that binary numeric promotion performs value set conversion (§5.1.13) and may perform unboxing conversion (§5.1.8).
- Otherwise, the second and third operands are of types S1 and S2 respectively. Let T1 be the type that results from applying boxing conversion to S1, and let T2 be the type that results from applying boxing conversion to S2.The type of the conditional expression is the result of applying capture conversion (§5.1.10) to lub(T1, T2) (§15.12.2.7).
At run-time, the first operand expression of the conditional expression is evaluated first. If necessary, unboxing conversion is performed on the result.
The resulting
boolean value is then used to choose either the second or the third operand expression:
The chosen operand expression is then evaluated and the resulting value is converted to the type of the conditional expression as determined by the rules stated above.
The operand expression not chosen is not evaluated for that particular evaluation of the conditional expression.
2012年2月20日 星期一
1. 用abstract 修飾類時，則此類被稱之為抽象類
2. 含有抽象方法的類必須被聲明為抽象類，抽象類必須被繼承、抽象方法必須被重寫，否則子類就也要定義為抽象類及含有抽象方法，若子類繼承後沒有重寫方法，則出現錯誤 (Cat is not abstract and does not override abstract method enjoy() in Animal)
3. 抽象類不能被實例化，也就是不能以new產生新的物件，否則會出現錯誤 (Animal is abstract; cannot be instantiated)
4. 抽象方法只需被聲明，不需被實現
You can create objects of concrete subclasses.
You can still create object variables of an abstract class, but such a variable must refer to an object of a nonabstract subclass. For example:
Person p = new Student ("Vince Vu," "Economic");
Here p is a variable of the abstract type Person that refer to an instance of the nonabstract subclass Student.
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