繼承
Java只有單繼承,最頂級的父類是Object。
子類會繼承父類的fields和methods,而不會繼承constructors,因爲constructors不屬於methods,但是子類可以通過super調用父類的constructor。
子類繼承父類的範圍是:public、protected、package-private
隱式轉換,子類轉父類(只有1個爸爸):
Object obj = new MountainBike();
顯示轉換,父類轉子類(有多個子女,所以要明確指定):
MountainBike myBike = (MountainBike)obj;
類有field,而接口沒有,所以在多繼承時就有問題:如果多個類有相同的field,那麼子類將不知道用哪一個,而接口不存在這個問題。Java不支持繼承多個類,但是可以實現多個接口。
重載
子類的方法跟父類有完全相同的簽名和返回類型(也可以是子類),將會覆蓋父類方法Override。
如果子類定義了1個static方法,跟父類完全相同,那麼父類方法會被隱藏Hide。
Override和Hide是不同的:
public class Animal {
public static void testClassMethod() {
System.out.println("The static method in Animal");
}
public void testInstanceMethod() {
System.out.println("The instance method in Animal");
}
}
public class Cat extends Animal {
public static void testClassMethod() {
System.out.println("The static method in Cat");
}
public void testInstanceMethod() {
System.out.println("The instance method in Cat");
}
public static void main(String[] args) {
Cat myCat = new Cat();
Animal myAnimal = myCat; // 隱式轉換
Animal.testClassMethod(); // 調的父類
myAnimal.testInstanceMethod(); // 父類對象引用,還是調的子類
}
}
The static method in Animal
The instance method in Cat
Override只會調子類方法,而Hide取決於調用方是父還是子,比如這裏的myCat,隱式轉換爲父類Animal後,會調父類的static方法,而調的實例方法卻是子類的。(如果不是隱式轉換,而是直接給父類實例化,那肯定還是調父類方法)
類instance方法優先於接口default方法:
public class Horse {
public String identifyMyself() {
return "I am a horse.";
}
}
public interface Flyer {
default public String identifyMyself() {
return "I am able to fly.";
}
}
public interface Mythical {
default public String identifyMyself() {
return "I am a mythical creature.";
}
}
public class Pegasus extends Horse implements Flyer, Mythical {
public static void main(String... args) {
Pegasus myApp = new Pegasus();
System.out.println(myApp.identifyMyself());
}
}
輸出爲I am a horse.
Override的優先:
public interface Animal {
default public String identifyMyself() {
return "I am an animal.";
}
}
public interface EggLayer extends Animal {
default public String identifyMyself() {
return "I am able to lay eggs.";
}
}
public interface FireBreather extends Animal { }
public class Dragon implements EggLayer, FireBreather {
public static void main (String... args) {
Dragon myApp = new Dragon();
System.out.println(myApp.identifyMyself());
}
}
輸出爲I am able to lay eggs
如果實現多接口,有同名的,需要顯示指定調用方:
public interface OperateCar {
// ...
default public int startEngine(EncryptedKey key) {
// Implementation
}
}
public interface FlyCar {
// ...
default public int startEngine(EncryptedKey key) {
// Implementation
}
}
public class FlyingCar implements OperateCar, FlyCar {
// ...
public int startEngine(EncryptedKey key) {
FlyCar.super.startEngine(key); // 顯示指定,並且使用super
OperateCar.super.startEngine(key);
}
}
總結下,如果子類方法簽名+return跟父類方法一樣,有以下4種情況:
注意compile-time error,static方法不能和instance方法一樣,因爲它們是不同級別的。
多態
MountainBike和RoadBike都繼承Bicycle,雖然都有printDescription,但它們有多樣的形態:
public class MountainBike extends Bicycle {
private String suspension;
public MountainBike(
int startCadence,
int startSpeed,
int startGear,
String suspensionType){
super(startCadence,
startSpeed,
startGear);
this.setSuspension(suspensionType);
}
public String getSuspension(){
return this.suspension;
}
public void setSuspension(String suspensionType) {
this.suspension = suspensionType;
}
public void printDescription() {
super.printDescription();
System.out.println("The " + "MountainBike has a" +
getSuspension() + " suspension.");
}
}
public class RoadBike extends Bicycle{
// In millimeters (mm)
private int tireWidth;
public RoadBike(int startCadence,
int startSpeed,
int startGear,
int newTireWidth){
super(startCadence,
startSpeed,
startGear);
this.setTireWidth(newTireWidth);
}
public int getTireWidth(){
return this.tireWidth;
}
public void setTireWidth(int newTireWidth){
this.tireWidth = newTireWidth;
}
public void printDescription(){
super.printDescription();
System.out.println("The RoadBike" + " has " + getTireWidth() +
" MM tires.");
}
}
所謂的虛擬方法調用,名字很高大上,其實質就是,子類重載了父類方法,在調用子類實例方法時,先調子類實現:
注意,子類的field如果和父類的一樣,那麼父類的field會被hide,即使type不一樣。如果要使用父類的field,需要關鍵字super。同名fileld是不好的設計,應該儘量避免。
super關鍵字
public class Superclass {
public void printMethod() {
System.out.println("Printed in Superclass.");
}
}
public class Subclass extends Superclass {
// overrides printMethod in Superclass
public void printMethod() {
super.printMethod();
System.out.println("Printed in Subclass");
}
public static void main(String[] args) {
Subclass s = new Subclass();
s.printMethod();
}
}
在子類constructor中,如果沒有顯式super,那麼會調用默認的super()
Object
Java中的所有類,都終極繼承了Object。(繼承鏈的頂端)
toString()
System.out.println(firstBook.toString());
equals()
public class Book {
String ISBN;
public String getISBN() {
return ISBN;
}
public boolean equals(Object obj) {
if (obj instanceof Book)
return ISBN.equals((Book)obj.getISBN());
else
return false;
}
}
hashCode()
如果兩個對象相等,那麼它們的hashCode一定相等。重寫equals(),必須重寫hashCode()
getClass()
void printClassName(Object obj) {
System.out.println("The object's" + " class is " +
obj.getClass().getSimpleName());
}
clone()
aCloneableObject.clone();
finalize()
垃圾回收時調用。它的調用時機是不確定的,不要使用它來做邏輯。
抽象
abstract class,抽象類,不能被實例化,但是可以繼承。
abstract void moveTo(double deltaX, double deltaY);
如果抽象類有method,那麼也必須是abstract:
public abstract class GraphicObject {
// declare fields
// declare nonabstract methods
abstract void draw();
}
子類必須抽象類的所有方法,否則子類也必須是abstract。
抽象類:(not static、final) field,(public、protected、private) method
接口:public static final field,public method
Java中,抽象類的例子是AbstractMap、接口的例子是HashMap實現的Serializable, Cloneable, and Map<K, V>接口。
如果類沒有實現接口中的所有方法,可以定義爲abstract,然後由子類來實現剩餘的全部方法:
abstract class X implements Y {
// implements all but one method of Y
}
class XX extends X {
// implements the remaining method in Y
}
X沒有實現全部方法,所以是abstract,子類XX實現剩餘全部方法。
參考資料:
Inheritance https://dev.java/learn/inheritance/