好記性不如爛筆頭,僅以此記錄
20200618
Prototype:原型模式(克隆模式)
注:一般用於一個對象的屬性已經確定,需要產生很多相同對象的時候
java中的原型模式(自帶)
1.實現原型模式需要實現標記型接口Cloneable,一般會重寫clone()方法
如果只是重寫clone()方法,而沒實現Cloneable接口,調用時會報異常 java.lang.CloneNotSupportedException、
例:
1.1 實現Cloneable,且重寫clone()
class Person implements Cloneable {
int age = 8;
int score = 100;
@Override
public Object clone() throws CloneNotSupportedException {
return super.clone();
}
}
public class Test {
public static void main(String[] args) throws Exception {
Person p1 = new Person();
Person p2 = (Person)p1.clone();
System.out.println(p2.age + " " + p2.score);
}
}
2.深克隆和淺克隆
淺克隆
例:當執行 p1.loc.street = "sh"; 時 ,p2.loc的street也會改變爲 "sh"
原因 : clone() 會把 p1 整個在內存中copy出一份,p1.loc 和p2.loc的指向地址是相同的,改變了一個兩個都會改變
public class Test {
public static void main(String[] args) throws Exception {
Person p1 = new Person();
Person p2 = (Person)p1.clone();
System.out.println(p2.age + " " + p2.score);
System.out.println(p2.loc);
System.out.println(p1.loc == p2.loc);
p1.loc.street = "sh";
System.out.println(p2.loc);
}
}
class Person implements Cloneable {
int age = 8;
int score = 100;
Location loc = new Location("bj", 22);
@Override
public Object clone() throws CloneNotSupportedException {
return super.clone();
}
}
class Location {
String street;
int roomNo;
@Override
public String toString() {
return "Location{" +
"street='" + street + '\'' +
", roomNo=" + roomNo +
'}';
}
public Location(String street, int roomNo) {
this.street = street;
this.roomNo = roomNo;
}
}
深克隆
例:loc也需要實現 Cloneable
原因 : clone()時,loc1 也會在內存中clone()一份loc2,兩個的地址不同,改變其中一個不會對另一個造成影響
注意: 如果不想clone的對象引用地址相同,需要實現Cloneable
public class Test {
public static void main(String[] args) throws Exception {
Person p1 = new Person();
Person p2 = (Person)p1.clone();
System.out.println(p2.age + " " + p2.score);
System.out.println(p2.loc);
System.out.println(p1.loc == p2.loc);
p1.loc.street = "sh";
System.out.println(p2.loc);
}
}
class Person implements Cloneable {
int age = 8;
int score = 100;
Location loc = new Location("bj", 22);
@Override
public Object clone() throws CloneNotSupportedException {
Person p = (Person)super.clone();
p.loc = (Location)loc.clone();
return p;
}
}
class Location implements Cloneable {
String street;
int roomNo;
@Override
public String toString() {
return "Location{" +
"street='" + street + '\'' +
", roomNo=" + roomNo +
'}';
}
public Location(String street, int roomNo) {
this.street = street;
this.roomNo = roomNo;
}
@Override
public Object clone() throws CloneNotSupportedException {
return super.clone();
}
}
問題:
1.String需要進一步深克隆嗎?
不需要,因爲 String指向的是常量池,本來就是共用的,修改其中一個不會修改常量池中的數據
2.new String()呢?
需要,因爲 new對象實在堆中創建,雖然此對象引用依然指向常量池,但是clone出來的new String 修改是修改的堆中數據,引用改了,元數據的引用跟着就改變了
勉強可以看的圖