初始JAVA中淺拷貝和深拷貝
1. 簡單變量的復制
public static void main(String[] args) { int a = 5; int b = a; System.out.println(a); System.out.println(b); }
八種基本數據類型的(int,boolean,char,byte,short,float,double.long)都適用於這種情況。
2. 對象的復制
對象的復制並沒有基本類型變量的復制這麽簡單,下面看一個例子
2.1 準備一個Student類
packagecom.jepson.java; /** * @author jepson * @create 2018-09-21 23:20 */ public class Student { private String name; public String getName() { return name; } public void setName(String name) { this.name = name; } }
2.2 測試Object的測試
package com.jepson.java;/** * @author jepson * @create 2018-09-21 23:21 */ public class TestObjectCopy { public static void main(String[] args) { Student stu1 = new Student(); stu1.setName("jepson"); Student stu2 = stu1; System.out.println("student1:"+stu1.getName()); System.out.println("student2:"+stu2.getName()); } }
2.3 運行結果
student1:jepson
student2:jepson
2.4 總結
這裏我們自定義了一個學生類,該類只有一個name字段。
我們新建了一個學生實例,然後將該值賦值給stu2實例。(Student stu2 = stu1;)
再看看打印結果,作為一個新手,拍了拍胸腹,對象復制不過如此,
難道真的是這樣嗎?
2.5 舉一反三
我們試著改變stu2實例的name字段,再打印結果看看
package com.jepson.java; /** * @author jepson * @create 2018-09-21 23:21 */ public class TestObjectCopy { public static void main(String[] args) { Student stu1 = new Student(); stu1.setName("jepson"); Student stu2 = stu1; stu2.setName("tom"); System.out.println("student1:"+stu1.getName()); System.out.println("student2:"+stu2.getName()); } }
打印結果
student1:tom
student2:tom
這就怪了,為什麽改變學生2的學號,學生1的學號也發生了變化呢?
原因出在(stu2 = stu1) 這一句。該語句的作用是將stu1的引用賦值給stu2,
這樣,stu1和stu2指向內存堆中同一個對象。如圖:
那麽,怎樣才能達到復制一個對象呢?
是否記得萬類之王Object。它有11個方法,有兩個protected的方法,其中一個為clone方法。
該方法的簽名是:
protected native Object clone() throws CloneNotSupportedException;
因為每個類直接或間接的父類都是Object,因此它們都含有clone()方法,但是因為該方法是protected,所以都不能在類外進行訪問。
要想對一個對象進行復制,就需要對clone方法覆蓋。
3 對象的淺拷貝(淺復制)
1. 被復制的類需要實現Clonenable接口(不實現的話在調用clone方法會拋出CloneNotSupportedException異常) 該接口為標記接口(不含任何方法)
2. 覆蓋clone()方法,訪問修飾符設為public。方法中調用super.clone()方法得到需要的復制對象,(native為本地方法)
3.1 修改Student類
1.實現Cloneable接口
2.重寫clone方法
3.訪問修飾符改為public
package com.jepson.java; /** * @author jepson * @create 2018-09-21 23:20 */ public class Student implements Cloneable{ @Override public Object clone() { Student stu = null; try{ stu = (Student)super.clone(); }catch(CloneNotSupportedException e) { e.printStackTrace(); } return stu; } private String name; public String getName() { return name; } public void setName(String name) { this.name = name; } }
3.2 修改測試類
1.調用clone方法。
package com.jepson.java; /** * @author jepson * @create 2018-09-21 23:21 */ public class TestObjectCopy { public static void main(String[] args) { Student stu1 = new Student(); stu1.setName("jepson"); Student stu2 = (Student)stu1.clone(); stu2.setName("淺拷貝-tom"); System.out.println("student1:"+stu1.getName()); System.out.println("student2:"+stu2.getName()); } }
打印結果
如果你還不相信這兩個對象不是同一個對象,那麽可以測試一下
package com.jepson.java; /** * @author jepson * @create 2018-09-21 23:21 */ public class TestObjectCopy { public static void main(String[] args) { Student stu1 = new Student(); stu1.setName("jepson"); Student stu2 = (Student)stu1.clone(); stu2.setName("淺拷貝-tom"); System.out.println("student1:"+stu1.getName()); System.out.println("student2:"+stu2.getName()); System.out.println(stu1==stu2); } }
打印結果
4 對象的深拷貝(深復制)
我們在學生類裏再加一個Address類。
Address類如下:
package com.jepson.java; /** * @author jepson * @create 2018-09-21 23:57 */ public class Address { private String address; public String getAddress() { return address; } public void setAddress(String address) { this.address = address; } @Override public String toString() { return "Address{" + "address=‘" + address + ‘\‘‘ + ‘}‘; } }
Student類如下:
package com.jepson.java; /** * @author jepson * @create 2018-09-21 23:20 */ public class Student implements Cloneable{ private String name; private Address address; public Address getAddress() { return address; } public void setAddress(Address address) { this.address = address; } public String getName() { return name; } public void setName(String name) { this.name = name; } @Override public Object clone() { Student stu = null; try{ stu = (Student)super.clone(); }catch(CloneNotSupportedException e) { e.printStackTrace(); } return stu; } }
測試類
package com.jepson.java; /** * @author jepson * @create 2018-09-21 23:21 */ public class TestObjectCopy { public static void main(String[] args) { Address address = new Address(); address.setAddress("成都市"); Student stu1 = new Student(); stu1.setName("jepson"); stu1.setAddress(address); Student stu2 = (Student)stu1.clone(); stu2.setName("深拷貝-tom"); System.out.println("student1:"+stu1.getName()+"::::"+stu1.getAddress()); System.out.println("student2:"+stu2.getName()+"::::"+stu2.getAddress()); System.out.println(stu1==stu2); } }
打印結果
乍一看沒什麽問題,真的是這樣嗎?
我們在main方法中試著改變addr實例的地址。
package com.jepson.java; /** * @author jepson * @create 2018-09-21 23:21 */ public class TestObjectCopy { public static void main(String[] args) { Address address = new Address(); address.setAddress("成都市"); Student stu1 = new Student(); stu1.setName("jepson"); stu1.setAddress(address); Student stu2 = (Student)stu1.clone(); stu2.setName("深拷貝-tom"); System.out.println("student1:"+stu1.getName()+"::::"+stu1.getAddress()); System.out.println("student2:"+stu2.getName()+"::::"+stu2.getAddress()); address.setAddress("重慶區"); stu2.setName("深拷貝-tom"); System.out.println("student1:"+stu1.getName()+"::::"+stu1.getAddress()); System.out.println("student2:"+stu2.getName()+"::::"+stu2.getAddress()); System.out.println(stu1==stu2); } }
打印結果
這就奇怪了,怎麽兩個學生的地址都改變了?
原因是淺復制只是復制了addr變量的引用,並沒有真正的開辟另一塊空間,將值復制後再將引用返回給新對象。
所以,為了達到真正的復制對象,而不是純粹引用復制。我們需要將Address類可復制化,並且修改clone方法,完整代碼如下:
1.修改Address類
1.實現Cloneable接口
2.重寫clone方法
package com.jepson.java; /** * @author jepson * @create 2018-09-21 23:57 */ public class Address implements Cloneable{ private String address; public String getAddress() { return address; } public void setAddress(String address) { this.address = address; } @Override public String toString() { return "Address{" + "address=‘" + address + ‘\‘‘ + ‘}‘; } @Override protected Object clone() { Address addr = null; try{ addr = (Address)super.clone(); }catch(CloneNotSupportedException e) { e.printStackTrace(); } return addr; } }
2. 修改Student類
clone方法中加入深度復制
package com.jepson.java; /** * @author jepson * @create 2018-09-21 23:20 */ public class Student implements Cloneable{ private String name; private Address address; public Address getAddress() { return address; } public void setAddress(Address address) { this.address = address; } public String getName() { return name; } public void setName(String name) { this.name = name; } @Override public Object clone() { Student stu = null; try{ stu = (Student)super.clone(); // 淺復制 }catch(CloneNotSupportedException e) { e.printStackTrace(); } stu.address = (Address)address.clone(); //深度復制 return stu; } }
3. 測試類
package com.jepson.java; /** * @author jepson * @create 2018-09-21 23:21 */ public class TestObjectCopy { public static void main(String[] args) { Address address = new Address(); address.setAddress("成都市"); Student stu1 = new Student(); stu1.setName("jepson"); stu1.setAddress(address); Student stu2 = (Student)stu1.clone(); stu2.setName("深拷貝-tom"); System.out.println("student1:"+stu1.getName()+"::::"+stu1.getAddress()); System.out.println("student2:"+stu2.getName()+"::::"+stu2.getAddress()); address.setAddress("重慶區"); stu2.setName("深拷貝-tom"); System.out.println("student1:"+stu1.getName()+"::::"+stu1.getAddress()); System.out.println("student2:"+stu2.getName()+"::::"+stu2.getAddress()); System.out.println(stu1==stu2); } }
這樣結果就符合我們的想法了。
總結:淺拷貝是指在拷貝對象時,對於基本數據類型的變量會重新復制一份,而對於引用類型的變量只是對引用進行拷貝,
沒有對引用指向的對象進行拷貝。
而深拷貝是指在拷貝對象時,同時會對引用指向的對象進行拷貝。
區別就在於是否對 對象中的引用變量所指向的對象進行拷貝。
初始JAVA中淺拷貝和深拷貝