淺談Java中的深拷貝和淺拷貝
淺談Java中的深拷貝和淺拷貝(轉載)
原文鏈接:
http://blog.csdn.net/tounaobun/article/details/8491392
假如說你想復制一個簡單變量。很簡單:
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- int apples = 5;
- int pears = apples;
int apples = 5; int pears = apples;
不僅僅是int類型,其它七種原始數據類型(boolean,char,byte,short,float,double.long)同樣適用於該類情況。
但是如果你復制的是一個對象,情況就有些復雜了。
假設說我是一個beginner,我會這樣寫:
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- class Student {
- private int number;
- public int getNumber() {
- return number;
- }
- public void setNumber(int number) {
- this.number = number;
- }
- }
- public class Test {
- public static void main(String args[]) {
- Student stu1 = new Student();
- stu1.setNumber(12345);
- Student stu2 = stu1;
- System.out.println("學生1:" + stu1.getNumber());
- System.out.println("學生2:" + stu2.getNumber());
- }
- }
class Student { private int number; public int getNumber() { return number; } public void setNumber(int number) { this.number = number; } } public class Test { public static void main(String args[]) { Student stu1 = new Student(); stu1.setNumber(12345); Student stu2 = stu1; System.out.println("學生1:" + stu1.getNumber()); System.out.println("學生2:" + stu2.getNumber()); } }
打印結果:
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- 學生1:12345
- 學生2:12345
學生1:12345 學生2:12345
這裏我們自定義了一個學生類,該類只有一個number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?
我們試著改變stu2實例的number字段,再打印結果看看:
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- stu2.setNumber(54321);
- System.out.println("學生1:" + stu1.getNumber());
- System.out.println("學生2:" + stu2.getNumber());
stu2.setNumber(54321); System.out.println("學生1:" + stu1.getNumber()); System.out.println("學生2:" + stu2.getNumber());
打印結果:
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- 學生1:54321
- 學生2:54321
學生1:54321 學生2:54321
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。
一般步驟是(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)
下面對上面那個方法進行改造:
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- class Student implements Cloneable{
- private int number;
- public int getNumber() {
- return number;
- }
- public void setNumber(int number) {
- this.number = number;
- }
- @Override
- public Object clone() {
- Student stu = null;
- try{
- stu = (Student)super.clone();
- }catch(CloneNotSupportedException e) {
- e.printStackTrace();
- }
- return stu;
- }
- }
- public class Test {
- public static void main(String args[]) {
- Student stu1 = new Student();
- stu1.setNumber(12345);
- Student stu2 = (Student)stu1.clone();
- System.out.println("學生1:" + stu1.getNumber());
- System.out.println("學生2:" + stu2.getNumber());
- stu2.setNumber(54321);
- System.out.println("學生1:" + stu1.getNumber());
- System.out.println("學生2:" + stu2.getNumber());
- }
- }
class Student implements Cloneable{ private int number; public int getNumber() { return number; } public void setNumber(int number) { this.number = number; } @Override public Object clone() { Student stu = null; try{ stu = (Student)super.clone(); }catch(CloneNotSupportedException e) { e.printStackTrace(); } return stu; } } public class Test { public static void main(String args[]) { Student stu1 = new Student(); stu1.setNumber(12345); Student stu2 = (Student)stu1.clone(); System.out.println("學生1:" + stu1.getNumber()); System.out.println("學生2:" + stu2.getNumber()); stu2.setNumber(54321); System.out.println("學生1:" + stu1.getNumber()); System.out.println("學生2:" + stu2.getNumber()); } }
打印結果:
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- 學生1:12345
- 學生2:12345
- 學生1:12345
- 學生2:54321
學生1:12345 學生2:12345 學生1:12345 學生2:54321
如果你還不相信這兩個對象不是同一個對象,那麽你可以看看這一句:
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- System.out.println(stu1 == stu2); // false
System.out.println(stu1 == stu2); // false
上面的復制被稱為淺復制(Shallow Copy),還有一種稍微復雜的深度復制(deep copy):
我們在學生類裏再加一個Address類。
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- class Address {
- private String add;
- public String getAdd() {
- return add;
- }
- public void setAdd(String add) {
- this.add = add;
- }
- }
- class Student implements Cloneable{
- private int number;
- private Address addr;
- public Address getAddr() {
- return addr;
- }
- public void setAddr(Address addr) {
- this.addr = addr;
- }
- public int getNumber() {
- return number;
- }
- public void setNumber(int number) {
- this.number = number;
- }
- @Override
- public Object clone() {
- Student stu = null;
- try{
- stu = (Student)super.clone();
- }catch(CloneNotSupportedException e) {
- e.printStackTrace();
- }
- return stu;
- }
- }
- public class Test {
- public static void main(String args[]) {
- Address addr = new Address();
- addr.setAdd("杭州市");
- Student stu1 = new Student();
- stu1.setNumber(123);
- stu1.setAddr(addr);
- Student stu2 = (Student)stu1.clone();
- System.out.println("學生1:" + stu1.getNumber() + ",地址:" + stu1.getAddr().getAdd());
- System.out.println("學生2:" + stu2.getNumber() + ",地址:" + stu2.getAddr().getAdd());
- }
- }
class Address { private String add; public String getAdd() { return add; } public void setAdd(String add) { this.add = add; } } class Student implements Cloneable{ private int number; private Address addr; public Address getAddr() { return addr; } public void setAddr(Address addr) { this.addr = addr; } public int getNumber() { return number; } public void setNumber(int number) { this.number = number; } @Override public Object clone() { Student stu = null; try{ stu = (Student)super.clone(); }catch(CloneNotSupportedException e) { e.printStackTrace(); } return stu; } } public class Test { public static void main(String args[]) { Address addr = new Address(); addr.setAdd("杭州市"); Student stu1 = new Student(); stu1.setNumber(123); stu1.setAddr(addr); Student stu2 = (Student)stu1.clone(); System.out.println("學生1:" + stu1.getNumber() + ",地址:" + stu1.getAddr().getAdd()); System.out.println("學生2:" + stu2.getNumber() + ",地址:" + stu2.getAddr().getAdd()); } }
打印結果:
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- 學生1:123,地址:杭州市
- 學生2:123,地址:杭州市
學生1:123,地址:杭州市 學生2:123,地址:杭州市
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。
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- addr.setAdd("西湖區");
- System.out.println("學生1:" + stu1.getNumber() + ",地址:" + stu1.getAddr().getAdd());
- System.out.println("學生2:" + stu2.getNumber() + ",地址:" + stu2.getAddr().getAdd());
addr.setAdd("西湖區"); System.out.println("學生1:" + stu1.getNumber() + ",地址:" + stu1.getAddr().getAdd()); System.out.println("學生2:" + stu2.getNumber() + ",地址:" + stu2.getAddr().getAdd());
打印結果:
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- 學生1:123,地址:杭州市
- 學生2:123,地址:杭州市
- 學生1:123,地址:西湖區
- 學生2:123,地址:西湖區
學生1:123,地址:杭州市 學生2:123,地址:杭州市 學生1:123,地址:西湖區 學生2:123,地址:西湖區
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:
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- package abc;
- class Address implements Cloneable {
- private String add;
- public String getAdd() {
- return add;
- }
- public void setAdd(String add) {
- this.add = add;
- }
- @Override
- public Object clone() {
- Address addr = null;
- try{
- addr = (Address)super.clone();
- }catch(CloneNotSupportedException e) {
- e.printStackTrace();
- }
- return addr;
- }
- }
- class Student implements Cloneable{
- private int number;
- private Address addr;
- public Address getAddr() {
- return addr;
- }
- public void setAddr(Address addr) {
- this.addr = addr;
- }
- public int getNumber() {
- return number;
- }
- public void setNumber(int number) {
- this.number = number;
- }
- @Override
- public Object clone() {
- Student stu = null;
- try{
- stu = (Student)super.clone(); //淺復制
- }catch(CloneNotSupportedException e) {
- e.printStackTrace();
- }
- stu.addr = (Address)addr.clone(); //深度復制
- return stu;
- }
- }
- public class Test {
- public static void main(String args[]) {
- Address addr = new Address();
- addr.setAdd("杭州市");
- Student stu1 = new Student();
- stu1.setNumber(123);
- stu1.setAddr(addr);
- Student stu2 = (Student)stu1.clone();
- System.out.println("學生1:" + stu1.getNumber() + ",地址:" + stu1.getAddr().getAdd());
- System.out.println("學生2:" + stu2.getNumber() + ",地址:" + stu2.getAddr().getAdd());
- addr.setAdd("西湖區");
- System.out.println("學生1:" + stu1.getNumber() + ",地址:" + stu1.getAddr().getAdd());
- System.out.println("學生2:" + stu2.getNumber() + ",地址:" + stu2.getAddr().getAdd());
- }
- }
package abc; class Address implements Cloneable { private String add; public String getAdd() { return add; } public void setAdd(String add) { this.add = add; } @Override public Object clone() { Address addr = null; try{ addr = (Address)super.clone(); }catch(CloneNotSupportedException e) { e.printStackTrace(); } return addr; } } class Student implements Cloneable{ private int number; private Address addr; public Address getAddr() { return addr; } public void setAddr(Address addr) { this.addr = addr; } public int getNumber() { return number; } public void setNumber(int number) { this.number = number; } @Override public Object clone() { Student stu = null; try{ stu = (Student)super.clone(); //淺復制 }catch(CloneNotSupportedException e) { e.printStackTrace(); } stu.addr = (Address)addr.clone(); //深度復制 return stu; } } public class Test { public static void main(String args[]) { Address addr = new Address(); addr.setAdd("杭州市"); Student stu1 = new Student(); stu1.setNumber(123); stu1.setAddr(addr); Student stu2 = (Student)stu1.clone(); System.out.println("學生1:" + stu1.getNumber() + ",地址:" + stu1.getAddr().getAdd()); System.out.println("學生2:" + stu2.getNumber() + ",地址:" + stu2.getAddr().getAdd()); addr.setAdd("西湖區"); System.out.println("學生1:" + stu1.getNumber() + ",地址:" + stu1.getAddr().getAdd()); System.out.println("學生2:" + stu2.getNumber() + ",地址:" + stu2.getAddr().getAdd()); } }
打印結果:
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- 學生1:123,地址:杭州市
- 學生2:123,地址:杭州市
- 學生1:123,地址:西湖區
- 學生2:123,地址:杭州市
學生1:123,地址:杭州市 學生2:123,地址:杭州市 學生1:123,地址:西湖區 學生2:123,地址:杭州市
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。
最後我們可以看看API裏其中一個實現了clone方法的類:
java.util.Date:
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- /**
- * Return a copy of this object.
- */
- public Object clone() {
- Date d = null;
- try {
- d = (Date)super.clone();
- if (cdate != null) {
- d.cdate = (BaseCalendar.Date) cdate.clone();
- }
- } catch (CloneNotSupportedException e) {} // Won‘t happen
- return d;
- }
/** * Return a copy of this object. */ public Object clone() { Date d = null; try { d = (Date)super.clone(); if (cdate != null) { d.cdate = (BaseCalendar.Date) cdate.clone(); } } catch (CloneNotSupportedException e) {} // Won‘t happen return d; }
該類其實也屬於深度復制。
淺談Java中的深拷貝和淺拷貝