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
假如說你想複製一個簡單變量。很簡單:
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。
但是如果你複製的是一個對象,情況就有些複雜了。
假設說我是一個beginner,我會這樣寫:
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:
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,作爲一個新手,拍了拍胸腹,對象複製不過如此,
難道真的是這樣嗎?
我們試着改變stu2實例的number字段,再打印結果看看:
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:
學生1:54321
學生2:54321
學生1:54321
學生2:54321
這就怪了,爲什麼改變學生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:
那麼,怎樣才能達到複製一個對象呢?
是否記得萬類之王Object。它有11個方法,有兩個protected的方法,其中一個爲clone方法。
該方法的簽名是:
protected native Object clone() throws CloneNotSupportedException;
因爲每個類直接或間接的父類都是Object,因此它們都含有clone()方法,但是因爲該方法是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(淺複製):
1. 被複制的類需要實現Clonenable接口(不實現的話在調用clone方法會拋出CloneNotSupportedException異常) 該接口爲標記接口(不含任何方法)
2. 覆蓋clone()方法,訪問修飾符設爲public。方法中調用super.clone()方法得到需要的複製對象,(native爲本地方法)
下面對上面那個方法進行改造:
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:
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,那麼你可以看看這一句:
System.out.println(stu1 == stu2); // false
System.out.println(stu1 == stu2); // false
上面的複製被稱爲淺複製(Shallow Copy),還有一種稍微複雜的深度複製(deep copy):
我們在學生類裏再加一個Address類。
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:
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
乍一看沒什麼問題,真的是這樣嗎?
我們在main方法中試着改變addr實例的地址。
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:
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,怎麼兩個學生的地址都改變了?
原因是淺複製只是複製了addr變量的引用,並沒有真正的開闢另一塊空間,將值複製後再將引用返回給新對象。
所以,爲了達到真正的複製對象,而不是純粹引用複製。我們需要將Address類可複製化,並且修改clone方法,完整代碼如下:
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(); //淺複製
}catch(CloneNotSupportedException e) {
e.printStackTrace();
}
stu.addr = (Address)addr.clone(); //深度複製
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(); //淺複製
}catch(CloneNotSupportedException e) {
e.printStackTrace();
}
stu.addr = (Address)addr.clone(); //深度複製
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:
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複製一份,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:
/**
* 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複製。