1.传递引用
在一个方法中将一个对象的引用传递给另外一个方法,引用指向的对象是同一个
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 | public class Person { int age; String name; public Person( int age, String name) { this .age = age; this .name = name; } public static void main(String[] args) { Person p= new Person( 18 , "tom" ); System.out.println( "main: " +p); f(p); } public static void f(Person p) { System.out.println( "f(): " +p); } } |
引用别名
1 2 3 4 5 6 7 | public static void main(String[] args) { Person p= new Person( 18 , "tom" ); Person p2=p; p2.age++; System.out.println(p.age); //19 } |
引用p和p2指向的是同一个对象,p2对对象的属性进行操作,当使用引用p访问对象的属性时当然也改变,同样的情况也发生在对象引用在方法之间的传递,如下面的代码:
1 2 3 4 5 6 7 8 | public static void main(String[] args) { Person p= new Person( 18 , "tom" ); f(p); System.out.println(p.age); //19 } public static void f(Person p) { p.age++; } |
2. 创建本地副本
几个概念:
- 引用别名会在方法参数是对象类型时自动发生
- 没有本地对象,只有本地引用(方法中创建的对象存在于堆中,只有引用变量存在于方法栈中)
- 引用是有作用域的,而对象没有
- Java中的对象的生命周期并不是一个问题(垃圾回收机制)
2.1 值传递
Java中方法之间只有值传递
传递基本类型时,传递的是基本类型的值的拷贝
1 2 3 4 5 6 7 8 | public static void main(String[] args) { int a= 100 ; change(a); System.out.println(a); //100 不受影响 } public static void change( int a) { a= 99 ; } |
传递对象时,传递的是对象的引用拷贝
1 2 3 4 5 6 7 8 | public static void main(String[] args) { Person p= new Person( 18 , "tom" ); f(p); System.out.println(p.age); //还是18 f()中p指向了一个新的对象 不影响main函数 } public static void f(Person p) { p= new Person( 20 , "bob" ); } |
传递对象时如果在另外一个方法中对对象的属性进行操作会对main方法产生“副作用”, 但是如果只是简单的对引用进行操作是没有影响的
2.2 对象克隆
步骤:
类实现Cloneable空接口,默认情况下不希望所有的类都有克隆能力,当需要某个类有克隆能力时就需要实现该接口作为一种“可克隆”的标记,否则克隆时会报错CloneNotSupportedException
1 2 | public interface Cloneable { } |
重写clone方法,clone方法是Object类中的,它在Object类中的是一个protected的本地方法,需要重写,加上public修饰符,否则只能在当前类中使用clone方法
1 | protected native Object clone() throws CloneNotSupportedException; |
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 | public class Person implements Cloneable { int age; String name; public Person( int age, String name) { this .age = age; this .name = name; } public Person clone() throws CloneNotSupportedException { return (Person) super .clone(); } public static void main(String[] args) throws CloneNotSupportedException { Person p1= new Person( 18 , "tom" ); Person p2=p1.clone(); System.out.println(p1+ " name: " +p1.name+ " age: " +p1.age); System.out.println(p2+ " name: " +p2.name+ " age: " +p2.age); } } |
重写clone方法时实际是就是调用Object类中的本地clone方法,Object类中的clone方法做了哪些工作?
Object类中clone方法负责创建正确大小的存储空间,并执行了从原始对象中所有二进制位到新对象内存中的按位复制。
2.3 浅拷贝问题
场景:Person类中增加一个引用类型的属性Country, 表示这个人所属的国家,然后进行克隆
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 | package test; class Country{ String nation; public Country(String nation) { super (); this .nation = nation; } } public class Person implements Cloneable { int age; String name; Country country; public Person( int age, String name,Country country) { this .age = age; this .name = name; this .country=country; } public Person clone() throws CloneNotSupportedException { return (Person) super .clone(); } public static void main(String[] args) throws CloneNotSupportedException { Country country= new Country( "China" ); Person p1= new Person( 18 , "tom" ,country); Person p2=p1.clone(); System.out.println(p1+ " name: " +p1.name+ " age: " +p1.age+ " country: " +p1.country); System.out.println(p2+ " name: " +p2.name+ " age: " +p2.age+ " country: " +p2.country); p1.name= "bob" ; p1.country.nation= "America" ; System.out.println(p1+ " name: " +p1.name+ " age: " +p1.age+ " country: " +p1.country.nation); System.out.println(p2+ " name: " +p2.name+ " age: " +p2.age+ " country: " +p2.country.nation); } } |
问题描述: 当Person类中有一个引用类型属性时,对于基本类型数据和String类型是对值进行拷贝的,因此改变p1的name不影响p2的name, 但是对于Country这种引用类型,在clone时仅仅是拷贝了一份对象的引用,拷贝的引用和原来的引用指向的是同一个对象,因此p1改变country的属性时,p2的country的属性也发生了改变
2.4 深拷贝
解决浅拷贝问题的关键点在于不仅仅是拷贝引用,而且要拷贝一份引用指向的对象,深拷贝有两种方式:
- 逐个对引用指向的对象进行浅拷贝
- 使用序列化方式进行深拷贝
2.4.1 引用类型逐个浅拷贝
如果一个类A中有多个引用类型,那么这些引用类型的类需要实现 Cloneable接口,在对A的对象进行克隆时,逐个浅拷贝其中的引用类型
eg: Person类中有Country引用类型,在进行clone时,单独对country进行浅拷贝
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 | package test; class Country implements Cloneable{ String nation; public Country(String nation) { super (); this .nation = nation; } public Country clone() throws CloneNotSupportedException { return (Country) super .clone(); } } public class Person implements Cloneable { int age; String name; Country country; public Person( int age, String name,Country country) { this .age = age; this .name = name; this .country=country; } public Person clone() throws CloneNotSupportedException { Person p= null ; p=(Person) super .clone(); p.country=p.country.clone(); //对country进行浅拷贝 return p; } public static void main(String[] args) throws CloneNotSupportedException { Country country= new Country( "China" ); Person p1= new Person( 18 , "tom" ,country); Person p2=p1.clone(); System.out.println(p1+ " name: " +p1.name+ " age: " +p1.age+ " country: " +p1.country); System.out.println(p2+ " name: " +p2.name+ " age: " +p2.age+ " country: " +p2.country); p1.name= "bob" ; p1.country.nation= "America" ; System.out.println(p1+ " name: " +p1.name+ " age: " +p1.age+ " country: " +p1.country.nation); System.out.println(p2+ " name: " +p2.name+ " age: " +p2.age+ " country: " +p2.country.nation); } } |
2.4.2 序列化方式进行深拷贝
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 | package test; import java.io.ByteArrayInputStream; import java.io.ByteArrayOutputStream; import java.io.IOException; import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.io.Serializable; class Country implements Serializable{ private static final long serialVersionUID = 1L; String nation; public Country(String nation) { super (); this .nation = nation; } } public class Person implements Cloneable,Serializable { private static final long serialVersionUID = 1L; int age; String name; Country country; public Person( int age, String name,Country country) { this .age = age; this .name = name; this .country=country; } public Person clone() throws CloneNotSupportedException { Person p= null ; ObjectInputStream ois= null ; ObjectOutputStream oos= null ; ByteArrayInputStream bais= null ; ByteArrayOutputStream baos= null ; try { baos= new ByteArrayOutputStream(); oos= new ObjectOutputStream(baos); oos.writeObject( this ); //Person对象序列化 序列化写入到baos流中 bais= new ByteArrayInputStream(baos.toByteArray()); ois= new ObjectInputStream(bais); //从baos流中读取数据到ois p=(Person) ois.readObject(); //ois读取对象数据 } catch (Exception e) { e.printStackTrace(); } finally { if (bais!= null ) try { bais.close(); } catch (IOException e) { e.printStackTrace(); } if (baos!= null ) try { baos.close(); } catch (IOException e) { e.printStackTrace(); } if (oos!= null ) try { oos.close(); } catch (IOException e) { e.printStackTrace(); } if (ois!= null ) try { ois.close(); } catch (IOException e) { e.printStackTrace(); } } return p; } public static void main(String[] args) throws CloneNotSupportedException { Country country= new Country( "China" ); Person p1= new Person( 18 , "tom" ,country); Person p2=p1.clone(); System.out.println(p1+ " name: " +p1.name+ " age: " +p1.age+ " country: " +p1.country); System.out.println(p2+ " name: " +p2.name+ " age: " +p2.age+ " country: " +p2.country); p1.name= "bob" ; p1.country.nation= "America" ; System.out.println(p1+ " name: " +p1.name+ " age: " +p1.age+ " country: " +p1.country.nation); System.out.println(p2+ " name: " +p2.name+ " age: " +p2.age+ " country: " +p2.country.nation); } } |
总结: 实现一个可克隆的类的步骤
- 实现Cloneable接口
- 重写clone方法
- 在重写的clone方法中调用super.clone()方法
- 在重写的clone方法中捕获异常
总结
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