目录
Part6:Java中的克隆
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***Example01:Java对象的假克隆
- 对象的克隆是Java中的一项高级技术,获得与其相同的对象。
基本数据类型可以使用“=”来进行克隆,此时两个变量除了相等是没有任何关系的。而对于引用类型数据不能简单地使用“=”进行克隆,这与Java的内存空间使用有关。 Java将内存空间分成两块,即栈和堆。在栈中保存基本类型和引用变量;在堆中保存对象。对于引用变量而言,使用“=”将修改引用,而不是复制堆中的对象。此时两个引用变量将指向同一个对象。因此,如果一个变量对其修改则会改变另一个变量。
运行结果:
代码实现:public class Employee { private String name; private int age; //省略set()和get()方法 @Override public String toString() { return "Employee{" + "name='" + name + '\'' + ", age=" + age + '}'; } public static void main(String[] args) { System.out.println("-----克隆之前:--------"); Employee employee1 = new Employee(); employee1.setName("hyn"); employee1.setAge(20); System.out.println("员工1的信息:\n"+employee1); System.out.println("-----克隆之后:--------"); Employee employee2 = employee1; //将employee1赋值给employee2 employee2.setName("azw"); employee2.setAge(21); System.out.println("员工1的信息:\n"+employee1); System.out.println("员工2的信息:\n"+employee2); }}
Example02:Java对象的浅克隆
在克隆对象时,如果对象的成员变量是基本数据类型,则使用浅克隆即可完成。如果对象的成员变量包括可变引用类型,则需要深克隆。
运行结果:
代码实现://Address.java public class Address { private String state; //所在国家 private String province; //所在省 private String city; //所在城市 public Address(String state, String province, String city) { this.state = state; this.province = province; this.city = city; } //省略set()和get()方法 @Override public String toString() { StringBuilder sb = new StringBuilder(); sb.append("国家:"+state+","); sb.append("省:"+province+","); sb.append("市:"+city); return sb.toString(); } }//Employee.java public class Employee implements Cloneable{ private String name; private int age; private Address address; public Employee(String name, int age, Address address) { this.name = name; this.age = age; this.address = address; } //省略set()和get()方法 @Override public String toString() { StringBuilder sb = new StringBuilder(); sb.append("姓名:"+name+","); sb.append("年龄:"+age+","); sb.append("\n地址:"+address); return sb.toString(); } @Override public Employee clone() throws CloneNotSupportedException { //实现浅克隆 Employee employee = (Employee) super.clone(); return employee; } }
测试代码:
class Test { public static void main(String[] args) throws CloneNotSupportedException { System.out.println("*****克隆之前:******"); Address address = new Address("中国", "湖北", "武汉"); Employee employee1 = new Employee("azw", 20, address); System.out.println("员工1的信息:\n" + employee1); //employee1的信息 System.out.println("*****克隆之后:******"); Employee employee2 = employee1.clone(); //使用克隆创建Employee2 employee2.getAddress().setState("中国"); //修改地址 employee2.getAddress().setProvince("黑龙江"); employee2.getAddress().setCity("哈尔滨"); employee2.setName("hyn"); employee2.setAge(21); System.out.println("员工1的信息:\n" + employee1); System.out.println("员工2的信息:\n" + employee2); }}
- 如果引用类型是不可变的,如String类对象,则不必进行深克隆。 ***
Example03:Java对象的深克隆
- 如果类的成员变量中包括可变引用类型,则需进行深克隆。
运行结果:
代码实现://Address.javapublic class Address implements Cloneable{ private String state; //所在国家 private String province; //所在省 private String city; //所在城市 public Address(String state, String province, String city) { this.state = state; this.province = province; this.city = city; } //省略set()和get()方法 @Override public String toString() { StringBuilder sb = new StringBuilder(); sb.append("国家:"+state+","); sb.append("省:"+province+","); sb.append("市:"+city); return sb.toString(); } //--------------------------- @Override public Address clone() throws CloneNotSupportedException { //Address类中的域不是基本类型就是不可变类型,所以可以直接使用浅克隆 Address address = (Address) super.clone(); return address; } //---------------------------}//Employee.javapublic class Employee implements Cloneable{ private String name; private int age; private Address address; public Employee(String name, int age, Address address) { this.name = name; this.age = age; this.address = address; } //省略set()和get()方法 @Override public String toString() { StringBuilder sb = new StringBuilder(); sb.append("姓名:"+name+","); sb.append("年龄:"+age+","); sb.append("\n地址:"+address); return sb.toString(); } @Override public Employee clone() throws CloneNotSupportedException { //实现深克隆 Employee employee = (Employee) super.clone(); //--------------------------------- employee.address = address.clone(); //--------------------------------- return employee; }}//测试代码同Example02测试代码.
- 要点:通常情况下,需要用到克隆对象时都需要使用深克隆。 ***
Example04:序列化与对象克隆
如果类的成员变量比较复杂,例如使用了多个可变的引用类型,使用clone()方法是非常麻烦的,所以可以考虑序列化的方式完成克隆。
运行结果:代码实现:import java.io.Serializable;public class Employee implements Serializable { //同Example04中Employee.java的代码}public class Address implements Serializable { //同Example04中Assress.java的代码}
测试代码:
class Test { public static void main(String[] args) throws IOException, ClassNotFoundException { System.out.println("*****序列化之前:******"); Address address = new Address("中国", "湖北", "武汉"); Employee employee1 = new Employee("azw", 20, address); System.out.println("员工1的信息:\n" + employee1); //employee1的信息 System.out.println("*****序列化之后:******"); Employee employee2 = null; ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("E:\\employee.txt")); out.writeObject(employee1); //将对象写入到本地文件中 ObjectInputStream in = new ObjectInputStream(new FileInputStream("E:\\employee.txt")); employee2 = (Employee)in.readObject(); //从本地文件中读取对象 if (employee2 != null) { employee2.getAddress().setState("中国"); //修改地址 employee2.getAddress().setProvince("黑龙江"); employee2.getAddress().setCity("哈尔滨"); employee2.setName("hyn"); employee2.setAge(21); System.out.println("员工1的信息:\n" + employee1); System.out.println("员工2的信息:\n" + employee2); } }}
要点:进行序列化的类需要实现Serializable接口,该接口中并没有定义任何方法,是一个标识接口。如果类中有可变的引用类型成员变量,则该变量需要实现Serializable接口。本实例采用将对象写入本地文件的方式完成序列化。
***Example05:深克隆和序列化的效率比较
- 通过使用这两种方式克隆100000个对象,并输出花费的时间来比较这两种方法的效率。
运行结果:
代码实现:import java.io.Serializable;public class Employee implements Cloneable,Serializable { private String name; private int age; public Employee(String name, int age) { this.name = name; this.age = age; } @Override public String toString() { StringBuilder sb = new StringBuilder(); sb.append("姓名:"+name+","); sb.append("年龄:"+age+","); return sb.toString(); } @Override public Employee clone() throws CloneNotSupportedException { //使用父类的clone()方法实现深克隆 Employee employee = (Employee) super.clone(); return employee; }}测试代码:import java.io.*;import java.util.ArrayList;import java.util.List;class Test { public static void main(String[] args) throws IOException, ClassNotFoundException, CloneNotSupportedException { Listemployees = new ArrayList (); //创建列表保存对象 Employee employee = new Employee("azw", 20); //创建对象 long currentTime = System.currentTimeMillis(); //获得当前系统时间 //使用克隆方式获得对象 for (int i = 0;i<100000;i++){ employees.add(employee.clone()); } System.out.println("克隆花费的时间:"+(System.currentTimeMillis()-currentTime)+"毫秒"); currentTime = System.currentTimeMillis(); //获得当前系统时间 for (int i = 0;i<100000;i++){ ByteArrayOutputStream bout = new ByteArrayOutputStream(); //创建字节数组输出流 ObjectOutputStream out = new ObjectOutputStream(bout); //创建对象输出流 out.writeObject(employee); //将对象写入到输出流中 //获得字节输出流内容 ByteArrayInputStream bin = new ByteArrayInputStream(bout.toByteArray()); ObjectInputStream in = new ObjectInputStream(bin); //创建对象输入流 employees.add((Employee) in.readObject()); //读取对象 } System.out.println("序列化花费的时间:"+(System.currentTimeMillis()-currentTime)+"毫秒"); }}
要点:使用ByteArrayOutputStream和ByteArrayInputStream可以将对象保存在内存中,这样就不必产生一个本地文件来完成序列化的功能。
***假克隆、浅克隆和深克隆的应用范围
假克隆 | 基本数据类型 |
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浅克隆 | 基本数据类型、不可变引用类型 |
深克隆 | 可变引用类型 |